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• Amir Izham

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Intake and Exhaust System Tuning © (Ramblings on exhaust and intake system modifications) InExTuning.htm-10

This is a discussion of some of the factors that go into the size, length, width, opening area, ETC., of the intake and exhaust system. It is not a how-to article, but offers a fair amount of not easily found information. This article is an updated version of one that was posted to the Airheads LIST: "">>>>Some time ago I promised to do an article on tuning of Intake and Exhaust systems, and then later posted that it was becoming so unwieldy that I ripped up my notes. I really wanted to avoid long engineering discussions & mathematics (which many folks dislike). After thinking about this, I decided that I could do the article in a way that would spell out some theory and some problems, and give some hints. ""....This is that article Due to VERY complex effects having mostly to do with camshaft timing and size and length of intake and exhaust systems, including the shape and size of combustion chamber items such as port and valve sizes, angles, flow, eddies, etc...., modifications for power or torque is a mix of science and art, even with modern computers.... needing an large amount of experimentation. Most folks seem to think that if the exhaust system is 'opened up', that in itself will give more power, more performance. They also seem to think that bigger carburetors will do the same. Same idea for perhaps 'freer flowing' air cleaners, or individual air cleaner elements at the carburetor mouths (and eliminating the airbox, etc.). Some changes really will give improvements across the rpm range. Colder air to the air cleaner, insulating the air cleaner interior. A good 3 angle valve job. Higher compression ratio within gasoline limits, and many more changes. However, some, if not many changes, will often REDUCE performance. Of course, one has to define performance: top speed? quick acceleration to some particular speed? tractability? Torque?... so as to not needing excessive shifting? Anecdotal evidence to the contrary, the only real way to 'prove' your "improvements" to horsepower and torque, top end performance, acceleration, etc., is either on the drag strip, or a known distanced layout, or, better yet, on a calibrated, or at least repeatable readings type of dynamometer. Certainly you can road test for characteristics like throttle response and smoothness, and acceleration. Dynamometer time is expensive. One could get together with some friends and build one, it need not be calibrated, just consistent. NOT expensive here. A few hundred dollars and a fair amount of time would get you a dyno. Purchasing one is likely to cost a minimum of $3000.00, and a really good setup will cost MANY times that amount. It is counter-productive to 'hop up' an engine, if the results are such that the lower and mid range torque are severely reduced....UNLESS you intend the engine purpose to be ONLY racing, where you might be able to 'stay on the pipes', or keep the rpm well up, all the time. This makes for a really bad engine for touring, even bad for sport touring.

HOWEVER, it IS possible to use a 'hotter' camshaft, which normally might make the engine 'peaky', and add exhaust and intake 'tuning' such that the resultant torque curve is really reasonably flat and the engine hardly peaky at all. THAT is likely the goal of many modifiers for street usage, whether they understand, or not. Doesn't hurt racing performance either! BMW designed our bikes for reliable touring under all conditions. Only the most serious street rodders would put up with a truly peaky engine. That being said, the following discussion will describe some of the details of engine design and tuning, and give some pointers.

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/midrange, than the 40 mm. Let me get into this a bit deeper. With carburetor sizes, and this applies also to FI 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 is needed to allow the carburetors 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 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 carburetor. 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 suddenly cracked open a large amount. 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 it 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. In ALL cases, the best power is available from the coolest possible temperature of the incoming air, since the vast portion of the burning mixture by weight, and volume, is 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). Generally that is not a problem once the

engine is warmed up. 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 over these characteristics. You WILL get more power, perhaps lots more, if you find a way to get cooling air to the airbox. It is 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. Ram air: Forget about it unless you are planning to ride over 130mph. Below 130 mph, the effects are extremely tiny, and effects even at 130 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. Again, the NASA duct works really well. Intake tract length: This is a particularly difficult idea to get across to some folks, and the same effects, in reverse, are in the exhaust system. Bear with me, I will try to explain this. 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, one full set per 2 rev per piston, 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 gas 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 ITEMS. MOST WERE VERY IMPRACTICAL. IN THE LAST 12 YEARS OR SO SOME CAR AND MOTORCYCLE MANUFACTURER'S CAME UP WITH SOME CLEVER WAYS TO DYNAMICALLY CHANGE THE EFFECTIVE LENGTH OF THE INTAKE TRACT. THIS WAS DONE IN QUITE A FEW DIFFERENT WAYS, BEGINNING WITH LONGER INTAKE PIPING, BUT THE BIGGEST GAINS WERE FROM SUCH AS MULTIPLE INTAKE VALVES THAT HAD A VALVE TURNED OFF/ON, VARIABLE CAMSHAFT TIMING, MULTIPLE INTAKE BUTTERFLIES/MULTIPLE INPUT PATHS, AND EVEN MORE ESOTERIC METHODS. SOME WERE SPECTACULARLY EFFECTIVE. IN PARTICULAR WERE THE LONGER INPUT TUBES, VARIABLE CAMSHAFT TIMING, AND TWO PATHS FOR THE INTAKE AIR VALVE. MANUFACTURER'S MANAGED TO GET SOME QUITE 'RACE-TYPE' TUNINGS, YET KEEP THE ENGINES VERY TRACTABLE AND WITH BROAD TORQUE CURVES. PHILOSOPHY VARIED WITH THE MANUFACTURER. GENERALLY SPEAKING, THE IDEA WAS TO GAIN A LOT OF MID-RANGE POWER AND SMOOTH THROTTLE RESPONSE ACROSS A WIDE RPM RANGE. Note that some engines were using very radical camshaft timing and had a low torque in the lower mid-range rpm area, and thus this type of intake tract tuning GREATLY enhanced performance by improving the lower and mid areas. Another way of saying this is that fancy plumbing ALLOWED more radical cam timing. IN A FEW INSTANCES SOME OF THE ENGINE TUNING WAS DONE FOR WIDE OPEN THROTTLE, HIGH RPM, HIGHEST TOP SPEED. THIS WAS, AFAIK, ONLY DONE WITH MOTORCYCLES. (I am NOT speaking about pure race

vehicles here). FOR CARS, HOWEVER, WITH A LOT OF DESIGN WORK, A MANUFACTURER MIGHT BE ABLE TO USE BOTH QUITE MILD AND QUITE RACY CAM TIMINGS, ESPECIALLY IF THERE WAS TWO OR THREE INTAKE VALVES PER CYLINDER, AND SOME COULD BE SEPARATELY CONTROLLED. MOST OF THESE MANUFACTURERS HAD SPECIAL IGNITION, EXHAUST, AND INTAKE SYSTEM CONTROLS, ALLOWING ALMOST RACE PERFORMANCE OUTPUT OF THE ENGINE WITHOUT DRIVEABILITY PROBLEMS. NEARLY THREE HORSEPOWER PER CUBIC ENGINES!, YET STREET DRIVEABLE, WERE OBTAINED BY A FEW MANUFACTURER'S....WITHOUT SUPERCHARGING/TURBOCHARGING. Modern metallurgy and casting techniques allowed the engines to have normal....even extended....life.

The Airheads, and some generalities: BMW has actually come out with bulletins, particularly on some R80 models, for drilling holes near the snorkel opening, to deal with a reflected pulse problem {at least I think it was the reflected pulse} that was not so perfect in the mid-range (of course, the technical terms details were left out of the bulletin). AND, just because it works on some R80 models (or some form was on R45/R65 models) does NOT mean that it works on other models!!! ...and, in fact, it does not work on most of the R100 models, without some other changes, the R100's being mostly already tuned correctly. Let me expound more on this intake system. Late models of the BMW airhead have intake 'snorkels' that APPEAR to restrict the intake opening size, and taper forward and expand outwards, a 'bell mouth'. This might APPEAR to greatly restrict performance. If those openings WERE small enough, this would certainly be true. However, IF BMW designed the taper and the bell mouth for the purpose of keeping laminar airflow and JUST the correct velocity...at the almost supersonic velocities in the system which are normal here, then BMW was VERY clever!! AND...it is my belief that they did just exactly that!, allowing only a relatively small amount of restriction. Laminar flow means that the incoming airflow (the reversion pulse too, but let's ignore that here) flows smoothly, with little if any burbling and eddy currents and swirling, etc., which would cause losses, during its way down the snorkels. I am trying to keep math out of this posting as much as possible, but some folks may be interested in the effects of changing the length of the intake system. This is OFTEN seen with those 'racer boys' (real or otherwise) who remove the airbox stuff and let each carburetor breath through a screen or a filter, right at the carburetor inlet itself. I thought some of you might like to know that the distance from the intake valve to the

intake opening [to the outside air], in inches, is equal to APPROXIMATELY 79200 divided by the rpm, for the maximum torque peak. One of the terms I managed to boil out of the formulas involved in getting that

simplification has a term in it that is a SQUARED function, meaning that things happen FAST with SMALL changes in some items in the formula. This simple version of the formula includes certain assumptions I made about the speed of sound inside the intake system, effective pressure, valve timing, and temperature, but still, it is pretty close. SO...IF you remove your airbox stuff, and put stubs on individual carb intakes, you STILL have a tuning effect, but the effect is now at a very much higher RPM, with the shortened intake length. The tube length affects the rpm for torque peak, in other words. I don't remember the figures I used from so many years ago, but I vaguely remember about 7 inches from valve to carb inlet. Let's be generous, assume 8 inches there, and 5 inches for a large inlet filter, shaped to act as a pipe. That gives 13 inches of tract length. That is 6092 rpm for the torque peak, and it is likely much higher. I think 9 inches is generous enough, for 8800 rpm. Seems about right, from what I remember. The engine will likely be peaky, unless other things are done...such as tuning the exhaust for a different rpm!! (amongst MANY other 'things'). One of my racing engines had such an intake, with specially formed bellmouths, and a rather restricted exhaust, and smallish carbs, but very light valve gear, and a special camshaft, and a very light flywheel and clutch. We had no problem with WELL over normal redline, to say the least! Not a good engine to go touring on! It had very good acceleration above 5300 or so.

The Exhaust System: On a practical basis, 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 apply. Please keep in mind that the things that happen in the exhaust system, 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 small diameter straight-through muffler does NOT perform well. 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, and 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. You can think of this as a series of rubber slugs if you wish, traveling in the exhaust system, with a bit of separation between them. They hit 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. On a PRACTICAL note, it would require a much TOO LONG exhaust system to take full advantage of this effect, 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, 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. 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). ****This is all VERY similar to tuning an organ pipe, or other types of musical instruments!!! In fact, engineers use the formulas for open and closed organ pipes in their design of exhaust and intake systems!! The other part of this is that waveforms interact in very known ways, depending on the part of the waveform being studied. AGAIN: 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, super computers and lots of measurements with sophisticated equipment, do not usually tell the whole story, just give one some direction, granted rather good direction. Let me restate this: 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 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 very well-experienced master tuners are in such high demand for commercial racing. Today's master tech's use very sophisticated computerized tools for analysis.

READ THIS TWICE!!: Back pressure is LESS important than the pulse timing ...and back pressure has LITTLE effect until IT is really high! Design of TRUE performance enhancing 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 DEcreases! For the airheads with two mufflers, one on left, one on right, you CAN put a hole in the baffle that ordinarily keeps gases from going straight out, without causing more than a nice little bit, not excessive, noise increase. BUTTT......This has only a VERY SMALL effect on performance, unfortunately. The stock BMW system is pretty good, overall. It is the size, shape, and construction of the stock mufflers that do the job. Particularly the large volume. IF

putting other 'mufflers' on your bike vastly improves performance, you have likely a poorly tuned motor to start with. A few more words on exhaust systems are in order. Many folks seem to have the idea that reducing pressure (often called back pressure, somewhat erroneously) 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 fuelair 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. I would estimate that this pressure is probably less than 100 pounds per square inch. 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. Thus, 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 and head, or carb inlets and intake of aircleaner. Ram tubes generally allow more radical cam timing, without the engine becoming too 'peaky'. Because of the BMW intake system, and the large diameter pistons, and some other effects, the 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. 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!!! 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 a fair amount. 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. 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

rather careful. Pay attention to the rocker arms, pushrods, etc. The engine will be happiest above 5500 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 95 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. 3. Flywheel, clutch: 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. I usually remove a pound or so. 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). 4. Intake system: you should consider modifying for cold air to AND THROUGH the airbox, [and not try to use individual screens or filters right at the carburetor mouths, thereby not using the airbox at all]. HOWEVER.....For racing at very high rpm, this will not be as effective as short inputs to the carburetor. 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, is recommended. Get rid of ALL smog items under the 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 hot metal comes in contact with the incoming air, as possible. 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. Again, 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 touring, if cost is no object. For racing, or very spirited riding at HIGH rpm, single plugs MIGHT be better for power, 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. The BMW stock electronic ignition is 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 those caps also lengthen the spark duration, 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. The ignition curve could 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 always at full advance. Note that the early model points can and usually will be unstable at highest rpm. Canisters have one major disadvantage...they are built such that the automatic advance units are MUCH harder to work on. The Hall device system is fine, once the automatic advance curve is worked out. This is not as difficult as it sounds. Frankly, the automatic advance curve is not really critical, and the stock one is probably just fine. For those insisting on VERY high rpm, and using the canister with points, a Porsche set of points will give less points bounding. See my ignition articles. Note that some $$$ sophisticated crankshaft triggered systems are available from German companies. Tuning for ping is sort-of acceptable, but certainly NOT optimum, and the BMW design is very nice about pinging for you, and detonation is usually not a problem, but a dyno is far better, and far safer. One final word about the automatic advance: The best power and performance, IN GENERAL, assuming (I truly mean 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.

Addendum #1: Modifying the stock early mufflers for sound and SMALL 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 softens 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. 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. 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. btw...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.