Designing and Building an Exhaust System

The main purpose of an exhaust system is undoubtedly to route the bunt air/fuel mixture out of the car's engine. Along the way it 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 does much more than that as it also affects engine performance and engine tuning!

An exhaust system generally consists of an exhaust manifold (which is also called an exhaust header), a front pipe, a catalyst converter, a main muffler or silencer, and a tail pipe. Of these items, the muffler is the easiest to deal with — simply replace the stock muffler with a performance muffler, such as a Flowmaster muffler, to create a free flow exhaust system. However, the performance muffler must have an inlet and an outlet that is the same size (diameter) as your front pipe and your tail pipe. Your front pipe and your tail pipe should be the same size. The rest of the exhaust system is complicated by questions of back pressure, your engine's power band, and your engine's maximum usable RPM.

BACK PRESSURE
Back pressure is an important consideration because too much back pressure will adversely affect top-end engine performance as it will restrict the flow rate of the exhaust gasses at high RPM. The car's engine will not be able to expel the burnt air/fuel mixture at the required rate. The burnt air/fuel mixture remaining in the cylinder at the next intake stroke will contaminate the fresh air/fuel mixture and will rob the engine of power. Thus, fitting a 1 inch pea-shooter to your engine in place of the exhaust pipe is not 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 exhaust fumes 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.

BASIC DESIGN
Generally speaking, when designing an exhaust system for a 4-cylinder engine, a 2¼ inch exhaust pipe is ideal but for a 6-cylinder engine, a 2½ inch pipe is ideal, though a 2000cc 4-cylinder race engine could do with a 3 inch exhaust pipe. The size of the exhaust header primary pipes of also influences 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 and your exhaust header design depends on your engine's power band, displacement and maximum usable RPM.

In our next section we take a closer look at primary pipe length and diameter, and at the exhaust header design and anti-reversion.