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Tuning Intake and Exhaust Systems.
Intake & exhaust systems functions & modifications,
including Ram Air.
Premium fuel versus mileage & combustion temperatures.
© Copyright 2020, R. Fleischer
https://bmwmotorcycletech.info/InExTuning.htm
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This article contains a discussion of 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. There are also sections on ram air and on premium (higher octane) fuels versus mileage and combustion temperatures.
Due to VERY complex effects having to do with such as camshaft lobe timing, size & length of intake & exhaust systems, & shape & size of combustion chamber items ...such as port and valve sizes, angles, flow, eddies, etc; ....design and/or modifications for power and/or torque is a mix of science & art. Even with modern high speed computers and specialized sensors, a large amount of experimentation is almost always required. Many 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 ...and same for 'freer flowing' air-cleaners, or individual air-cleaner elements at the carburetor mouths (and eliminating the airbox, etc.). Many commonly held beliefs are WRONG, or, there is much more to it!
Some changes really will, almost always, give improvements across the rpm range. Colder air to the air cleaner, even as little as just insulating the air cleaner interior. A good 3 angle valve job. Higher compression ratio within gasoline limits. Sometimes just gearing changes or additional transmission gear ratios to better match engine characteristics.
MANY CHANGES WILL OR CAN REDUCE PERFORMANCE. One has to define performance. Top speed? Quick acceleration to some particular speed? Tractability? Flatter torque curve so as to not needing excessive shifting? Better fuel mileage? Ability to use lower octane fuel?
The only accurate way to 'prove' your "improvements" to horsepower and torque, top end performance, acceleration, etc., is either measurements on known sections of road with test instruments or on the drag strip; or, much better, on a calibrated, or at least repeatable-readings, type of dynamometer. Certainly you can road test for characteristics like 'perceived' throttle response and smoothness. Dynamometer time can be expensive. One could get together with some friends and build one, it need not be calibrated, just consistent. Several hundred dollars and a lot of labor would get you a simple dyno. Purchasing one is likely to cost a minimum of $5000.00; a really good setup will cost much more.
It is counter-productive to 'hop up' an engine if the results are such that the lower & mid range torque are severely reduced....UNLESS you intend the engine purpose to be for mostly or only racing; or, where you might be able to 'stay on the cam or pipes', that is, you keep the rpm well up, all or most of the time. A peaky engine makes for a really bad engine for touring, & can be unpleasant for sport touring. What you may end up with is an engine that can produce more top rpm horsepower, but requires a lot of shifting of the transmission at high RPM. Only the more serious street rodders would put up with a truly quite peaky engine. It is not unusual to see engines that are so peaky that they may, too easily, stall off the starting line unless exceptionally high rpm is used while at the starting gate....perhaps even to keep the engine running. In some instances, a heavier flywheel is used in such circumstances....but everything takes its toll on something else.
It IS possible to use a 'hotter' camshaft, which, mostly alone, would normally make the engine 'peaky', to which you add higher compression ratio, exhaust & intake 'tuning', perhaps a modified ignition, etc., such that the resultant horsepower output is increased and the resultant torque curve is usable enough & the engine not very peaky. This, over-all, is likely the goal of many modifiers for street usage, whether they understand it, or not. Doesn't hurt modest racing performance either!
Intake system:
Increasing the size of the carburetors is not always a good thing to do. As carburetor throat (venturi) diameter is increased, it becomes increasingly difficult 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 BMW R80 and R100 machines well know, the 32 mm carburetors provide better throttle feel and even acceleration into midrange & sometimes a fair amount higher, than the 40 mm carburetors. With carburetor sizes (& this also applies to fuel injection throat size), smaller throats give better low to mid-range throttle response, sometimes even upper 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 & high rpm to very high rpm, the largest carburetors or FI units that will allow decent atomization, are usually the best for JUST 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 suffering can be a considerable amount. 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. Many have installed Mikuni carburetors to get that SNAP effect. What is not often realized is that over-all acceleration over a distance may be the same. Some have installed springs on top of the slides of the Bing CV carburetors (stock on some late Airheads versions), and the characteristics of the spring add some of the snap feel to the engine. Often the springs are added to improve the positive return of the slide to the bottom mechanical stop (including the speed/velocity of the slide return). This spring can fool a lot of people, regarding what is really going on. Having a motorcycle that has a snappy jump off the line, and then peters out some, is an example. For some, they will replace the stock CV carburetor with a non-CV type, for the 'improved power' slide-only type of carburetor, and may well get a snappier feel, but less over-all acceleration to a much higher speed goal. For pure slide carburetors, the operator needs a good feel for the carburetor throttle amount, lest the engine lean-out, during the desired throttle positioning.
It is more important than often considered by those after more and more power, that in the mid-range of rpm, good response is needed on a touring or sports-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 opened a large amount. The CV carburetor tends to greatly minimize that effect, but in some instances it will still be evident. In quite a few instances, the effect is sometimes described as 'the engine acts like it is not fully warmed up'. This is the leaning-out effect. Some will make the situation even worse, by adding richness to any available acceleration pump output. It can be tricky to determine just what the problem(s) are.
Still, it is important that the combination of enough quantity of mixture, AND the throat velocity, insures an fully or nearly fully atomized mixture gets into the cylinder. If the velocity is too low, the next intake stroke of the piston will not allow much mixture into the cylinder. It is a simplification, but the total 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 (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 part of the 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 large effect. A definite and noticeable increase in power is available from getting cool air to the carburetor intakes. However, if this air is not warm enough, or at a critical temperature to be more truthful, the carburetor venturi can ice up; 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; or, as is said differently, the density altitude is higher. 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; this is all especially so with carburetors, and much less so with a good fuel injection system. That is why carbureted vehicles have 'chokes', or enricheners, particularly 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 will compensate for oxygen in the exhaust, air pressure, temperature of engine cooling water, etc. BMW Airheads, ALL models, have warmish to warm+ air coming to the carburetors, due to the central air-cleaner, and how that air cleaner gets its air. The pathway is quite different for this heated air for the clamshell models versus the twin snorkle rectangular air cleaner models, but the effect is the same, but not necessarily the amount of the effect.You WILL get more power, perhaps lots more, if you can find a way to get enough cooling/cooled air to ...and through ...the airbox and to/through, the carburetors. It is often difficult to do this neatly and avoid ingestion of rain, leaves, etc. A NASA type duct, including the angle separation system used on turbine motors (airplanes) works well ...sometimes. Few ever use NASA ducts on piston engines.
Ram Air:
Sometimes folks ask questions or make statements about ram air possibilities for "no-cost supercharged horsepower". I suggest you forget about it unless you are planning to ride over 150 mph. Below 130 mph, the effects are extremely small, and effects even at 150 mph are just noticeable. The effects above 150 mph are noticeable and often worthwhile. No, ...a larger or extremely large 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 not correct here. There is some information on intake areas velocity in my formulas article: https://bmwmotorcycletech.info/formulas.htm
Any specific dynamic air pressure on a moving object is treated by engineers as a specific size of flat plate moving through the air at some specified 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. Read that again and again!
An example:
Consider your Airhead or any other motorcycle or vehicle, 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 from the speed. At 200 feet per second (about 136 miles per hour) the pressure is 48 pounds per square foot. The drag is 4 times higher for a doubling of speed (I also put this information here so you'd know why gasoline usage increases so fast with increasing speed). If one was to divide 48 by 144, you would have the pressure per square inch, in this case 0.33 PSI. That is basically almost nothing (2%) compared to atmospheric pressure forcing itself into the cylinders (about 15.0 psi at sea level). Thus the 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 to quite high road speeds, and this includes some very fancy types of ducting that has an added drag that is quite low.
Intake tract length:
The effects of intact tract length (from air opening to the intake valve) are usually difficult ideas to get across to many folks, and the same effects, sort-of in reverse, are seen in the exhaust system. These effects are noticeable at all throttle settings, but the effect varies considerably with those settings. I'll give it a go here.
When the intake valve is in its 'open, or at least somewhat open' position, & in conjunction with the time just after the piston reaches bottom, & 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, but engine rotating, 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 & pressure, depending on the valve opening & more complicated things. Pressure here means absolute pressure; or, if you will, referenced to atmospheric. To simplify this, let us start at the beginning of the intake stroke and assuming that the valve is opening, & the throttle is open at least some. 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 after the piston reverses direction a bit. Since this all happens at a fast rate, even at idle, the intake flow is in PULSES. The incoming air slows when that intake valve closes, and this slowdown occurs VERY suddenly. View this as a slug of air slamming against a closed valve, if you want to 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 these pulses, some complex things happen in the intake system, and the one that I want to discuss first is a fairly complicated one, the reflected pulse.
The intake system gas-flow pileup will act like any one specific pulse we wish to discuss, as if it is partially reflected backwards. This is not inertia I am speaking of, and if you cannot picture it in your mind yet, just accept it as fact. I will discuss this from a different viewpoint later, in the exhaust discussion, which might be easier for you to understand. This intake system 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 length so that the REFLECTED pulse, which is time-related, arrives at the intake valve at exactly a particular moment of time, IN RELATION TO a INcoming pulse of fuel mixture. This will have the effect of boosting power, as more fuel/air mixture will be forced into the cylinder ...at NO meaningful cost whatsoever in loss of energy, etc. Of course, you can ...and will ...use more fuel! FREE POWER!!! ...except for the gasoline usage! I once actually made up a VERY crude sliding tubes system for the intake, one of these replaced both snorkels, and on a dyno, moved a short tube from a toilet paper roll, in relation to a similar longer tube from a paper towel roll. I noted very distinct changes. The effect on rpm/power had to be seen to be believed. It was necessary to jockey the throttle and tube length and throttle to keep the rpm more or less constant. A change of as much as 8 horsepower was noted. I want to caution here (especially due to the unexpectedly large horsepower effect) that I did this very 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.
Engines have been built with variable length (or, two switchable lengths) intake tracts, controlled by throttle position, rpm, and other things. In the past, most of these ideas were not very practical, although a few worked pretty well. In the last 20 or so years, some car and motorcycle manufacturer's have managed to incorporate such things. There have been some clever engineering done 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 are the quite common now various methods of variable camshaft timing. Some have even had mechanical alternate air paths to the fuel injection. Engine designers have found ways to INcrease compression ratios far higher than in the long-ago past. What would have previously been called RACE tuning for camshafts has now become 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 are using ...what in the past would be regarded ...as quite radical camshaft timing ...which in the past would have meant low torque and lousy idling, all in the lower mid-range rpm area, except that now the various automatic cam timing changes and intake tract tuning GREATLY enhances performance by improving those lower and mid areas. These engines manage to keep high rpm performance; all this on regular grade fuel, with some engines having even better performance on 91 octane, yet could run on 87 octane, due to sensors and automatic adjustments. SOME OF THESE ENGINES NOW MAKE THREE HORSEPOWER PER CUBIC INCH!...and have a pretty nice torque curve (vs. rpm) too. You won't be able to do this to your Airhead.
Take a look at this updated URL for Mez Porting's calculator. This, with appropriate input from you, will give you a lot of information. Even if you do not plan on doing the calculations or using the page, take a good look at it ...the things that it asks for; and, the bottom comments, will prove interesting. This calculator is for singles, twins and four cylinder engines:
http://www.mezporting.com/exhaust_length.html
The Airheads:
BMW has Service Information bulletins, particularly on some R80 models, for drilling holes near the snorkel opening, to deal with a reflected pulse velocity problem (poor fuel mileage) (at least I think it was the reflected velocity pulse) that was not so perfect in the mid-range (of course, the technical effects details were left out of the bulletin). BMW was certainly not trying to restrict power! Just because it works to improve fuel mileage 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 tuned correctly right from the factory. My article about the intake on the R80 models is on this website ...it is article #9 of the Technical Articles List (in the fuel system section):
https://bmwmotorcycletech.info/R80-fuel-mileage-problems.htm
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Late models of BMW Airheads have intake 'snorkels' to the airbox that restrict the intake opening size to a smallish taper. There is a bell-mouth associated with that. The section with a taper seems, to many folks, to go in the wrong direction! For some strange reason (I leave YOU to think about this), the snorkels are NOT the same cross-section near the bell-mouth. LOOK AT THEM! All this might, at first glance, to greatly restrict performance & if openings were small enough, this would certainly be true. However, if BMW designed the taper & the bell mouth to keep laminar (smooth) airflow & the correct velocity ...at the almost supersonic velocities in the system (which are normal), then BMW was very clever .... & ...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, eddy currents & swirling, etc., which would cause losses, during its way down the snorkels. |
Laminar air flow? Reversion pulses? Sounds like I'm getting techy, and, yes, I am ...but will try to keep things relatively simple.
YOU may be interested in the effects of changing the length of the intake system. This is OFTEN SEEN with 'racer boys' or builders of Café bikes who remove airbox items & let each carburetor separately breath through a screen or a filter, right at the carburetor inlet itself. 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 remove (from 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. My simplified version of the formula includes certain assumptions I made about the speed of sound inside the intake system, effective pressure, valve timing, & temperature but it is close to real world.
IF you remove your airbox-to-carburetor items & put stubs on individual carb intakes, you certainly do have a tuning effect, but the effect is now at a very much higher RPM, with the shortened intake length. NOTE that this mathematical effect is exactly what testing shows, and it is exactly what the result is from using the individual cone-shaped filter units that are popular on Racer-Boy carburetor intakes. There can be other effects, from the weight causing fuel frothing from vibration, and the need for jetting changes ...but I will stay with the torque curve effect for the immediate discussion.
Keep in mind, again, that the length of the intact tract, overall, is from the beginning of the air flow into the system, through the filter, any tubing and connections, through the carburetor, through the head adaptor, and to the inlet valve.
This intake system length ALWAYS affects the rpm for torque peak.
About 7 inches from valve to carburetor inlet to the intake valve was what I used decades ago for Airheads, with the stock air cleaner and tubing removed, for calculations for dyno work. Let's be slightly 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 (& 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 bell-mouths, & a rather restricted exhaust, & smallish carbs, but very light valve gear, & a special camshaft, & a very light flywheel & clutch. We had no problem with WELL over normal redline, to say the least! Not a good engine to go touring on! It had fairly good acceleration above 5300 or so, and above about 6200 it was fierce. But, how often do YOU want to be at or above 6200 RPM? Can your engine stay together at 8000+ ...???
The bottom line is that if you shorten the intake over-all length, the torque peak will rise in RPM, & once it gets high enough, the 'tractable' torque at the lower rpm area is AWFUL. With a wilder camshaft on an Airhead, the bottom end torque can be so bad that you must slip the clutch a LOT when taking off from a stop. If other things are reasonable, the torque peak will not only be at a higher rpm, but it may be a considerably higher peak too; which is often the goal. Since this happens at higher rpm, then the horsepower output is increased, sometimes very considerably. The cost will typically be poor low rpm performance.
The Exhaust System:
At this point you might be thinking that all of the intake effects discussed also happen in the exhaust system, due to complex relationships with the exhaust valve, length of system, intake opening, etc. YES ...the same sort of formulas & things generally really do apply. I'll try to explain this in a gross simplification.
The effect of exhaust system SIZE is generally a rather small effect (particularly on 4 stroke cycle engines), & what is usually most important is the effective cross-section area from cylinder head port to the outlet ...or at least to the muffler intake. A smaller diameter exhaust header pipe (meaning all of the piping, from engine exhaust port to the muffler) can HELP midrange torque, but may or may not hinder top end. It is generally important that the muffler interior volume be substantial, the reasoning for this is quite complex. A noisy small diameter straight-through muffler does NOT perform well! Many have replaced the BMW mufflers on the Twin-Shock Airhead models & found that the now loud exhaust does NOT equate with more horsepower, in fact, performance may suffer ....sometimes quite a lot. BMW did their design work rather well on the old twin rear shock models mufflers. The BMW Airhead mufflers or pre-muffler collectors that are under the engine/frame, do NOT perform as well as the old BMW twin mufflers, so BMW made other changes, to help compensate for this. That help was not perfect, as seen by the hp and torque figures for those bikes ....but, it is OK.
Let's get into this a bit deeper:
The exhaust valve begins to open, & the mostly burned gases begin to exit, until just AFTER the exhaust valve closes, where the gases pile-up, but can't exit. Each power stroke results in a pulse of gases, of uneven shape. Multiple firings of the cylinder means a 'train' of unevenly shaped pulses are going down the exhaust pipe. At higher & higher RPM, the pulses get shorter & shorter, but more often. 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 exact exit point. The gases see atmospheric pressure, atmospheric temperatures, increased effective diameter of the pipe (that is, NO pipe!!) ...ETC. The gases are confused, & think they have run up against a a solid wall, which then gives way, modestly smoothly, & allows the rest of the pulse to go to 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 seemingly instantaneous manner, unless you have instruments to measure it. At the "gas confusion point", the gases are reflected back up the pipe, all the way to the exhaust valve & even into the cylinder if the valve is open. The reflection can be full, partial, partial distance, or a combination. One of the complicating factors is that ANY inconsistencies in smooth pipe interiors 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, ETC. 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. This is just ONE of the reasons that exhaust system design is part art & part engineering.
You might think that 'old-fashioned' exhaust mufflers are for 'show', or just for big noise reductions, but a LOT of engineering went into REAL fishtail & other 'weird' looking mufflers, for engine power output performance.
You can think of the exhaust gases as a series of soft rubber slugs, traveling in the exhaust system, with a bit of separation between them. They hit an inconsistency ...or, especially, the outside air, and SOME rubber bounces back, pushing each previous one ahead of it, and squeezing them thinner ...and so on. So, what we have is a reversal/reversion of SOME of the exhaust energy. This wave is at relatively low pressure, so you might think it has little real-world effect and that it would or could be a bad thing, except that the pressure is a decent amount compared to the atmospheric pressure on the intake system; and, 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 RPM area. On a PRACTICAL basis, 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.
Yes, it is true, if the exhaust system was extended, perhaps by a FEW FEET behind the motorcycle, the results would be very nice for torque output, and would likely be used to enhance the lower rpm torque range. This is mostly disregarded by many UNknowledgeable 'hot rodders', who SHORTEN and use OPEN exhausts! I have had some experience in this regard in a practical situation, not just a dyno, when I raced at Bonneville, and someone had an engine that would not work very well that day, and I temporarily added a pipes extension (a long piece, very heavy, just a fence pipe we picked-up!). The owner was astounded!
NOTE! The careful reader may well say, what about using a LONG intake, and a SHORT exhaust (or, vice-versa)? ...well, that would be workable, but the intake, if long, will have more problems staying cool, and have many other problems, including the FACT that the exhaust must exit someplace not near the rider's knees! Changes have repercussions. On a practical basis, due to gas expansion, mechanical location problems, and MANY other things, the exhaust system is always longer than the intake system on stock vehicles, and the exhaust tuning on 4 stroke engines is usually done mostly by pipe diameter, and one or more crossover pipes, and by some muffler innards that are often rather clever ...or, at least, have vastly more real engineering in them, than just for sound reduction.
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In some systems, the exhaust flow starts out with a small diameter and then 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, to provide what is called an Expansion Chamber. The outlet of such an exhaust system can be of very small diameter, and OFTEN IS! There is an amazing amount of power produced by the small old 2 stroke engines for their displacement, and the exhaust exit is quite small. How does this all fit in with your ideas about muffler and pipes size and 'open pipes' theories? |
I have NOT, so far, except in reports of experiments, heard of anyone using a truly wide-range dynamically 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 and with high engine output.
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. Engineers use similar formulas for open and closed pipes in organs for many 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 for each pulse, or call it a wave, the result CAN reduce the pressure (or increase it) in the pipe at that particular point, and THIS effect then travels, again! .... 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 during 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 help 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 everything, including, ESPECIALLY, RPM, but also throttle position, 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 and tiny sensors, 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 still extremely complicated; and a big complicating factor is that SOUND levels are a major reason for exhaust mufflers....and, incidentally, sound generation or muffling of sound, is somewhat dependent on other things besides what is inside a muffler. Even today, computers and lots of measurements with sophisticated equipment do not usually tell the whole story, just give some directions and/or hints, granted rather good ones ...usually.
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 if you change any one thing, it seems to change many more things. That is why well-experienced master tuners/modifiers are in such high demand for commercial racing. Today's engineers and some master tech's use very sophisticated computerized tools for analysis. The factories have very sophisticated dynamometers; and methods of changing almost any part of the intake, exhaust, or engine internals. YOU will not have such a sophisticated multi-million dollar setup. The factories can change various things until they get the particular performance they want...>>and for touring, that usually means a high torque output, with a relatively flat curve.
If you consider what you have been reading, then you should now understand that "Back Pressure" in the exhaust system is LESS important than the pulse timing; back pressure has LITTLE effect until it is really quite high! More on this further down this page.
Design of TRUE performance enhancing exhaust systems, especially the mufflers, used to be part art, as well as part science. Today, vehicle manufacturer's have LOTS of tiny pressure transducers ....and visual transducers! .... plumbed into prospective 'mufflers' and other exhaust components, and results analyzed on very high speed computers, capable of many dozens of sensor outputs to be displayed at exactly the same time; as is horsepower, RPM, and torque (and fuel flow, etc.). That is why I am so skeptical about those who sell aftermarket exhaust systems, unless they have AT LEAST a dyno and sound measuring gear.
You might be surprised to find out that antique 'fishtail' exhausts (not the much later ones for appearance sake only) 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 exhaust tip outlet is then restricted, changing the velocity, and reflection pulse. Does this begin to sound a bit like an intake system too?
Megaphones of the tapered larger outlet types are not practical on the street, unless internally muffled, which tends to defeat their performance. A proper megaphone for wide rpm range 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. I am particularly 'amused' at those folks who use a type of aftermarket muffler that has a number of restrictor discs (you can change the number, sometimes the type). These do not sound good, and the restrictions are just that, and the reversion wave is usually close to non-existent. These things are a total waste of money, IMO.
SOME of the BMW airhead models with muffler-collectors located under the transmission can be modified, at considerable effort. Usually the sound increases some, and the torque MIGHT or might not increase; and, can actually decrease. It is difficult to improve on the BMW stock setup.
A general rule is that IF putting other 'mufflers' on your BMW motorcycle vastly improves performance, you likely had 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 a prominent and properly timed extraction effect at the exhaust valve ....due to the effect this will have on the incoming fuel-air mixture. The holy grail would be to have the good effect be over a quite wide rpm range. That comes, as best possible, by careful design of the engine and intake and exhaust system as a whole. Typical exhaust back pressure has a rather small effect on power ...within limits of course. Perhaps a few more words will help in this area:
Shortly after the spark plug tip area ionization and then spark ignites the fuel-air mixture, the flame begun by this spreads rapidly, and the very rapid expansion of 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 very 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 to 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 cylinder. 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 has good usable piston pressure makes a BIG power output difference ...but there are practical limits. Another way of thinking is that a given pressure in the exhaust pipe for the exhaust pipe diameter, is very much smaller than for the much larger piston area. In that sense, there is an effect similar to your hydraulic braking system, with its small master cylinder piston versus the much larger disc brake cylinder size. It is way beyond that idea, but, well ...understand better now?
Many decades ago, 'boxes' were put where we now think of 'muffler', and that box, which was NOT a TRUE expansion chamber, collected the exhaust mixture, quieted the exhaust, and had little restriction. Early engines mostly used that system, but the rpm was low, and the stroke long. An improvement on this, especially for the increasing rpm engines were capable of, 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, with design mostly just for appearance-sake and are all without the needed 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 a very complex waveform developed 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 next step in early designs was the adding of smallish holes in internal baffles. This caused many complications, which were eventually solved. In fact, it was these complications (that needed fixing) that brought about a large amount of knowledge about muffler, collector, and pipes design. It was then possible to have a exhaust system that muffled AND performed. The innards of some exhaust systems contained an expansion chamber, improving performance. BMW uses that method, and more, on the Airheads, particularly the twin muffler types. I know this seems absolutely wrong, common sense seems to tell us that restrictions ALWAYS mean lower power, but it is true. There is a photo of a cutaway BMW Airhead muffler in my SuperTech Article:
https://bmwmotorcycletech.info/SuperTech.htm
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. 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 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 sport cam with a compression increase is usually 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 such as more vibration and a bit more clutch slipping at initial takeoff. Some sporty camshafts, the BMW mild one included, EFFECTIVELY raise the dynamic-effective compression ratio DURING high rpm operation. I will NOT get into why ...and there are several effects. You should 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 lower RPM performance will SUFFER. The BMW sport camshaft will considerably detract from performance if you do not. The sport cam will STILL only START to "perform" above 5000 rpm ...typically the improvement is mostly notable from 6000 or so. I 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, harder to quantify effects, come from combining changes to the intake and exhaust. In our BMW Airhead 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. That effect varies with throttle opening and rpm, etc. However, properly designed, the over-all effects can IMPROVE THE TORQUE CURVE OR PEAK HORSEPOWER. When it is the horsepower increase that one is after, that gives higher top speed.
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! That means that the exhaust system....and MORE...was costly to design for the ST and GS models. OK OK OK ...so there are other reasons for raising the exhaust height.
The sales department was probably responsible for the rear disc brake on the older RT and RS models; the brake is not very good compared to the drum version.
What all this means, on a practical basis, is that you really need a dyno to properly 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!
Specific recommendations and statements, that I feel safe in making:
These apply to stock, relatively stock, somewhat modified Airheads, and well modified Airheads. In every instance I mean for the street ....although much can apply to all-out racing. It makes little difference the engine size, compression ratio, piston changes, milling the heads; carburetion and/or exhaust changes; nor, even installing the BMW sport camshaft; these comments generally apply.
1. Camshaft:
A BMW sport camshaft can be used. The BMW Sport Cam has a deleterious effect at lower rpm. This is a timing cam, not a big high lift cam, and use of even higher lift, by incorporating high ratio rocker arms, is probably possible, but one must be VERY careful. Pay attention to the rocker arms, pushrods, piston interference, etc. The performance improvement will be best above 5700 rpm. This means that without a LOT of other changes, you will LOSE performance lower down. There are companies in Germany making a variety of camshafts for the Airhead. Do consider them. If you use a sport camshaft, try to have at least 9.5 compression ratio.
2. Compression ratio ....and other things:
The higher the better, within reason for fuel octane, etc. On a practical basis, probably about 9.8 is the highest with a stock camshaft, and maybe 10.8 with a sport camshaft. This includes dual-plugging and with a really clean combustion chamber with no sharp places, and everything tuned to the Nth. Even 11.3 might be possible on 100 octane or better. The problem is mostly 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 all 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 cylinders themselves....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., including depth, and so on....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 reduced by balancing of the assembly and engine. 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 a flywheel:
https://bmwmotorcycletech.info/lightened-flywheel.htm
4. Intake system:
You should strongly consider modifications to obtain cold air to and through the airbox. Except for a near race or actual race motor, OR A CAFE BIKE for appearances, I suggest you NOT eliminate the stock airbox entirely....that is, I suggest you do NOT use individual screens or filters right at the carburetor mouths (thereby not using the airbox at all), although individual screens are certainly better when obtaining cold air to the carburetors. If you do go to individual filters, AND ARE AFTER PERFORMANCE, NOT JUST LOOKS, BE SURE TO 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 vacuum at the filters. IT IS VERY IMPORTANT THAT THE ADDITIONAL WEIGHT OF THE AFTERMARKET CONE OR OTHER TYPE OF INTAKE FILTERS (MOUNTED AT THE CARBURETOR INTAKE) NOT VIBRATE, WHICH WILL FROTH UP THE FUEL IN THE FLOAT BOWL AND CAUSE YOU ENDLESS PROBLEMS BECAUSE YOU DON'T KNOW ABOUT IT AND TRY EVERYTHING ELSE AS A 'FIX'. Use of a length of plastic pipe between the individual screen filter and the carburetor will improve mid RPM to a bit of lower RPM area, depending on the length. Don't allow disturbances inside the piping, etc.
Use of K & N air filters WILL decrease engine life. 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 to be solved. The engine will be less tractable, due to the bad effect on the stock 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 (especially durometer rating), & mechanical support for the carburetor & the individual air cleaners/screens is needed. More power IS available from individual air cleaners, but the torque peak is moved considerably upwards in rpm, making for a more peaky engine. You can, as mentioned, move that torque peak down some, with an intake pipe insertion. NOTE that using JUST a bell-mouth adaptor, with no filtering other than a non-restrictive large hole screen, will give the best performance for peak RPM horsepower. This is what you would do for setting speed records, like at Bonneville.
NOTE that a bell-mouth is really going to be necessary for any sort of decent performance. Even if not going for a top speed record, a bell-mouth, all other things considered, will eliminate MUCH of the air disturbance problems of a straight pipe or bare carburetor input. Remember what I had previously said about the exhaust pulse as it hits the air at the muffler output? That SAME thing happens at the carburetor intake, particularly if there is no smoothing, tapered, bell-mouth.
With regards to cold-air:
Avoid effectively lengthening the intake system, unless you WANT 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 that. 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 situation 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 BMW clamshell box ...and think that it should be eliminated. Get rid of ALL smog and other items under the air filter on the later rectangular 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 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 can be cut back, a bit at a time, or, removed entirely, and you will see the BAD effect any shortening 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 in rpm. Generally, if the carburetor size is stock, you can do minor modifications 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. If your engine has two sizes of snorkles, try using the larger one for both. Try the reverse too. You will need jetting changes for each change. 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 to 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 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 and also dual plugging:
For racing, or very spirited riding at VERY HIGH rpm, single plugs MIGHT be better than dual-plugs, & this is NOT cut and dried. You may run into octane problems if your CR is high and you are staying in a normal RPM range, but any problem that dual plugging helps with does tend to fade at the highest rpm to being almost non-existent; but, there are exceptions!
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 7300+-. Install Porsche points in the canister and you may go even to 8000 or so, but the latest stock BMW numbered points for the canister are decent for stock red-line rpm. The BMW stock electronic ignition as far as Hall device, module, and coil(s), are quite adequate for 8000 or bit more rpm. Only the most modified engines require tighter controlled and more powerful ignitions. The spark plug resistance caps can be eliminated on points models. For the BMW electronic ignition, I suggest the latest ignition module (see my ignition article), with two each 6 volt coils as used on RS and RT in the early eighties, and with 5000 ohm spark plug caps (NOT lower), with high quality silicon rubber covered stranded copper wires. You can NOT use 1000 ohm caps with the BMW electronic ignition model, without some goodly danger of frying the ignition components (especially the Hall device in the canister). 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 ideal setup is as just described, but use dual-plugging and use the Accel twin tower coils as noted in my other articles. If you have unlimited funds, you can use a crankshaft-mounted ignition trigger, but in my experience, none of them are worth the money, assuming you already have a BMW electronic ignition.
Ignition curve:
This can be modified to match the rest of the engine, particularly if the engine is heavily modified. On a mild to moderate performance enhanced Airhead, this can be as simple as bending the ears in or out a bit, and/or using a slightly stiffer spring on one weight. In some instances, lightening the weight to lower the rpm for maximum advance is done, but usually not. On some early bikes any change will be simply using a different model of advance unit; this is particularly so on the first of the /5 motorcycles, where the advance completes at about 2000 RPM, and you suffer from pinging (pinking) problems. Restricting the advance amount can be done with post shims or other methods . For a RACE bike, much of these things is moot, as you are well out of idle rpm area, and thus always at full advance, and the advance unit is for starting the engine. You might also have a performance ignition driven off the crankshaft for best stability, and near perfect ignition as the timing chain and sprockets, tensioner guides, etc., wear.
One final word about the automatic advance: The best power and performance, IN GENERAL, assuming early use of the throttle (idle and above), is that the octane of the gasoline is adequate for the compression ratio, etc., and will be obtained with a fairly 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, but is of no help if the engine is being revved-up in preparation for a fast higher rpm take-off! If accelerating the engine rapidly, a higher advance limiting RPM is sometimes a good idea. Consideration of what to do about the ATU can be important...or not. In general, for Airheads, I prefer a fast ATU, with maximum advance obtained at around 2000, sometimes a bit higher...or lower. All, if gasoline octane permits.
6. Use of a two-into-one exhaust system will likely REDUCE performance.
7. The Super-Trapp type of exhaust is VERY likely, even with a large numbers of discs, to perform MUCH less well than the stock BMW exhaust...and be loud.
Addendum #1:
Modifying the stock twin mufflers for a more pleasing sound & a rather small power increase:
1. On the NEARLY FLAT REAR FACE of the stock mufflers, make a small sharp dimple as a drill guide, then drill 1/4" holes in each muffler at TWO places, one at top, one at bottom, centered in the flat area. Start with a smaller drill bit if you want to. Finish the holes by using a counter-sink tool in your electric drill motor chuck, to give a small outer depression.
2. Obtain an old drill bit meant for metal, not wood, of about 5/8 to 3/4" diameter 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 the brazing or welding is secure. If you have a bolt-together drill bit extended used by some electricians in house wiring, you can use that, but be SURE the bit can NOT become separated from the extension ...it is then difficult to remove from the muffler innards! Drill from the muffler OUTLET end (this has nothing to do with the holes in 1.), into the way far deep inside center, & drill it through. If you were clever, you could do this from the inlet side, which might be easier, but not always!...the mufflers VARY in construction at that end ...but IF you CAN drill it there, try it ...the problem is usually that you see a dome, and it is hard to drill into a dome. I've done it with a very long punch, then drilling the punch mark.
3. At the rear outlet, at both 9 O'clock and 3 O'clock, punch-prick a starting indent, INSIDE the outlet, about 3/4" inwards from the flat face ....and drill on an ANGLE, leaving the drilled part elongated (elongate 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 drill bit side flutes cutting the metal of the muffler. Do NOT overdo this. The drill size suggested for this elliptical hole is 1/4" to start with ....up to 3/8" maximum.
4. Make no carburetion jetting or other change unless you are already on the very edge of running lean. If the leanness is for wide-open-throttle, go up '5' on the main jet. If the leanness is in the mid-range, go to one step larger on the needle JET (NOT needle position). If the leanness is at idle, then adjust the idle mixture screw. Check for recommendations for the UK version of your bike (assuming SAME size carburetors, 32 or 40 mm). If not loud enough, add TWO more holes, 90 degrees to the others at the outlet flat face, but NO MORE elliptical holes. If still not loud enough, change those 4 flat area holes to about 3/8". This mod helps power SLIGHTLY, moves the torque peak up about 200 to 300 rpm, is definitely louder ...all with relatively stock appearing mufflers to "most inspection folks", especially after some miles to get the shiny hole metal blackened.
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 just do TWO each 1/4" holes ...holes at top and bottom, not sides.
An extra advantage is that condensation inside the muffler is GREATLY decreased, and the muffler will last longer.
Addendum #2:
Krauser 4 valve heads:
I do not want to get deeply into these here. I will just make some quickie generalizations. They are tricky to get to work properly because they were never fully machined and completed. Once that 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 a fair amount. Here is my article on them: https://bmwmotorcycletech.info/Krauser-Heads.htm
Addendum #3:
Premium fuels versus mileage and combustion temperatures.
There is an old controversy over possible increases in combustion chamber temperatures, etc, when using premium gasoline's in lower compression BMW airhead engines, where lower octane is supposedly not needed. Gasoline burns at about the same rate under normal, that is, not detonating, etc., conditions. The output (BTU) per gallon of premium gasoline is potentially ....or even likely ...to be a small amount LOWER than for regular grade of gasoline. Therefore, I think it is likely that SOME premium gasoline's WILL give LOWER gas mileage than a regular gasoline will ....assuming here that the engine will run properly on Regular grade gasoline in the first place. Temper this with the fact that gasoline formulas change, and also change between summer and winter grades of gasoline's. Winter gasoline tends to have rather volatile things like butane or propane in them ...etc. In the West, California in particular, oxygenates were added to most fuels for decades. These additions GENERALLY cause ~5% (or more) POORER gas mileage. They are NOT good for your engine, carburetors, hoses, etc. Nowadays, ethanol is added to nearly all road fuels. Without jetting changes, ethanol in the fuel makes an engine run leaner. The fuel mileage decreases even more, perhaps another 3 to 5% when you rejet for performance with such fuels. When dealing with this subject on a practical basis, especially on a stock engine, one of the first things to check is the EURO (UK) jetting and needle position, for the same size of carburetor. The UK jetting is almost always better, richer, for performance.
Addendum #5: MISCL
Nothing here at the moment!
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