The main purpose of an exhaust system is undoubtedly to route the spent exhaust gas out of the car's engine. Along the way the exhaust gasses may be used to drive a turbocharger and now-a-days it will most definitely incorporate a catalyst converter to reduce carbon dioxide emissions. But on a high performance car, such as a modified street car, or a modified race car, the exhaust system is much more important as it has a direct effect on engine performance and engine power. As a result, the exhaust system, and particularly the exhaust header design, plays an important part in both engine tuning and car tuning.
In general terms, an exhaust system consists of an exhaust manifold (which is sometimes called an exhaust header), a front pipe, a catalyst converter, a main muffler or silencer, and a tail pipe with an exhaust tip. In terms of tuning the exhaust system, the muffler is the easiest to deal with it's simply a matter of replacing the stock muffler with a free-flow or high performance muffler, such as a Flowmaster muffler. The result is a free flow exhaust system. However, the performance muffler must have an inlet and an outlet pipe that is the same size (diameter) as your front pipe and your tail pipe. Your front pipe and your tail pipe should also have the same diameter. The rest of the exhaust system is much more complicated as you need consider back pressure, your engine's power band, and your engine's maximum usable RPM.
The amount of back pressure produced by the exhaust system is crucial as too much back pressure will have a negative effect on your engine's top-end performance as it will restrict the flow rate of the exhaust gasses at high RPM. The result would be the engine not being able to expel the spent exhaust gasses fast enough to prevent spent exhaust gasses from contaminating the fresh air/fuel mixture that is drawn into the engine on the next intake stroke. Ultimately, this will result in reduced engine power! Therefore, attaching a little 1-inch pea-shooter to your engine instead of an exhaust system is not such a good idea! But then neither is fitting a 10-inch sewage pipe. If the exhaust pipe is too large, you will get reduced flow velocity of the exhaust gasses. The flow velocity of the exhaust gasses assists with the scavenging of the spent exhaust gasses as well as the amount of air/fuel mixture that can be drawn into the combustion chamber on the next intake stroke. This is because the flow velocity of the exhaust creates a low pressure immediately behind it that sucks more gasses out of the combustion chamber. The trick is thus to get the back pressure just right.
Our exhaust header design section will have more specific information, but generally speaking, a 2¼ inch exhaust pipe is ideal for an exhaust system for a 4-cylinder street car, but a 2½ inch exhaust pipe is a better fit for a 6-cylinder street car. However, a 2000cc 4-cylinder modified race car would do much better with a 3-inch exhaust pipe! The size of the exhaust header primary pipes is also important as it influences both back pressure and flow velocity; while the length of the primary pipes affect the power band of your engine. The size and length of the primary pipes of the exhaust manifold, as well as your exhaust header design depends on your engine's displacement and maximum usable RPM, as well as the power band you want from the engine.
In our next section we take a closer look at ideal primary pipe length and diameter of the exhaust manifold, at the exhaust header design and at anti-reversion techniques.