INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
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Janu
Mamut
cml
urbandrag
Manna
Edum
10 participantes
INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
Edum 3/11/2014, 11:09
es muy interesante, todos nosotros estamos interesados en mayor performance y en cambiar los escapes...
esta en ingles...y es muy largo para traducir..... podria pasarle un traductor online pero suele quedar incomprensible...pero bueno cualquiera puede hacerlo despues
primero unas palabras sobre la admision para luego pasar al escape
Intake system:
Increasing the size of the carburetors is not always the best thing to do. AS carburetor throat (venturi) diameter is increased, it becomes harder and harder to obtain enough velocity through the carburetors (for that, one usually needs increased rpm and/or displacement) to enable them to atomize fuel correctly. In some cases, as some owners of R80 and R100 machines well know, the 32 mm carburetors provide better jump/lower to mid midrange and even higher, than the 40 mm. With carburetor sizes, and this also applies to fuel injection throat size, smaller throats give better low to mid-range throttle response, sometimes even upper mid-range performance. That is because high air velocity through the carburetors or FI is needed to allow them to do their job. At JUST the extreme top end, let us say at or near wide open throttle and high rpm, the largest carburetors that will cause decent atomization, are usually the best for power output. A manufacturer usually strikes a balance in sizes. Unless one can stay in this high throttle/rpm range (with a more peaky cam you'd also want close ratio gears), OVERALL acceleration may suffer. The Bing CV carburetors TEND to act like a rather modest variable venturi, but do not provide the almost instantaneous 'snap' of a directly controlled slide or butterfly carburetor. What is not often realized is that over-all acceleration may be the same.
It is more important than often considered, that in the mid-range of rpm, good response is needed on a touring bike. If the carburetor is too large, all sorts of strange effects will occur, including the engine simply refusing to accelerate smoothly when the throttle is opened a large amount, especially suddenly. The CV carburetor tends to greatly minimize that effect, but in some instances it will still be evident. Still, it is the combination of enough quantity of mixture, AND the throat velocity, to ENABLE an atomized mixture to get into the cylinder that is important. If the velocity is too low, the next intake stroke of the piston will not allow much mixture into the cylinder. It is the total sum quantity of atomized mixture that gets into the cylinder, at the right time, that is the important thing (this is a simplification).
In ALL cases, the best power is available (assuming jetting can be modified if need-be) from the coolest possible temperature of the incoming air (within reason), since the vast portion of the burning mixture by weight, and volume, is the oxygen in the incoming air. Cooler air has FAR more oxygen, allowing more power, if the gasoline quantity/mixture is adjusted to match. This is a BIG effect. A definite increase in power is available from getting cool air to the carburetor intakes. However, if this air is not warm enough, one can freeze up the carburetor (venturi icing); especially if the air contains a lot of moisture. Contrary to popular belief, MOISTURE in the air (high humidity) means LESS power, as the air has LESS usable oxygen. Generally cold air is not a problem once the engine is warmed up....or not with air intake systems that are warming the incoming air. When cold, the engine may not run well as the fuel is not atomizing well, condenses on cylinder walls and other parts, and is no longer a vapor, especially with cold parts. That is why vehicles have 'chokes'...for cold weather operation, allowing lots more richness, really a brute force method. Fuel injection systems can have far better control, as they can automatically compensate for air density; and most compensate for oxygen in the exhaust, air pressure, temperature of cooling water, ETC.
You WILL get more power, perhaps lots more, if you find a way to get cooling air to...and through... the airbox. It is often difficult to do it neatly, and to avoid ingestion of rain, leaves, etc. A NASA type duct, including the angle separation system used on turbine motors (airplanes) works well...sometimes.
Ram air: Sometimes folks ask questions about RAM air possibilities for no-cost supercharged horsepower. Forget about it unless you are planning to ride over 150mph. Below 130 mph, the effects are EXTREMELY small, and effects even at 150 are just barely noticeable. The effects above 150 are noticeable and worthwhile. And, no,... a larger scoop, does NOT mean you will see improvement. You could make a scoop/funnel the width of the motorcycle, and all it would do is likely add a lot of drag. Yes, that sounds wrong, 'common sense' is otherwise, eh? There is some information on intake areas velocity in my formulas article.
Dynamic air pressure by or on a moving object are treated by engineers as a certain size of flat plate moving through the air at some speed. Dynamic air pressure is proportional to the SQUARE of speed, but the horsepower to attain that speed goes up as the CUBE of that speed change.
Consider your Airhead or any other motorcycle, moving at 68 statute miles per hour. That is easily converted to feet per second, and the value is 99.7, let us just say 100 feet per second. There is about 12 pounds of pressure per square foot of surface area. At 200 feet per second (about 136 miles per hour) the pressure is 48 pounds per square foot. (see, I did the calculations for you!!). Thus, the drag is 4 times higher for a doubling of speed (I put this information here so you'd know why gasoline use skyrockets with increasing speed). If one was to divide by 144, you would have the pressure per square inch, in this case 0.33 PSI. That is basically almost nothing compared to atmospheric pressure forcing itself into the cylinders (about 15.0 psi at sea level). Thus ram-air supercharging effect is very small until speed gets VERY high. It starts to make some reasonably usable difference around 150 mph. Thus, ram air pressure does NOT help at ordinary road speeds.
Intake tract length: This is a particularly difficult idea to get across, and the same effects, in reverse, are in the exhaust system. This effect is very noticeable at all throttle settings, but the effect varies greatly with those settings:
When the intake valve is in its 'open, at least somewhat' phase, and in conjunction with a time just after the piston reaches bottom, and the valve is still not yet closed (we are not going to get into cam timing theory here), the air coming through the intake system is NOT a steady flow. In fact, at any time, intake valve open or not, there is still some flow into the intake system, as this flow pressurizes against itself, readying itself for the next opening of the intake valve, so to speak. This flow is varying in velocity and pressure, depending on the valve opening, and a few other more esoteric things. Pressure here means absolute pressure, or, if you will, referenced to atmospheric. To picture this simplified, let us start at the beginning of the intake stroke. The valve is opening, the throttle is open a bit or more. As the piston lowers, it reduces the atmospheric pressure in the cylinder, allowing the outside air pressure to push air/fuel mixture into the cylinder. View it as sucking the outside air/mixture inwards if you must. The piston eventually stops lowering, and in modern engines, the valve closes a bit later. Since this happens at a fast rate, even at idle, the intake flow is in PULSES. The incoming air slows when that valve closes, and this slowdown occurs VERY suddenly. View this as a slug of air slamming against a closed valve, if you must think of it that way. These pulses are described by engineers in a type of complex interacting mathematics dealing with 'waveforms'. You can think of it as the air piling up on itself. Because of this pulse, well, really lots of pulses, some complex things happen in the intake system, and the one that I want to discuss is the most complicated one, the reflected pulse.
The intake system will act like 'any one specific pulse is partially reflected backwards'. It is not inertia, and if you cannot picture it yet, just accept it as fact! I will discuss this from a different viewpoint later, in the exhaust, which might be easier to understand. This reversion/reflection effect occurs at the point of intake, which in the later airheads, as an example, is approximately at the tips of the snorkels!! If, at any one given and constant throttle and constant rpm, one could adjust the LENGTH of the effective intake system, one could find a correct length so that the REFLECTED pulse hits the intake valve at exactly a particular moment of time, IN RELATION TO an existing normal INcoming pulse of mixture. This will have the effect of boosting power, as more fuel/air mixture will be forced into the cylinder...at NO cost whatsoever in loss of energy, etc. Of course, you will use more fuel! FREE POWER!!!...except for the gasoline usage! I once actually made up a VERY crude sliding tube system for the intake, one of these replaced both snorkels, and on a dyno, moved the tube-inside-a-tube, noting very distinct changes. I made a second one up, guess-tuned it INITIALLY for 5000 rpm, and varied it while on the dyno. Believe it or not, this crude device was made from the inner cardboard 'roll' of a roll of kitchen counter wipes. The effect on rpm/power had to be seen to be believed. It was necessary to jockey the throttle and tube length to keep the rpm more or less constant. A change of as much as 8 horsepower was noted. I want to CAUTION here that I did this only crudely, and a truly more vigorous AND PROPER analysis/testing would have REQUIRED changing the carburetor jetting and an exhaust analysis at each try, and I did not do this, due to lack of time that day.
NOTE: Engines have been built with variable length intact tracts, controlled by throttle position, rpm, and other things. In the past, most of these things were not very practical. In the last 15 or 20 years, some car and motorcycle manufacturer's have managed to incorporate some of these things, and more. There have been some clever ways to dynamically change the effective length and size, of the intake tract. There have even been engines with multiple intake valves that had a valve closed at times other than normal, and especially notable is the various methods of variable camshaft timing and multiple intake butterflies. Manufacturer's found ways to INcrease compression ratios far higher than if you were an old timer and had only 87-91 octane fuel available. RACE tunings on the camshafts became relatively common, yet the engines maintained nice torque curves and were very tractable, that is, smooth throttle response, lots of low and mid-range power, and plenty of top end power.
Many engines were using rather radical camshaft timing and would otherwise have low torque in the lower mid-range rpm area, except that the various cam timing changes and intake tract tuning GREATLY enhanced performance by improving those lower and mid areas; yet kept the high rpm performance; all this on regular grade fuel, with some engines having even bigger performance on 91 octane. A few of these engines made NEARLY THREE HORSEPOWER PER CUBIC INCH!
A few manufacturer's added turbochargers (a very few used superchargers) and kept reasonably high CR.
Modern metallurgy and casting techniques allowed the engines to have normal....even extended....life!
The Exhaust System:
On a practical basis, you could well think that all of the intake effects already discussed ALSO happen in the exhaust system, due to complex relationships with the exhaust valve, length of system, intake opening, etc. The same sort of formulas and things generally really do apply. Please keep in mind that the things that happen in the exhaust system, also happen in the intake system. It is just harder for most folks to understand it in the intake system, so I will delve here into the exhaust. Let me try to explain this in a laypersons viewpoint (I hope), in a gross simplification:
Exhaust sizes: The effect is generally small, and what is usually most important is the effective cross section area from cylinder head port to the outlet...or at least the muffler intake. A smaller diameter exhaust header pipe (meaning all of the piping, up to the muffler) can HELP midrange torque, but may or may not hinder top end. It is important, GENERALLY, that the muffler interior volume be substantial, the reasoning for this is quite complex. YES, all this means that, in general, a noisy small diameter straight-through muffler does NOT perform well! Many have replaced the BMW mufflers on the Twin-Shock Airhead models and found that loud exhaust does NOT equate with more horsepower, in fact, performance may suffer....a LOT. BMW did their homework on the mufflers. UNfortunately the mufflers that are under the frame, GS type, etc., do NOT perform as good as the old BMW twin mufflers.
Let's get into this a bit deeper:
The exhaust valve begins to open, and the mostly burned gases begin to exit, and the process continues, until a little bit AFTER the exhaust valve closes, due to inertia of the gases. Each power stroke results in a pulse of gases, of uneven shape. Multiple firings of the cylinder means a 'train' of unevenly shaped pulses going down the exhaust pipe. At higher and higher RPM, the pulses get shorter and shorter, but more often. YES, exactly the same type of thing as the incoming air for the intake system. Now, speaking of any single pulse, when some portion of this pulse...for our purposes, let us say the very beginning,,,, reaches the exit mouth of the muffler tip (or unmuffled pipe...etc), the gases, which have accommodated themselves to the pipe system before this, now suddenly see vastly different conditions at that very exit point. The gases see atmospheric pressure, atmospheric temperatures, increased effective diameter of the pipe (NO pipe!!)...ETC. The gases are confused, and think they have run up against a brick wall, which then gives way, modestly smoothly, and allows the rest of the pulse to go to the outside air. It does not 'give way' instantaneously, it seems so, but is not so. It gives way in proportion to the intensity of the gas pulse, basically suddenly strong, then fades slowly, all this happening in an instantaneous manner, unless you have instruments to measure it. At the confusion point, the gases are reflected back up the pipe, all the way to the exhaust valve...and even into the cylinder if the valve is open. The reflection can be partial, partial distance, or a combination. One of the very complicating factors is that ANY inconsistencies in smooth pipe insides will promote a point of reflection, which may be quite minor, or could be major. This includes the change in effective pipe diameter as the header pipe enters the muffler, or before, crossover pipes with poor smoothness of the curve that should really exist where they meet the main header pipe, and so on. Every little jog or anything not smooth to the flow, creates a change in the traveling exhaust, perhaps a detrimental reversion wave, small or large.
You can think of the exhaust gases as a series of rubber slugs if you wish, traveling in the exhaust system, with a bit of separation between them. They hit in inconsistency...or, especially, the outside air, and SOME rubber bounces back, pushing each one ahead of it...and so on. So, what we have is a reversal/reversion of SOME of the exhaust energy. This would or could be a bad thing, except that the pipe length can be made such that a pulse can arrive at the valve at any portion of that pulse, or in-between pulses at any given single rpm. On a PRACTICAL note, it would require a much TOO LONG exhaust system to take full advantage of this effect for lower rpm, so the manufacturer must deal with that in other ways.
NOTE! The careful reader may well say, what about a LONG intake, and a SHORT exhaust. ...well, that would be workable, but the intake will have more problems staying cool, and many other problems....including the FACT that the exhaust must exit someplace not near the rider's knees! On a practical basis, due to gas expansion, mechanical location problems, and MANY other things, the exhaust system is longer than the intake system, and the exhaust tuning on 4 stroke engines is usually done mostly by pipe diameter, and one or more crossover pipes, and some muffler innards that are often VASTLY more clever...or, at least, have vastly more real engineering in them, than just for sound reduction. In some systems, the exhaust flow starts out with a small diameter and is a constantly expanding taper...this can be made to work very well indeed....but is expensive to fabricate. It was very commonly done with 2-stroke engines, with many welded-in sections, called Expansion Chambers. The outlet of such can be a very small diameter!
I have NOT, so far, except in reports of experiments, seen anyone with a truly variable tuned exhaust system...although some single bypass systems are in use, and there are now appearing some mechanically controlled ones with more than a fixed one-two change. If the mechanical problems could be sorted out, it might be possible to combine a variable camshaft, separate pair of intake valves for differing cam lobes, variable intake length and cross-section; together with a variable exhaust, all electronically controlled, and obtain an almost unbelievably flat torque curve (or, super peaky race engine.....; whatever).
As you have read, the length and size of intake and exhaust systems have big effects. In quite a few ways, some of these things use the same formulas and are similar to tuning a pipe organ, at least in some areas. Engineers use the same formulas for open and closed pipes in organs for some of the needed calculations.
If this pulse (or lack of pulse; that is, in-between pulses) of reverse energy meets another oncoming (from and out of cylinder) pulse at exactly the right moment, it CAN reduce the pressure (or increase it) in the pipe at that particular point, and THIS effect then travels back down the pipe. With a lowered pressure, the exhaust is extracted from the cylinder far better....and when this occurs with the intake tract still flowing, as it is with valve overlap timing, MORE intake charge goes into the cylinder as well. Almost like a supercharger. This means that a tuned exhaust can extract more exhaust AND suck in more mixture to be burned! This seems like the best of both worlds, and CAN be, but the effects vary with just about every other thing, including the rpm, throttle position (usually), entire intake system design, the camshaft timing for the valves, shape and sizes of things, and dozens of other parameters. Many decades ago, before computers helped the engineers, all this was done by first cutting and trying (with some engineering formulas). Nowadays with high speed computers and special instruments, all able to analyze and correlate hundreds of changing characteristics at the SAME time...there is far less 'cut and try'. However, overall, exhaust and intake design is extremely complicated. Even today, computers and lots of measurements with sophisticated equipment do not usually tell the whole story, just give one some directions/hints, granted rather good ones, usually.
SO: In the INtake system, it is desirable to fashion the system so that the reflected pulse enhances the flow by creating even more of a vacuum than normally would exist, or a longer length of the gases slug. In the exhaust system, the reflected pulse, from the exhaust outlet tip, is returned to the exhaust valve area, and if timed correctly, will REDUCE exhaust system pressure, and thereby, in a complex manner, allow MORE INtake of fuel mixture to occur, due to the valve overlap period, including the effects of the intake charge pushing the remnants of the diluted exhaust out the exhaust valve...AND reducing 'back pressure', allowing more flow in the first place.
If you are getting the idea that back pressure IN ITSELF is NOT the most important thing...you are absolutely correct.
The problem is in getting it all to work together, as you change one thing, and that seems to change many more things. That is why well-experienced master tuners are in such high demand for commercial racing. Today's master tech's use very sophisticated computerized tools for analysis.
Back pressure is LESS important than the pulse timing; back pressure has LITTLE effect until it is really high!
MORE on this a bit further down this page.
Design of TRUE performance enhancing exhaust systems, especially the mufflers, is a real art, as well as science. Today, vehicle manufacturer's have LOTS of pressure transducers....and visual transducers!.... plumbed into prospective 'mufflers' and other exhaust components, and results analyzed on very high speed computers. You might be surprised to find out that a PROPERLY designed 'fishtail' exhaust can be a good enhancer of performance, as are several other designs, IF properly done for performance, and not for just appearance. Generally speaking, it is desirable to have the exhaust system gases constantly expanding, even if ever so slightly, in cross-section (that means larger and larger inside diameter of the round pipe...or, its equivalent in other shapes), from exhaust valve to outlet. In some designs the very tip is then restricted, changing the velocity, and reflection pulse. Does this begin to sound a bit like an intake system too? Megaphones are not practical on the street, unless internally muffled, which tends to defeat their performance. A proper megaphone for performance on a stock airhead would be quite long, with a SLOW taper, and it PROBABLY would stick WAY out and beyond the end of the motorcycle. SOME types of megaphones have a reverse tapered outlet, which enhance the performance, whilst others are sales gimmicks. Many folks find that the BMW stock muffler is better, performance-wise, than they thought, after spending lots of $$$ on aftermarket exhaust systems. SOME of the BMW airhead models with mufflers/collectors located under the transmission can be modified, usually the sound increases some, and the torque MIGHT or might not increase/decrease. In other instances, you can make major improvements.
IF putting other 'mufflers' on your bike vastly improves performance, you likely have a poorly tuned motor to start with.
Most folks seem to have the idea that reducing back pressure in the exhaust system is the holy grail of design. This is NOT so. What is REALLY desired is the extraction effect at the exhaust valve....and the effect this will have on the incoming fuel-air mixture. True back pressure has a small effect on power,,,,within limits of course. Perhaps a few more words will help in this area:
Shortly after the spark plug ignites the mixture, the flame begun by this spreads rapidly, and the very rapid expansion of the gases produces a very high pressure, which is very effective in moving the piston downwards. As the piston moves downwards this pressure reduces by a huge amount. By the time the exhaust valve begins to open, the pressure is fairly low. The pressure then is probably under 100 pounds per square inch; still enough to do some work. The pressure in the header and exhaust pipe, due to expansion into that area, is much lower, especially at low rpm, but still fairly low even at high rpm. It is only a FEW pounds per sq. inch. If you do not believe this, have someone put the throttle up for 5000 rpm, and cover both exhausts with your hands, try to stop the engine, while your friend turns the throttle wide open. There is no way that any internal pressure, without your hands, would in itself reduce engine power greatly, compared to the very high combustion pressures. A horsepower is 550 foot pounds per second...do you REALLY think that some sort of pressure on your hands is relative here? (ok, so that is a bad analogy, so what). It is the effect of the waves, in a properly designed system, that reduces this pressure somewhat, but reduces it AT the valve area, in synchronization with the incoming charge (remember: valve overlap...the intake valve is opening to allow incoming charge while the exhaust valve is still partially open), thereby allowing MORE mixture to be packed into the cylinders. THAT 'packing of mixture' is where the largest portion of the power increase is noted, with a good exhaust system, from the improvement in the force on the piston during the early portion of the power stroke......AND HOPEFULLY CONTINUING DOWN THAT STROKE A LONGER DISTANCE THAN PREVIOUSLY. Just a very small amount of lengthening of the amount of the stroke that usable pressure is on the piston makes a BIG power output difference.
Many decades ago, 'boxes' were put where we think of 'muffler', and this box, which was NOT a TRUE expansion chamber, collected the exhaust mixture, quieted the exhaust, and had not much restriction. Early engines mostly used that system. An improvement on this was the Brooklands 'muffler', which developed into the full-blown type called a 'fishtail'. This design used a slot (unknown to many who have seen fishtail mufflers...likely because those in the U.S. are GARBAGE designs without the slot) in the outlet, the resultant of which was that the AREA of the outlet was somewhat LESS than the input pipe area...which reduced the sound level. (hmmm...whatcha think about the smaller inlet size of snorkels now??)
Due to very complex waveform development inside the design, the fishtail muffler had a greatly reduced pipe pressure, compared to an OPEN PIPE! In fact, it was roughly 50% better! YES..this means MEASURED RESTRICTION meant LESS restriction in OPERATION! If the fishtail was removed, and only the 'box' used, the results were in-between an open pipe and with the fishtail.
The very next step in early design was the adding of smallish holes in internal baffles, and the complications this made were eventually solved. It was then possible to have a exhaust system that muffled AND performed. I know this sounds absolutely wrong, common sense seems to tell us that restrictions ALWAYS mean lower power, but it is true.
In the intake system, the reflected pulse can be timed, within reason, to boost effective amounts of actual cylinder filling fuel/air mixture. This effect is why RAM TUBES are seen on so many vehicles these days. They lengthen the intake system, adding more power robbing drag (from wall effects, and hopefully keeping laminar flows), but boosting far greater the reflected pulse effect. With LOTS of dyno time, well-designed ram tubes can work well, whether between carbs/FI and head, or carb/FI inlets and intake of aircleaner. Ram tubes generally allow more radical cam timing, without the engine becoming too 'peaky'.
BMW factory-sold 'sport camshafts' are rather versatile, performing fairly well for the mostly stock motor, without any necessity for radical camshaft timing, (radical camshaft timing usually results in a very rough loping idle and VERY poor low end performance), and there is no dangerous super high valve lifts either. Use of the factory type sport cam with a compression increase is helpful. It is also helpful to reduce the clutch/flywheel weight, to allow the engine to accelerate a bit faster, although this has some drawbacks. BTW, some sporty camshafts, the BMW mild one included, EFFECTIVELY raise the compression ratio, DURING high rpm operation. We will NOT get into why!...there are several effects. You can NOT just change the camshaft in your Airhead, without other work. If you do install such as the BMW sport camshaft, be prepared to do LOTS more, or your bottom end performance will SUFFER...a LOT. The BMW sport camshaft will greatly detract from performance if you not do lots of other things. Even with lots of other modifications, the sport cam will STILL only START to "perform" above 5000 rpm....typically the improvement is notable from 6000 or so. I just can NOT recommend the BMW sport cam (often called 'the 336") for any sort of street Airhead for pleasurable touring characteristics. For a Cafe Bike, OK. For racing, yes.
Other, hard to quantify effects, come from combining the intakes. This effect is in the exhaust also. In our BMW bikes, the intake filter housing feeds BOTH carburetors. Hence there is now a complicating factor (VERY complicating) of the unequal 'sucking' strokes/pulses of the two out of phase cylinders, and the effect THIS has on the incoming wave charge. AND, that effect varies with throttle opening and rpm, ETC.
Just the thought of a sales department telling engineering management that 'a two into one exhaust is needed for appearance-sake', for an upcoming model, means hundreds of test and design hours, on intake and exhaust!
What all this means, on a practical basis, is that you really need a dyno to REALLY hop up a bike....OR, someone who already has done exactly this, or has been very lucky, to give you all the exacting and exciting details!!!
Here is that updated URL for Mez Porting's calculator again, I had it near the top of this article too.
[Tienes que estar registrado y conectado para ver este vínculo]
Now, specific recommendations, that I feel I am safe in making:
Before we get into things, below, a few flat-out statements, here in RED; and these apply to stock, relatively stock, somewhat modified airheads, and fairly well modified airheads...........in every instance I mean for the street....NOT for full-out racing. It makes no difference the engine size, compression ratio, piston changes, milling the heads; nor, making carburetion changes, nor even installing the BMW sport camshaft....my RED comments apply to all:
A. Use of a two-into-one exhaust system will likely REDUCE performance.
B. The Super-Trapp type of exhaust is tricky, hard to get it to perform correctly, and is VERY likely, even with large numbers of discs, to perform MUCH less well than the stock BMW exhaust.
C. Use of K & N air filters is NOT going to improve performance, and WILL decrease engine life.
D. Use of individual air cleaners, screens or filters, of any type, at each carburetor throat, instead of using the stock air cleaner system, will CAUSE PROBLEMS.....and make the engine less tractable, due to the bad effect on the flatness of the torque curve. In some cases the weight will cause vibrations that will cause the fuel to FOAM. To get around this, the proper type of rubber intake hoses, and a support for the carburetor and perhaps the individual air cleaners/screens is needed. More power IS available from individual air cleaners, but the torque peak is moved upwards in rpm, making for a more peaky engine.
1. Camshaft: A BMW sport camshaft can be used, unless you have access to a dyno equipped camshaft grinder company for your specific application. The BMW Sport Cam has a deleterious effect at lower rpm. This is a timing cam, not a lift cam, and use of higher lift, by incorporating high ratio rocker arms, is probably possible, but one must be VERY careful. Pay attention to the rocker arms, pushrods, etc. The engine will be happiest above 5700 rpm. This means that without a LOT of other changes, you will LOSE performance lower down.
2. Compression ratio: the higher the better. On a practical basis, probably about 9.5 is the highest with a stock camshaft, and maybe 10.5 with the sport camshaft. This includes dual-plugging. With a really clean combustion chamber, with no sharp places, and everything tuned to the Nth, even 11.0 might be possible on 98 octane or better, with dual plugs. The problem is definitely the octane of available gasoline. If you have the time and do not mind experimenting, the squish area, as in the 1977 airhead, can be played with, with some improvement available if it is reduced, or re-incorporated. One problem in any hopping up, is that if you are using a R100 engine, it is already running quite hot. If you are willing to do valve jobs more often, you can go pretty far, approaching early Oilhead output. The larger valves of the early models, with the larger carburetors, are going to be needed with higher compression and a sport camshaft. A good 3 angle valve job, ...careful valve guide work, ... are necessary, except for the most extreme motors, where polishing, reducing weights, shaving guides, modifying lifters and pushrods, etc., are helpful. I do not think going to a larger bore via special expensive aftermarket kits is a good idea. BUT...if you already need cylinders, maybe going to 1050 cc or so is OK, but do NOT increase the bore on the R65....it will probably be UNreliable. Keep in mind that going over 9:5 in CR, or going to large bores, will, or can, cause reliability problems. Very careful attention to the valves, valve springs, etc., is going to be NECESSARY.
3. Flywheel, clutch, transmission: The early heavy flywheel/clutch assembly is fine for most uses, but not for best acceleration and fast shifting, where lighter items are better, but give more vibration, which can be somewhat (ONLY somewhat) reduced by balancing. Even the later 'flywheel' (carrier) and clutch is too heavy for the most spirited sport riding and racing. Some of the later clutch parts are well known to us wrenches as going out of balance, from warping (like the plate ring), so balancing is relatively important. Vibration robs power, not just annoying to you physically. SOME may want to go into the transmission and modify it for easier/faster shifting (particularly at highest rpm). There is an article on this website on how to PROPERLY lighten the flywheel: CLICK
4. Intake system: you should consider modifying for cold air to AND THROUGH the airbox. Except for a near race or actual race motor, I suggest you NOT use individual screens or filters right at the carburetor mouths (thereby not using the airbox at all). If you do go to individual filters, do some work with tufting or smoke, to ensure a good airflow to those filters, and that airflow coming from the forward area is not being curved excessively, and creating a light vacuum at the filters.
With regards to cold-air: Avoid effectively lengthening the intake system, unless you PLAN to lower the rpm for torque peak. If the cross section areas are too small you will run into non-laminar flow effects, so avoid this. You want to avoid supersonic velocities. You MUST run tests to be sure that the area of cold air pickup really is in an area that is not affected by the front end/fender/etc...and surrounding parts...that can actually cause loss of effective air pressure there. AGAIN, generally unless engine is heavily massaged, use of the stock airbox, modified for smooth flow and cooler air, is recommended. I have reservations, however, about using the clamshell box...and think that it should be eliminated. Get rid of ALL smog items under the air filter on the later models. Rework that airbox for a SMOOOOOTH flow of air internally. Yes, you want turbulence, but you want it AFTER the carburetor...INSIDE the cylinder! Insulate as required with cambric, etc., so that as little as possible hot metal comes in contact with the incoming air. HINT: Sometimes you will get a bigger improvement from making the intake system smaller, IF it remains cool! If you keep the intake system from being excessively heated by the engine you will gain a LARGE amount of power. The power gain from cold air modifications is REALLY significant. This was one of my personal secrets.
Snorkels: They can be cut back, a bit at a time, or, removed entirely, and you will see the BAD effect THAT usually has on a stock machine. They can also be opened, but not much, unless you are going to very high rpm consistently. Shortening them will move the torque peak upwards. Generally, if the carburetor size is stock, you can do minor mods on the snorkels on both the R80 and R100 machines, but watch for loss of midrange torque, which you will want to make up elsewhere if you go too far. You will need jetting changes along the way here. Gas mileage can suffer if the rpm position for the snorkel effect is off, BMW found that out with the R80, and R65, and then modified the snorkels with holes...and later, UNequal length snorkels. For machines WITH a fair number of modifications, you likely will want to chop off enough of the snorkel to increase the cross section area of the intake, but do not go too far. As you chop the snorkel, you will greatly affect low mid-range. I have found that putting some holes in the inside length of the snorkels works well even with the stock lengths...but while this is true, some turbulence is generated, and cutting the snorkel a inch and a half or so, and bell mouthing the remainder, works very well, and far better than the holes. If you are truly anal, you might try modifying the two snorkels to one larger one, and making it adjustable in length...and somewhat in cross-section. This is NOT all that difficult!
5. Ignition: Dual plugs. For racing, or very spirited riding at VERY HIGH rpm, single plugs MIGHT be better, this is NOT cut and dried, but you may run into octane problems if your CR is high, but this problem tends to fade at the highest rpm to being almost non-existent. The stock BMW ignition systems are fully capable of good performance. The NON-canister points models are not so good at 6000+ rpm. The points canister models are probably OK to 7000 or 7500. Install Porsche points in the canister and you may go even to 8000 or so. The BMW stock electronic ignition as far as Hall device, module, and coil(s), are quite adequate for 8000+ rpm. Only the most modified engines require tighter controlled and more powerful ignitions. The spark plug resistance caps can be eliminated on the NON electronic models, although I think you will find that 1000 ohm caps are nice for spark plug gap longevity, and I think the spark waveform is better, than with zero ohm caps, so I would NOT eliminate the 1000 caps. You can NOT use 1000 ohm caps on the electronic ignition models, without some danger of frying the ignition components. I think it can be done with some shielding and filtering, but have NOT tried it, nor experimented with it, which I once planned to do with my oscilloscope...the idea being to prevent any voltage spikes getting to the Hall elements.
The ignition curve should be modified to match the rest of the engine, particularly if the engine is heavily modified. On a mild performance enhanced bike, this can be as simple as bending the ears in or out a bit, and/or using a slightly stiffer spring on one weight....on some early bikes it will be simply using a different model of advance unit. Restricting advance can be done with shims too. The combination of points and all the various automatic advance models and springs that have been used on the /5/6/7 models, allow quite a few versions, and might be adequate to make up a matching advance, or you can modify the weights. One can certainly modify the springs and weights to get just about any curve one wants. Frankly, I think that at this point some dyno time will allow one to make a custom advance curve. One or two springs and sometimes reshaping of the weights are all that is really usually necessary. For a RACE bike, all this is moot, as you are nearly always at full advance.
One final word about the automatic advance: The best power and performance, IN GENERAL, assuming that the octane of the gasoline is adequate for the compression ratio, etc., will be obtained with a very fast advance curve. Granted that very little power is on hand below 2500, still, a fast advance will help acceleration from the low rpm point. If possible, the use of the stock early /5 advance unit, or springs, or simply reducing the weight of the stock ignition advance unit weights (sometimes just one weight or spring), allowing a faster (lower rpm for maximum advance) advance, will be of some help.
I think THE answer to all the ignition problems with a VERY hotted-up bike, is to simply purchase one of the CRANKSHAFT-triggered aftermarket ignitions.
Addendum #1:
Modifying the stock early mufflers (twin shock models) for sound and SMALL (miniscule) power increase:
1. On the FLAT rear face of the stock muffler, drill 1/4" holes, TWO places, one at top, one at bottom,
centered in the flat area.
2. Obtain an old metal drill bit of about 5/8 to 3/4" size. Heat soften the shank end for an inch or three,
and BRAZE or WELD this bit to a long piece of material. Be SURE it is secure. If you have a
bolt-together drill bit extended used by some electricians in house wiring, use that, but be SURE the bit
can NOT become separated from the extension....it is then damn difficult to remove from the muffler
innards! Drill from the muffler outlet end, into the way far deep inside center, drill it through. (if you were
real clever, you could do this from the inlet side, easier...not always!...the mufflers VARY in
construction at that end...but if you CAN drill it there, try that....the problem is usually that you see a
dome, hard to make mark or drill a dome when inside something.
3. At the rear outlet, at both 9 O'clock and 3 O'clock, use a punch to punch-prick a starting tit, INSIDE the
outlet, about 3/4" IN from the flat face....and drill on an ANGLE, leaving the drilled part elongated....by
moving the electric drill motor sideways, after the hole is first drilled through....this makes the final
inside hole sort of elliptical, by means of the side flutes cutting the metal of the muffler. Do NOT overdo
this. The drill suggested for this elliptical hole is 1/4" to start with....up to 3/8" maximum.
4. Make no jetting change unless you are on the very edge of being lean. If on the edge, go up '5' Bing
numbers on the main jet. Possibly go to one step larger on the needle JET. If not loud enough, add
TWO more holes, 90 degrees to the others at the outlet flat face. NO MORE elliptical holes.
If still not loud enough, change those 4 holes flat area holes to about 3/8". This mod helps
power SLIGHTLY, moves the torque peak up about 300 rpm, is definitely louder....all with stock
appearing mufflers to inspection folks.
5. For those that want a slightly throatier sound (this is for a street bike, not racer), do the 5/8" way-deep
hole, and NO elliptical holes, and TWO each 1/4" holes...holes at top and bottom, not sides.
One extra advantage is that condensation inside the muffler is GREATLY decreased, and the muffler will last for many decades.
Addendum #2: Krauser 4 valve heads:
I do not want to get deeply into these. I will just make some quickie generalizations. They are tricky to get to work properly because they were never fully machined and completed. Once the work is done, they can work well, but may need more fiddling with such as valve clearances, etc. They DO offer increased intake area, and WILL boost horsepower output.
esta en ingles...y es muy largo para traducir..... podria pasarle un traductor online pero suele quedar incomprensible...pero bueno cualquiera puede hacerlo despues
primero unas palabras sobre la admision para luego pasar al escape
Intake system:
Increasing the size of the carburetors is not always the best thing to do. AS carburetor throat (venturi) diameter is increased, it becomes harder and harder to obtain enough velocity through the carburetors (for that, one usually needs increased rpm and/or displacement) to enable them to atomize fuel correctly. In some cases, as some owners of R80 and R100 machines well know, the 32 mm carburetors provide better jump/lower to mid midrange and even higher, than the 40 mm. With carburetor sizes, and this also applies to fuel injection throat size, smaller throats give better low to mid-range throttle response, sometimes even upper mid-range performance. That is because high air velocity through the carburetors or FI is needed to allow them to do their job. At JUST the extreme top end, let us say at or near wide open throttle and high rpm, the largest carburetors that will cause decent atomization, are usually the best for power output. A manufacturer usually strikes a balance in sizes. Unless one can stay in this high throttle/rpm range (with a more peaky cam you'd also want close ratio gears), OVERALL acceleration may suffer. The Bing CV carburetors TEND to act like a rather modest variable venturi, but do not provide the almost instantaneous 'snap' of a directly controlled slide or butterfly carburetor. What is not often realized is that over-all acceleration may be the same.
It is more important than often considered, that in the mid-range of rpm, good response is needed on a touring bike. If the carburetor is too large, all sorts of strange effects will occur, including the engine simply refusing to accelerate smoothly when the throttle is opened a large amount, especially suddenly. The CV carburetor tends to greatly minimize that effect, but in some instances it will still be evident. Still, it is the combination of enough quantity of mixture, AND the throat velocity, to ENABLE an atomized mixture to get into the cylinder that is important. If the velocity is too low, the next intake stroke of the piston will not allow much mixture into the cylinder. It is the total sum quantity of atomized mixture that gets into the cylinder, at the right time, that is the important thing (this is a simplification).
In ALL cases, the best power is available (assuming jetting can be modified if need-be) from the coolest possible temperature of the incoming air (within reason), since the vast portion of the burning mixture by weight, and volume, is the oxygen in the incoming air. Cooler air has FAR more oxygen, allowing more power, if the gasoline quantity/mixture is adjusted to match. This is a BIG effect. A definite increase in power is available from getting cool air to the carburetor intakes. However, if this air is not warm enough, one can freeze up the carburetor (venturi icing); especially if the air contains a lot of moisture. Contrary to popular belief, MOISTURE in the air (high humidity) means LESS power, as the air has LESS usable oxygen. Generally cold air is not a problem once the engine is warmed up....or not with air intake systems that are warming the incoming air. When cold, the engine may not run well as the fuel is not atomizing well, condenses on cylinder walls and other parts, and is no longer a vapor, especially with cold parts. That is why vehicles have 'chokes'...for cold weather operation, allowing lots more richness, really a brute force method. Fuel injection systems can have far better control, as they can automatically compensate for air density; and most compensate for oxygen in the exhaust, air pressure, temperature of cooling water, ETC.
You WILL get more power, perhaps lots more, if you find a way to get cooling air to...and through... the airbox. It is often difficult to do it neatly, and to avoid ingestion of rain, leaves, etc. A NASA type duct, including the angle separation system used on turbine motors (airplanes) works well...sometimes.
Ram air: Sometimes folks ask questions about RAM air possibilities for no-cost supercharged horsepower. Forget about it unless you are planning to ride over 150mph. Below 130 mph, the effects are EXTREMELY small, and effects even at 150 are just barely noticeable. The effects above 150 are noticeable and worthwhile. And, no,... a larger scoop, does NOT mean you will see improvement. You could make a scoop/funnel the width of the motorcycle, and all it would do is likely add a lot of drag. Yes, that sounds wrong, 'common sense' is otherwise, eh? There is some information on intake areas velocity in my formulas article.
Dynamic air pressure by or on a moving object are treated by engineers as a certain size of flat plate moving through the air at some speed. Dynamic air pressure is proportional to the SQUARE of speed, but the horsepower to attain that speed goes up as the CUBE of that speed change.
Consider your Airhead or any other motorcycle, moving at 68 statute miles per hour. That is easily converted to feet per second, and the value is 99.7, let us just say 100 feet per second. There is about 12 pounds of pressure per square foot of surface area. At 200 feet per second (about 136 miles per hour) the pressure is 48 pounds per square foot. (see, I did the calculations for you!!). Thus, the drag is 4 times higher for a doubling of speed (I put this information here so you'd know why gasoline use skyrockets with increasing speed). If one was to divide by 144, you would have the pressure per square inch, in this case 0.33 PSI. That is basically almost nothing compared to atmospheric pressure forcing itself into the cylinders (about 15.0 psi at sea level). Thus ram-air supercharging effect is very small until speed gets VERY high. It starts to make some reasonably usable difference around 150 mph. Thus, ram air pressure does NOT help at ordinary road speeds.
Intake tract length: This is a particularly difficult idea to get across, and the same effects, in reverse, are in the exhaust system. This effect is very noticeable at all throttle settings, but the effect varies greatly with those settings:
When the intake valve is in its 'open, at least somewhat' phase, and in conjunction with a time just after the piston reaches bottom, and the valve is still not yet closed (we are not going to get into cam timing theory here), the air coming through the intake system is NOT a steady flow. In fact, at any time, intake valve open or not, there is still some flow into the intake system, as this flow pressurizes against itself, readying itself for the next opening of the intake valve, so to speak. This flow is varying in velocity and pressure, depending on the valve opening, and a few other more esoteric things. Pressure here means absolute pressure, or, if you will, referenced to atmospheric. To picture this simplified, let us start at the beginning of the intake stroke. The valve is opening, the throttle is open a bit or more. As the piston lowers, it reduces the atmospheric pressure in the cylinder, allowing the outside air pressure to push air/fuel mixture into the cylinder. View it as sucking the outside air/mixture inwards if you must. The piston eventually stops lowering, and in modern engines, the valve closes a bit later. Since this happens at a fast rate, even at idle, the intake flow is in PULSES. The incoming air slows when that valve closes, and this slowdown occurs VERY suddenly. View this as a slug of air slamming against a closed valve, if you must think of it that way. These pulses are described by engineers in a type of complex interacting mathematics dealing with 'waveforms'. You can think of it as the air piling up on itself. Because of this pulse, well, really lots of pulses, some complex things happen in the intake system, and the one that I want to discuss is the most complicated one, the reflected pulse.
The intake system will act like 'any one specific pulse is partially reflected backwards'. It is not inertia, and if you cannot picture it yet, just accept it as fact! I will discuss this from a different viewpoint later, in the exhaust, which might be easier to understand. This reversion/reflection effect occurs at the point of intake, which in the later airheads, as an example, is approximately at the tips of the snorkels!! If, at any one given and constant throttle and constant rpm, one could adjust the LENGTH of the effective intake system, one could find a correct length so that the REFLECTED pulse hits the intake valve at exactly a particular moment of time, IN RELATION TO an existing normal INcoming pulse of mixture. This will have the effect of boosting power, as more fuel/air mixture will be forced into the cylinder...at NO cost whatsoever in loss of energy, etc. Of course, you will use more fuel! FREE POWER!!!...except for the gasoline usage! I once actually made up a VERY crude sliding tube system for the intake, one of these replaced both snorkels, and on a dyno, moved the tube-inside-a-tube, noting very distinct changes. I made a second one up, guess-tuned it INITIALLY for 5000 rpm, and varied it while on the dyno. Believe it or not, this crude device was made from the inner cardboard 'roll' of a roll of kitchen counter wipes. The effect on rpm/power had to be seen to be believed. It was necessary to jockey the throttle and tube length to keep the rpm more or less constant. A change of as much as 8 horsepower was noted. I want to CAUTION here that I did this only crudely, and a truly more vigorous AND PROPER analysis/testing would have REQUIRED changing the carburetor jetting and an exhaust analysis at each try, and I did not do this, due to lack of time that day.
NOTE: Engines have been built with variable length intact tracts, controlled by throttle position, rpm, and other things. In the past, most of these things were not very practical. In the last 15 or 20 years, some car and motorcycle manufacturer's have managed to incorporate some of these things, and more. There have been some clever ways to dynamically change the effective length and size, of the intake tract. There have even been engines with multiple intake valves that had a valve closed at times other than normal, and especially notable is the various methods of variable camshaft timing and multiple intake butterflies. Manufacturer's found ways to INcrease compression ratios far higher than if you were an old timer and had only 87-91 octane fuel available. RACE tunings on the camshafts became relatively common, yet the engines maintained nice torque curves and were very tractable, that is, smooth throttle response, lots of low and mid-range power, and plenty of top end power.
Many engines were using rather radical camshaft timing and would otherwise have low torque in the lower mid-range rpm area, except that the various cam timing changes and intake tract tuning GREATLY enhanced performance by improving those lower and mid areas; yet kept the high rpm performance; all this on regular grade fuel, with some engines having even bigger performance on 91 octane. A few of these engines made NEARLY THREE HORSEPOWER PER CUBIC INCH!
A few manufacturer's added turbochargers (a very few used superchargers) and kept reasonably high CR.
Modern metallurgy and casting techniques allowed the engines to have normal....even extended....life!
The Exhaust System:
On a practical basis, you could well think that all of the intake effects already discussed ALSO happen in the exhaust system, due to complex relationships with the exhaust valve, length of system, intake opening, etc. The same sort of formulas and things generally really do apply. Please keep in mind that the things that happen in the exhaust system, also happen in the intake system. It is just harder for most folks to understand it in the intake system, so I will delve here into the exhaust. Let me try to explain this in a laypersons viewpoint (I hope), in a gross simplification:
Exhaust sizes: The effect is generally small, and what is usually most important is the effective cross section area from cylinder head port to the outlet...or at least the muffler intake. A smaller diameter exhaust header pipe (meaning all of the piping, up to the muffler) can HELP midrange torque, but may or may not hinder top end. It is important, GENERALLY, that the muffler interior volume be substantial, the reasoning for this is quite complex. YES, all this means that, in general, a noisy small diameter straight-through muffler does NOT perform well! Many have replaced the BMW mufflers on the Twin-Shock Airhead models and found that loud exhaust does NOT equate with more horsepower, in fact, performance may suffer....a LOT. BMW did their homework on the mufflers. UNfortunately the mufflers that are under the frame, GS type, etc., do NOT perform as good as the old BMW twin mufflers.
Let's get into this a bit deeper:
The exhaust valve begins to open, and the mostly burned gases begin to exit, and the process continues, until a little bit AFTER the exhaust valve closes, due to inertia of the gases. Each power stroke results in a pulse of gases, of uneven shape. Multiple firings of the cylinder means a 'train' of unevenly shaped pulses going down the exhaust pipe. At higher and higher RPM, the pulses get shorter and shorter, but more often. YES, exactly the same type of thing as the incoming air for the intake system. Now, speaking of any single pulse, when some portion of this pulse...for our purposes, let us say the very beginning,,,, reaches the exit mouth of the muffler tip (or unmuffled pipe...etc), the gases, which have accommodated themselves to the pipe system before this, now suddenly see vastly different conditions at that very exit point. The gases see atmospheric pressure, atmospheric temperatures, increased effective diameter of the pipe (NO pipe!!)...ETC. The gases are confused, and think they have run up against a brick wall, which then gives way, modestly smoothly, and allows the rest of the pulse to go to the outside air. It does not 'give way' instantaneously, it seems so, but is not so. It gives way in proportion to the intensity of the gas pulse, basically suddenly strong, then fades slowly, all this happening in an instantaneous manner, unless you have instruments to measure it. At the confusion point, the gases are reflected back up the pipe, all the way to the exhaust valve...and even into the cylinder if the valve is open. The reflection can be partial, partial distance, or a combination. One of the very complicating factors is that ANY inconsistencies in smooth pipe insides will promote a point of reflection, which may be quite minor, or could be major. This includes the change in effective pipe diameter as the header pipe enters the muffler, or before, crossover pipes with poor smoothness of the curve that should really exist where they meet the main header pipe, and so on. Every little jog or anything not smooth to the flow, creates a change in the traveling exhaust, perhaps a detrimental reversion wave, small or large.
You can think of the exhaust gases as a series of rubber slugs if you wish, traveling in the exhaust system, with a bit of separation between them. They hit in inconsistency...or, especially, the outside air, and SOME rubber bounces back, pushing each one ahead of it...and so on. So, what we have is a reversal/reversion of SOME of the exhaust energy. This would or could be a bad thing, except that the pipe length can be made such that a pulse can arrive at the valve at any portion of that pulse, or in-between pulses at any given single rpm. On a PRACTICAL note, it would require a much TOO LONG exhaust system to take full advantage of this effect for lower rpm, so the manufacturer must deal with that in other ways.
NOTE! The careful reader may well say, what about a LONG intake, and a SHORT exhaust. ...well, that would be workable, but the intake will have more problems staying cool, and many other problems....including the FACT that the exhaust must exit someplace not near the rider's knees! On a practical basis, due to gas expansion, mechanical location problems, and MANY other things, the exhaust system is longer than the intake system, and the exhaust tuning on 4 stroke engines is usually done mostly by pipe diameter, and one or more crossover pipes, and some muffler innards that are often VASTLY more clever...or, at least, have vastly more real engineering in them, than just for sound reduction. In some systems, the exhaust flow starts out with a small diameter and is a constantly expanding taper...this can be made to work very well indeed....but is expensive to fabricate. It was very commonly done with 2-stroke engines, with many welded-in sections, called Expansion Chambers. The outlet of such can be a very small diameter!
I have NOT, so far, except in reports of experiments, seen anyone with a truly variable tuned exhaust system...although some single bypass systems are in use, and there are now appearing some mechanically controlled ones with more than a fixed one-two change. If the mechanical problems could be sorted out, it might be possible to combine a variable camshaft, separate pair of intake valves for differing cam lobes, variable intake length and cross-section; together with a variable exhaust, all electronically controlled, and obtain an almost unbelievably flat torque curve (or, super peaky race engine.....; whatever).
As you have read, the length and size of intake and exhaust systems have big effects. In quite a few ways, some of these things use the same formulas and are similar to tuning a pipe organ, at least in some areas. Engineers use the same formulas for open and closed pipes in organs for some of the needed calculations.
If this pulse (or lack of pulse; that is, in-between pulses) of reverse energy meets another oncoming (from and out of cylinder) pulse at exactly the right moment, it CAN reduce the pressure (or increase it) in the pipe at that particular point, and THIS effect then travels back down the pipe. With a lowered pressure, the exhaust is extracted from the cylinder far better....and when this occurs with the intake tract still flowing, as it is with valve overlap timing, MORE intake charge goes into the cylinder as well. Almost like a supercharger. This means that a tuned exhaust can extract more exhaust AND suck in more mixture to be burned! This seems like the best of both worlds, and CAN be, but the effects vary with just about every other thing, including the rpm, throttle position (usually), entire intake system design, the camshaft timing for the valves, shape and sizes of things, and dozens of other parameters. Many decades ago, before computers helped the engineers, all this was done by first cutting and trying (with some engineering formulas). Nowadays with high speed computers and special instruments, all able to analyze and correlate hundreds of changing characteristics at the SAME time...there is far less 'cut and try'. However, overall, exhaust and intake design is extremely complicated. Even today, computers and lots of measurements with sophisticated equipment do not usually tell the whole story, just give one some directions/hints, granted rather good ones, usually.
SO: In the INtake system, it is desirable to fashion the system so that the reflected pulse enhances the flow by creating even more of a vacuum than normally would exist, or a longer length of the gases slug. In the exhaust system, the reflected pulse, from the exhaust outlet tip, is returned to the exhaust valve area, and if timed correctly, will REDUCE exhaust system pressure, and thereby, in a complex manner, allow MORE INtake of fuel mixture to occur, due to the valve overlap period, including the effects of the intake charge pushing the remnants of the diluted exhaust out the exhaust valve...AND reducing 'back pressure', allowing more flow in the first place.
If you are getting the idea that back pressure IN ITSELF is NOT the most important thing...you are absolutely correct.
The problem is in getting it all to work together, as you change one thing, and that seems to change many more things. That is why well-experienced master tuners are in such high demand for commercial racing. Today's master tech's use very sophisticated computerized tools for analysis.
Back pressure is LESS important than the pulse timing; back pressure has LITTLE effect until it is really high!
MORE on this a bit further down this page.
Design of TRUE performance enhancing exhaust systems, especially the mufflers, is a real art, as well as science. Today, vehicle manufacturer's have LOTS of pressure transducers....and visual transducers!.... plumbed into prospective 'mufflers' and other exhaust components, and results analyzed on very high speed computers. You might be surprised to find out that a PROPERLY designed 'fishtail' exhaust can be a good enhancer of performance, as are several other designs, IF properly done for performance, and not for just appearance. Generally speaking, it is desirable to have the exhaust system gases constantly expanding, even if ever so slightly, in cross-section (that means larger and larger inside diameter of the round pipe...or, its equivalent in other shapes), from exhaust valve to outlet. In some designs the very tip is then restricted, changing the velocity, and reflection pulse. Does this begin to sound a bit like an intake system too? Megaphones are not practical on the street, unless internally muffled, which tends to defeat their performance. A proper megaphone for performance on a stock airhead would be quite long, with a SLOW taper, and it PROBABLY would stick WAY out and beyond the end of the motorcycle. SOME types of megaphones have a reverse tapered outlet, which enhance the performance, whilst others are sales gimmicks. Many folks find that the BMW stock muffler is better, performance-wise, than they thought, after spending lots of $$$ on aftermarket exhaust systems. SOME of the BMW airhead models with mufflers/collectors located under the transmission can be modified, usually the sound increases some, and the torque MIGHT or might not increase/decrease. In other instances, you can make major improvements.
IF putting other 'mufflers' on your bike vastly improves performance, you likely have a poorly tuned motor to start with.
Most folks seem to have the idea that reducing back pressure in the exhaust system is the holy grail of design. This is NOT so. What is REALLY desired is the extraction effect at the exhaust valve....and the effect this will have on the incoming fuel-air mixture. True back pressure has a small effect on power,,,,within limits of course. Perhaps a few more words will help in this area:
Shortly after the spark plug ignites the mixture, the flame begun by this spreads rapidly, and the very rapid expansion of the gases produces a very high pressure, which is very effective in moving the piston downwards. As the piston moves downwards this pressure reduces by a huge amount. By the time the exhaust valve begins to open, the pressure is fairly low. The pressure then is probably under 100 pounds per square inch; still enough to do some work. The pressure in the header and exhaust pipe, due to expansion into that area, is much lower, especially at low rpm, but still fairly low even at high rpm. It is only a FEW pounds per sq. inch. If you do not believe this, have someone put the throttle up for 5000 rpm, and cover both exhausts with your hands, try to stop the engine, while your friend turns the throttle wide open. There is no way that any internal pressure, without your hands, would in itself reduce engine power greatly, compared to the very high combustion pressures. A horsepower is 550 foot pounds per second...do you REALLY think that some sort of pressure on your hands is relative here? (ok, so that is a bad analogy, so what). It is the effect of the waves, in a properly designed system, that reduces this pressure somewhat, but reduces it AT the valve area, in synchronization with the incoming charge (remember: valve overlap...the intake valve is opening to allow incoming charge while the exhaust valve is still partially open), thereby allowing MORE mixture to be packed into the cylinders. THAT 'packing of mixture' is where the largest portion of the power increase is noted, with a good exhaust system, from the improvement in the force on the piston during the early portion of the power stroke......AND HOPEFULLY CONTINUING DOWN THAT STROKE A LONGER DISTANCE THAN PREVIOUSLY. Just a very small amount of lengthening of the amount of the stroke that usable pressure is on the piston makes a BIG power output difference.
Many decades ago, 'boxes' were put where we think of 'muffler', and this box, which was NOT a TRUE expansion chamber, collected the exhaust mixture, quieted the exhaust, and had not much restriction. Early engines mostly used that system. An improvement on this was the Brooklands 'muffler', which developed into the full-blown type called a 'fishtail'. This design used a slot (unknown to many who have seen fishtail mufflers...likely because those in the U.S. are GARBAGE designs without the slot) in the outlet, the resultant of which was that the AREA of the outlet was somewhat LESS than the input pipe area...which reduced the sound level. (hmmm...whatcha think about the smaller inlet size of snorkels now??)
Due to very complex waveform development inside the design, the fishtail muffler had a greatly reduced pipe pressure, compared to an OPEN PIPE! In fact, it was roughly 50% better! YES..this means MEASURED RESTRICTION meant LESS restriction in OPERATION! If the fishtail was removed, and only the 'box' used, the results were in-between an open pipe and with the fishtail.
The very next step in early design was the adding of smallish holes in internal baffles, and the complications this made were eventually solved. It was then possible to have a exhaust system that muffled AND performed. I know this sounds absolutely wrong, common sense seems to tell us that restrictions ALWAYS mean lower power, but it is true.
In the intake system, the reflected pulse can be timed, within reason, to boost effective amounts of actual cylinder filling fuel/air mixture. This effect is why RAM TUBES are seen on so many vehicles these days. They lengthen the intake system, adding more power robbing drag (from wall effects, and hopefully keeping laminar flows), but boosting far greater the reflected pulse effect. With LOTS of dyno time, well-designed ram tubes can work well, whether between carbs/FI and head, or carb/FI inlets and intake of aircleaner. Ram tubes generally allow more radical cam timing, without the engine becoming too 'peaky'.
BMW factory-sold 'sport camshafts' are rather versatile, performing fairly well for the mostly stock motor, without any necessity for radical camshaft timing, (radical camshaft timing usually results in a very rough loping idle and VERY poor low end performance), and there is no dangerous super high valve lifts either. Use of the factory type sport cam with a compression increase is helpful. It is also helpful to reduce the clutch/flywheel weight, to allow the engine to accelerate a bit faster, although this has some drawbacks. BTW, some sporty camshafts, the BMW mild one included, EFFECTIVELY raise the compression ratio, DURING high rpm operation. We will NOT get into why!...there are several effects. You can NOT just change the camshaft in your Airhead, without other work. If you do install such as the BMW sport camshaft, be prepared to do LOTS more, or your bottom end performance will SUFFER...a LOT. The BMW sport camshaft will greatly detract from performance if you not do lots of other things. Even with lots of other modifications, the sport cam will STILL only START to "perform" above 5000 rpm....typically the improvement is notable from 6000 or so. I just can NOT recommend the BMW sport cam (often called 'the 336") for any sort of street Airhead for pleasurable touring characteristics. For a Cafe Bike, OK. For racing, yes.
Other, hard to quantify effects, come from combining the intakes. This effect is in the exhaust also. In our BMW bikes, the intake filter housing feeds BOTH carburetors. Hence there is now a complicating factor (VERY complicating) of the unequal 'sucking' strokes/pulses of the two out of phase cylinders, and the effect THIS has on the incoming wave charge. AND, that effect varies with throttle opening and rpm, ETC.
Just the thought of a sales department telling engineering management that 'a two into one exhaust is needed for appearance-sake', for an upcoming model, means hundreds of test and design hours, on intake and exhaust!
What all this means, on a practical basis, is that you really need a dyno to REALLY hop up a bike....OR, someone who already has done exactly this, or has been very lucky, to give you all the exacting and exciting details!!!
Here is that updated URL for Mez Porting's calculator again, I had it near the top of this article too.
[Tienes que estar registrado y conectado para ver este vínculo]
Now, specific recommendations, that I feel I am safe in making:
Before we get into things, below, a few flat-out statements, here in RED; and these apply to stock, relatively stock, somewhat modified airheads, and fairly well modified airheads...........in every instance I mean for the street....NOT for full-out racing. It makes no difference the engine size, compression ratio, piston changes, milling the heads; nor, making carburetion changes, nor even installing the BMW sport camshaft....my RED comments apply to all:
A. Use of a two-into-one exhaust system will likely REDUCE performance.
B. The Super-Trapp type of exhaust is tricky, hard to get it to perform correctly, and is VERY likely, even with large numbers of discs, to perform MUCH less well than the stock BMW exhaust.
C. Use of K & N air filters is NOT going to improve performance, and WILL decrease engine life.
D. Use of individual air cleaners, screens or filters, of any type, at each carburetor throat, instead of using the stock air cleaner system, will CAUSE PROBLEMS.....and make the engine less tractable, due to the bad effect on the flatness of the torque curve. In some cases the weight will cause vibrations that will cause the fuel to FOAM. To get around this, the proper type of rubber intake hoses, and a support for the carburetor and perhaps the individual air cleaners/screens is needed. More power IS available from individual air cleaners, but the torque peak is moved upwards in rpm, making for a more peaky engine.
1. Camshaft: A BMW sport camshaft can be used, unless you have access to a dyno equipped camshaft grinder company for your specific application. The BMW Sport Cam has a deleterious effect at lower rpm. This is a timing cam, not a lift cam, and use of higher lift, by incorporating high ratio rocker arms, is probably possible, but one must be VERY careful. Pay attention to the rocker arms, pushrods, etc. The engine will be happiest above 5700 rpm. This means that without a LOT of other changes, you will LOSE performance lower down.
2. Compression ratio: the higher the better. On a practical basis, probably about 9.5 is the highest with a stock camshaft, and maybe 10.5 with the sport camshaft. This includes dual-plugging. With a really clean combustion chamber, with no sharp places, and everything tuned to the Nth, even 11.0 might be possible on 98 octane or better, with dual plugs. The problem is definitely the octane of available gasoline. If you have the time and do not mind experimenting, the squish area, as in the 1977 airhead, can be played with, with some improvement available if it is reduced, or re-incorporated. One problem in any hopping up, is that if you are using a R100 engine, it is already running quite hot. If you are willing to do valve jobs more often, you can go pretty far, approaching early Oilhead output. The larger valves of the early models, with the larger carburetors, are going to be needed with higher compression and a sport camshaft. A good 3 angle valve job, ...careful valve guide work, ... are necessary, except for the most extreme motors, where polishing, reducing weights, shaving guides, modifying lifters and pushrods, etc., are helpful. I do not think going to a larger bore via special expensive aftermarket kits is a good idea. BUT...if you already need cylinders, maybe going to 1050 cc or so is OK, but do NOT increase the bore on the R65....it will probably be UNreliable. Keep in mind that going over 9:5 in CR, or going to large bores, will, or can, cause reliability problems. Very careful attention to the valves, valve springs, etc., is going to be NECESSARY.
3. Flywheel, clutch, transmission: The early heavy flywheel/clutch assembly is fine for most uses, but not for best acceleration and fast shifting, where lighter items are better, but give more vibration, which can be somewhat (ONLY somewhat) reduced by balancing. Even the later 'flywheel' (carrier) and clutch is too heavy for the most spirited sport riding and racing. Some of the later clutch parts are well known to us wrenches as going out of balance, from warping (like the plate ring), so balancing is relatively important. Vibration robs power, not just annoying to you physically. SOME may want to go into the transmission and modify it for easier/faster shifting (particularly at highest rpm). There is an article on this website on how to PROPERLY lighten the flywheel: CLICK
4. Intake system: you should consider modifying for cold air to AND THROUGH the airbox. Except for a near race or actual race motor, I suggest you NOT use individual screens or filters right at the carburetor mouths (thereby not using the airbox at all). If you do go to individual filters, do some work with tufting or smoke, to ensure a good airflow to those filters, and that airflow coming from the forward area is not being curved excessively, and creating a light vacuum at the filters.
With regards to cold-air: Avoid effectively lengthening the intake system, unless you PLAN to lower the rpm for torque peak. If the cross section areas are too small you will run into non-laminar flow effects, so avoid this. You want to avoid supersonic velocities. You MUST run tests to be sure that the area of cold air pickup really is in an area that is not affected by the front end/fender/etc...and surrounding parts...that can actually cause loss of effective air pressure there. AGAIN, generally unless engine is heavily massaged, use of the stock airbox, modified for smooth flow and cooler air, is recommended. I have reservations, however, about using the clamshell box...and think that it should be eliminated. Get rid of ALL smog items under the air filter on the later models. Rework that airbox for a SMOOOOOTH flow of air internally. Yes, you want turbulence, but you want it AFTER the carburetor...INSIDE the cylinder! Insulate as required with cambric, etc., so that as little as possible hot metal comes in contact with the incoming air. HINT: Sometimes you will get a bigger improvement from making the intake system smaller, IF it remains cool! If you keep the intake system from being excessively heated by the engine you will gain a LARGE amount of power. The power gain from cold air modifications is REALLY significant. This was one of my personal secrets.
Snorkels: They can be cut back, a bit at a time, or, removed entirely, and you will see the BAD effect THAT usually has on a stock machine. They can also be opened, but not much, unless you are going to very high rpm consistently. Shortening them will move the torque peak upwards. Generally, if the carburetor size is stock, you can do minor mods on the snorkels on both the R80 and R100 machines, but watch for loss of midrange torque, which you will want to make up elsewhere if you go too far. You will need jetting changes along the way here. Gas mileage can suffer if the rpm position for the snorkel effect is off, BMW found that out with the R80, and R65, and then modified the snorkels with holes...and later, UNequal length snorkels. For machines WITH a fair number of modifications, you likely will want to chop off enough of the snorkel to increase the cross section area of the intake, but do not go too far. As you chop the snorkel, you will greatly affect low mid-range. I have found that putting some holes in the inside length of the snorkels works well even with the stock lengths...but while this is true, some turbulence is generated, and cutting the snorkel a inch and a half or so, and bell mouthing the remainder, works very well, and far better than the holes. If you are truly anal, you might try modifying the two snorkels to one larger one, and making it adjustable in length...and somewhat in cross-section. This is NOT all that difficult!
5. Ignition: Dual plugs. For racing, or very spirited riding at VERY HIGH rpm, single plugs MIGHT be better, this is NOT cut and dried, but you may run into octane problems if your CR is high, but this problem tends to fade at the highest rpm to being almost non-existent. The stock BMW ignition systems are fully capable of good performance. The NON-canister points models are not so good at 6000+ rpm. The points canister models are probably OK to 7000 or 7500. Install Porsche points in the canister and you may go even to 8000 or so. The BMW stock electronic ignition as far as Hall device, module, and coil(s), are quite adequate for 8000+ rpm. Only the most modified engines require tighter controlled and more powerful ignitions. The spark plug resistance caps can be eliminated on the NON electronic models, although I think you will find that 1000 ohm caps are nice for spark plug gap longevity, and I think the spark waveform is better, than with zero ohm caps, so I would NOT eliminate the 1000 caps. You can NOT use 1000 ohm caps on the electronic ignition models, without some danger of frying the ignition components. I think it can be done with some shielding and filtering, but have NOT tried it, nor experimented with it, which I once planned to do with my oscilloscope...the idea being to prevent any voltage spikes getting to the Hall elements.
The ignition curve should be modified to match the rest of the engine, particularly if the engine is heavily modified. On a mild performance enhanced bike, this can be as simple as bending the ears in or out a bit, and/or using a slightly stiffer spring on one weight....on some early bikes it will be simply using a different model of advance unit. Restricting advance can be done with shims too. The combination of points and all the various automatic advance models and springs that have been used on the /5/6/7 models, allow quite a few versions, and might be adequate to make up a matching advance, or you can modify the weights. One can certainly modify the springs and weights to get just about any curve one wants. Frankly, I think that at this point some dyno time will allow one to make a custom advance curve. One or two springs and sometimes reshaping of the weights are all that is really usually necessary. For a RACE bike, all this is moot, as you are nearly always at full advance.
One final word about the automatic advance: The best power and performance, IN GENERAL, assuming that the octane of the gasoline is adequate for the compression ratio, etc., will be obtained with a very fast advance curve. Granted that very little power is on hand below 2500, still, a fast advance will help acceleration from the low rpm point. If possible, the use of the stock early /5 advance unit, or springs, or simply reducing the weight of the stock ignition advance unit weights (sometimes just one weight or spring), allowing a faster (lower rpm for maximum advance) advance, will be of some help.
I think THE answer to all the ignition problems with a VERY hotted-up bike, is to simply purchase one of the CRANKSHAFT-triggered aftermarket ignitions.
Addendum #1:
Modifying the stock early mufflers (twin shock models) for sound and SMALL (miniscule) power increase:
1. On the FLAT rear face of the stock muffler, drill 1/4" holes, TWO places, one at top, one at bottom,
centered in the flat area.
2. Obtain an old metal drill bit of about 5/8 to 3/4" size. Heat soften the shank end for an inch or three,
and BRAZE or WELD this bit to a long piece of material. Be SURE it is secure. If you have a
bolt-together drill bit extended used by some electricians in house wiring, use that, but be SURE the bit
can NOT become separated from the extension....it is then damn difficult to remove from the muffler
innards! Drill from the muffler outlet end, into the way far deep inside center, drill it through. (if you were
real clever, you could do this from the inlet side, easier...not always!...the mufflers VARY in
construction at that end...but if you CAN drill it there, try that....the problem is usually that you see a
dome, hard to make mark or drill a dome when inside something.
3. At the rear outlet, at both 9 O'clock and 3 O'clock, use a punch to punch-prick a starting tit, INSIDE the
outlet, about 3/4" IN from the flat face....and drill on an ANGLE, leaving the drilled part elongated....by
moving the electric drill motor sideways, after the hole is first drilled through....this makes the final
inside hole sort of elliptical, by means of the side flutes cutting the metal of the muffler. Do NOT overdo
this. The drill suggested for this elliptical hole is 1/4" to start with....up to 3/8" maximum.
4. Make no jetting change unless you are on the very edge of being lean. If on the edge, go up '5' Bing
numbers on the main jet. Possibly go to one step larger on the needle JET. If not loud enough, add
TWO more holes, 90 degrees to the others at the outlet flat face. NO MORE elliptical holes.
If still not loud enough, change those 4 holes flat area holes to about 3/8". This mod helps
power SLIGHTLY, moves the torque peak up about 300 rpm, is definitely louder....all with stock
appearing mufflers to inspection folks.
5. For those that want a slightly throatier sound (this is for a street bike, not racer), do the 5/8" way-deep
hole, and NO elliptical holes, and TWO each 1/4" holes...holes at top and bottom, not sides.
One extra advantage is that condensation inside the muffler is GREATLY decreased, and the muffler will last for many decades.
Addendum #2: Krauser 4 valve heads:
I do not want to get deeply into these. I will just make some quickie generalizations. They are tricky to get to work properly because they were never fully machined and completed. Once the work is done, they can work well, but may need more fiddling with such as valve clearances, etc. They DO offer increased intake area, and WILL boost horsepower output.
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
Invitado 3/11/2014, 11:14
MUY BUENO!
THE CAT IS ON THE DOG BUT NEAR THE TABLE!
Gaita english teacher voy a domicilio!
THE CAT IS ON THE DOG BUT NEAR THE TABLE!
Gaita english teacher voy a domicilio!
Invitado- Invitado
Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
Manna 3/11/2014, 11:22
Yo voy a esperar a que salga la película. Es muy largo para leerlo todo.
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
Edum 3/11/2014, 11:28
ya salio manna.. se llama escape de nueva york
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
Invitado 3/11/2014, 11:30
CON LLUVIA NADIE LABURA!
Invitado- Invitado
Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
urbandrag 3/11/2014, 11:36
Resumiendo y en castellano,
El que necesite mas potencia, torque y velocidad en una GS, tiene la moto equivocada.
El que necesite mas potencia, torque y velocidad en una GS, tiene la moto equivocada.
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
cml 3/11/2014, 12:45
O sea: es al ñudo que lo fajen al que nace barrigón. No gasten al pedo!
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
Mamut 3/11/2014, 13:21
Jajajja me cago de risa, seguro nadie lo leyó por lo extenso, pero le vamos a encontrar la síntesis, yo de ingles solo Hello, así que digan que tengo que ponerle. Abrazo
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
Janu 3/11/2014, 13:26
+1 Mammut, lei hasta el "hola a todos"
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chapita y .- Cantidad de envíos : 871
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
fede12 3/11/2014, 15:22
jajaja muy buena la cara de Casey Wonder Messi
agarren el traductor de google, queda tarzanezco, [Tienes que estar registrado y conectado para ver este vínculo]
agarren el traductor de google, queda tarzanezco, [Tienes que estar registrado y conectado para ver este vínculo]
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
vincent 3/11/2014, 16:09
cuando llego a casa me fijo si tengo el Manual LERU de este tema y se los paso
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
emilio 3/11/2014, 17:00
Para mi lo unico importante de los escapes es que hacen ruido y si los tocas cuando estan calientes te quemas, el resto...... uauuuuuu (bostezo)
Última edición por ebara el 3/11/2014, 18:31, editado 1 vez
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Re: INTERESANTE ESTUDIO SOBRE ESCAPES DE NUESTRAS MOTOS
Edum 3/11/2014, 17:21
es para el q tenga ganas de leer algo tecnico.. si es largo.. pero bueno
tambien es para el q tenga ganas de hacer chistes y no poner nada util..
to be or not teletubi
tambien es para el q tenga ganas de hacer chistes y no poner nada util..
to be or not teletubi
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