Now I just know I'm going to make myself very unpopular with an array of people by doing this article, but I can't help myself. It is something that plagues me continually.
It does not seem to matter which way I try and explain it, or how I represent it, I always meet a barrage of either unsubstantiated or uneducated reason and arguments. So now I have the stage to myself, I will put this as I see it from a wealth of experiment and reasoning over the past twenty years or so.
Exhausts. A subject that a great deal is known about in theory, yet a diminutive amount is actually applied in practice as far as the poor little A series engine is concerned. In fact, the only country where I have seen any money/time put into developing exhaust systems (and that means from cylinder head to the tailpipe) for the venerable little motor, is in America. It is a consequence of those that race and seek the ultimate fully expect to have to pay for it. Whether they do it themselves or whether they purchase something someone else has properly developed. And that is a key word 'properly'. I am sad to say that few specialists in this country carry out conclusive and meaningful tests.
I have all too often heard the immortal words "It just gets rid of the exhaust gases". If that is so, how come the motorcycle industry and the higher echelons of automotive race development engineers invest so much time and effort in developing special exhaust systems?? Yamaha motorcycles have a sophisticated system that enables their motorbike engines to produce exceptional torque figures over a much broader power band than the others. Formula One developments reached such a peak that having developed a system with variable exhaust geometry, it got banned before it was used. And that is the crux of the problem really - development costs money. And time, but certainly lots of money - time is money after all. And that is one of the reasons we have so many duff and garbage exhaust systems on the market for Mini enthusiasts.
Oh, fashion plays a big part in this - the upsurge of the drainpipe - mine is bigger than yours syndrome - has only itself to blame. Some have manufactured purely for this market, with absolutely no consideration to anything else - including performance. Many of those who have produced such exhausts for their customers do not even understand exactly what goes on in an internal combustion engine, so couldn't start to think about where to start developing and producing an effective exhaust system. Even major manufacturers pander to the 'tart' market, rather than including anything resembling a performance-enhancing feature in their products. You know who you are.
I am afraid all I can say to those who buy a system, or part system for its street cred, is that you pays your money and takes your choice. This is one particular place where big ain't necessarily beautiful. In fact, big is definitely ugly in terms of decent performance. Ever wondered why the RC40 hasn't changed its design, to have some humungous rear box/pipe diameter, just to cater for the fashion conscious? To be blunt, it doesn't need to be because it was properly designed in the first place - a situation where performance was the sole and outstanding feature. So, for all those out there that have asked me a million questions on this subject, and all those I have upset one way or another along the way, I will try to explain, in the plainest English I can, how this thing works. Initially, I am going to just try to explain in the simplest terms I can, just what an exhaust system is all about. Read on and ingest this - it'll do you good, I promise!!
I have to confess that after so much time and money has gone into the development of the trickiest of cams, heads and other associated engine components, that it is hard to see that an inert piece of steel pipe could make any difference to performance. It is only after carefully listening to those in the know, reading of some literature, and some small amounts of development on my part, that I can appreciate the importance of the 'holistic' view of research. Much like a Formula 1 car, it isn't any one thing that makes one car better than all the others, but a sum total of differences. These differences are discovered during extensive and expensive development programs, the parameters of which can be changed with the introduction of new rules, and guess-ology rarely produces worthwhile results. Having invested a certain amount of (usually) hard earned cash on their engines and induction set-ups, why do people fall at the last hurdle when it comes to the exhaust? I wish I knew - but as previously expressed, fashion and street cred have a lot to do with it. It may be a sobering thought for those of you guilty of this, that the fancy looking system you just bought may be costing you a large chunk of horsepower and drivability. The exhaust system in its entirety affects both.
When the exhaust valve opens, it released the hot pressurised and noisily energetic gases behind it. The piston is charging up the bore and helps expel these gases in a torrent, sending them hurtling sown the pipe to freedom and the outside world. The size, shape and length of the system will affect how efficiently this is done and the effect it has on how the engine behaves.
As long as the piston is moving up the bore, it gives the exhaust gases momentum to keep traveling out of the combustion chamber. Trouble is, when it gets near top dead centre then starts its journey back down getting into ' intake' mode, the gases momentum is momentarily lost. In fact is almost reversed as it starts down again. It is here that exhaust design can influence engine behaviour during the overlap period when the inlet and exhaust valves are open briefly at the same time. This situation worsens with longer period cams when trying to improve power output; the more power required from any particular engine size is dependent on its volumetric efficiency at higher rpm. The more rpm you can turn the more horsepower you can make. Higher running rpm means longer duration cams, which in turn requires greater overlap periods.
Longer duration cams that open the inlet valve earlier and for longer, doing likewise to the exhaust valve, means that this situation is also occurring at low engine speed, adversely affecting performance at the lower engine speed ranges. This is largely because the volumetric efficiency (VE) suffers. VE is basically how well the cylinder is filled for each cycle on the engine. The more fuel/air mixture you can get in, the better the engine will run, the more power it will develop. VE can be adversely affected by contamination, particularly by any exhaust gases that either didn't make it out of the chamber or got sucked back in. On seriously long duration cams with badly matched exhaust systems, exhaust gases can actually travel part way up into the inlet tract as a result of high pressure. Hang on, we're getting a bit technical too early here, back to the basics…
Erm, oh yeah, so, a re-cap. Engines running at higher rpm need to have their inlet valves open for longer so that a sufficient fuel/air mix can be drawn into the chamber. Unfortunately this means that the exhaust valves are open a similar amount, and all are open the same amount at low rpm. Extending the cam timing for high rpm exhaust valve closing occurs later and inlet opening occurs earlier, increasing the amount of time that both valves are open at the same time. During this overlap period, the incoming charge can be contaminated by residual exhaust gas that didn't get out in time. This wastes space in the new combustion mixture, reducing the efficiency of the engine. This can become so bad that the fresh charge can be forced back up in to the intake tract. So something has to be done to keep all the gases flowing in the right direction.
Low pressure near the exhaust valve at this point would help considerably, the exhaust can assist in this goal. For illustration in the simplest form, I will describe this as it was put to me. It certainly helps get a grip on what is going on. Enter 'the river'. The exhaust system deals with the physical flow of exhaust gas much as water flows down a river. Where the river is deep and wide, the flow is slow, where it is shallow and narrow it speeds up; where there are sudden changes in direction, turbulence is created - as in white water. Exhaust gas in a system however differs as the flow is not steady and continuous. The opening and closing of the exhaust valve creates a high pressure pulse followed by a longer low pressure period, bit like a wave running downstream. Added to this are the smaller pressure waves that run through the stream both with the flow and against it. These ripples are caused by the flow hitting something.
In the exhaust, every time these pressure waves hit something they reflect up and down the exhaust at the speed of sound (faster usually because of the temperature). Now if these reflections can be timed right, they can help scavenge the burnt gases, saving the engine the effort of pushing it out. As previously intimated, size, length and shape can influence when and where this all happens. A simple straight open pipe was deemed to be the best years ago - no restrictions, see. However if it is too wide or too short, the gases expand and get turbulent, creating a jumble of pressure waves going in every direction, and decreases effective flow. They need to be better-organised if they are to help. Incidentally, these pressure waves are known technically as condensations and rarefractions. Anyway, back to the plot…
Each release of exhaust gas by the opening valve is referred to by most as a 'slug' of gas, so I'll stick with this for simplicity. When each slug exits the open end of the exhaust pipe in it's high pressure state, it sends a low pressure wave back up the pipe then changes from low to high, back to low a few times, each time getting weaker and weaker. This wave reflection is returned each time it hits the closed exhaust valve. At different speeds it gets there when the valve is still open. With careful design this should coincide with a low pressure wave, creating the required low pressure in the exhaust valve/combustion chamber vicinity. This will change with different system designs and lengths, and at different rpm levels. The idea is to design a system to suit your needs. When developing this on the engine dyno, the right and wrong timing can be seen as peaks and troughs in the power curve. You must then develop a system to give the best where you want it, which will be different on a road car to a race car. As with everything there is give and take.
Incidentally, modern motorbikes now use pressurised air boxes and tuned intake lengths to much the same effects, combining the effects of both intake and exhaust to maximise power across a broad rpm band. It is possible to tune both to give absolute maximum power, but will narrow the power band drastically. A compromise is usually struck where a slight mismatch between intake and exhaust which sacrifices the peak of the power curve, but gives a far broader output of power, and fills in some of the troughs, making the engine more flexible.
OK, so the open end of the pipe causes a reflection of the pressure wave and reverses their sense, thus the high-pressure wave that came out goes back up the pipe as a low-pressure wave. A dead end or sudden restriction creates the opposite situation. High is reflected as high, low as low. An expanding pipe creates the opposite situation. High is reflected as high, low as low. An expanding pipe also creates reflection waves as a larger pipe slows the flow down. These expanding pipes are commonly called megaphones (one going the opposite way, i.e. tapering down is called a nozzle - thought you might like to know that!), a short and large taper angle appears like an open end to the wave so has little effect. However, a longer tapered section creates useful waves over a wide speed range, fooling the engine into believing it has a variable length. This is because the gas needs to expand at a certain rate before a reflection can be generated. The point at which this occurs will be affected by the speed and pressure as well as the angle of the taper in the pipe. This in turn means that the reflections will come at different positions throughout different engines speeds and loads. At higher engine speeds, the slug of gas will have more momentum.
Now where have we seen this type of exhaust recently?
It is not necessarily the best exhaust to fit to everything, but is best suited to race engines or extensively modified road engines where a less peaky power output is desired. Having said that, I am sure that if this was fully developed, a new breed of systems would evolve to assist the A-Series to be more civilised in a higher state of tune, in keeping with more modern machinery. Course, they've been doing this kind of thing for years with motorbikes. That's where I started with all this in 1976!!