Exhaust System

A well designed branch exhaust system is one of the cheapest ways of increasing engine efficiency, and should give good results on almost any engine, depending on how good its standard setup is. Remember that on a four stroke engine, we only have one stroke out of the four that does work - the power stroke. The other three strokes - intake, compression and exhaust - all absorb some of the power that was made on the power stroke. If we can minimise the amount of power that is lost by these idle strokes, we will have more power available to drive the wheels, which is what the engine is supposed to be doing. To know why there is worthwile gains to be had from fitting a good performance exhaust system, we will start by carefully looking at the factory fitted system.


Most of the standard exhaust manifolds are made of cast-iron, because it is vastly cheaper and much quicker to manufacture than a crafted branch exhaust manifold. The weight and the thermal characteristics of the cast-iron, however, limits the length of the individual runners, and its shape causes the gasses to follow some abrupt turns. The silencers are also mostly not built to enhance the gasflow out of the combustion chamber. This means that the engine has to force the exhaust gasses out of the combustion chamber on the exhaust stroke, with the result that the chamber still has some spent gas inside when the fresh charge of air/fuel mixture arrives. This residual gas (which has done it's work, and will not burn again) takes up space in the chamber which could otherwise be filled by a healthy, combustible mixture that can produce power. The situation worsens: as engine revolutions increase, so does the back pressure in the exhaust system, because the engine has to pump more gas through the restrictive outlet. It is not uncommon to see back pressure rise to 5psi on some engines at peak power. At the end of the exhaust stroke, the spent gas still inside the combustion chamber, remains at that pressure. Next the intake valve opens, and this pressurised exhaust gas pops out through the intake valve into the inlet tracts, pushing back the fresh charge of combustible mixture. When the piston has travelled down far enough to draw in the intake charge, we now have a very much diluted mixture, bringing the efficiency of the engine right down.



It is thus clear that there are real gains to be had from making sure that the exhaust gasses are effectively removed from the combustion chamber. A well designed exhaust system can even "draw" the gasses out of the chamber, using the momentum of the gas travelling down the pipe to suck the residual gasses out of the combustion chamber. If this sounds too good to be true, the principle behind it can be demonstrated with a simple garden hose : Connect the hose to the tap, open the tap, and when its running well, pull the hose from the tap. Quickly put your finger over he end of the hose, and feel the suction created by the water travelling down the hose. (Now call someone to bring you a towel to dry yourself with.) The same principle applies to a well designed exhaust system. The gas travelling down the pipe creates an area of low pressure behind it. This not only purges the combustion chamber, but also draws more mixture into the chamber during the valve overlap period. So, instead of having high pressure exhaust gas popping into the inlet tracts, we now have a partial vacuum inside the combustion chamber, which pulls the fresh charge into the chamber when the intake valve opens.

to understand how the engine work. Watch the video

Unfortunately we have to add that not all free flow performance exhaust systems are created equal. I regularly see aftermarket systems producing less power than the standard systems they replace. Some are unpleasantly noisy, and their gasflow potantial are dismal.


There are suppliers out there who sell a well-designed, efficient product which is usually unobtrusive in sound and appearance. The bottom line is to locate a reputable supplier before parting with your hard-earned dinero. And after reading the above, it will be clear to you that changing to a more efficient exhaust system can cause the engine to require different settings for optimum performance. It is likely that the engine will be able to run with a leaner part-throttle or cruising mixture, often resulting in a significant gain in fuel economy. If the fuel system isn't matched to it's new environment, you will never enjoy the full benefit of your new exhaust system. And the best way to optimise your engine, is on a loading type dynamometer with an accurate exhaust gas analyser. But you knew that.