KEY POINTS:
The main purpose of an exhaust system is undoubtedly to get the burned air/fuel mixture out of the car's engine as quickly and efficiently as possible. The system may also be used to drive a turbocharger and, in modern cars, will incorporate a catalyst converter to reduce carbon dioxide emissions.
But on a high-performance car, the exhaust system does more than that - it is an integral part of the performance package.
A standard exhaust system consists of a manifold (also known as a header), a front pipe, a catalyst converter, a main muffler and a tail pipe.
To increase performance, the muffler is the easiest to deal with. Replace the stock muffler with a performance muffler to create a free-flow exhaust system.
Keep in mind that there are noise emission standards that have to be adhered to by law, so make sure whatever muffler you fit complies with noise regulations.
The performance muffler must have an inlet and an outlet that is the same diameter as your front pipe and your tail pipe - and they should be the same size.
Back pressure is an important consideration because too much will adversely affect top-end engine performance by restricting the flow rate of the exhaust gases at high revs. The car's engine will not be able to expel the burned air/fuel mixture at the required rate for optimum performance. The burned air/fuel mixture remaining in the cylinder at the next intake stroke will contaminate the fresh air/fuel mixture, robbing the engine of power. So, fitting a small diameter system to the car will reduce power.
Fitting an oversize drain pipe will also affect engine performance. If the exhaust pipe is too large, you will get reduced flow velocity of the exhaust gases. The flow velocity assists with scavenging 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 gases out of the combustion chamber.
When designing an exhaust system for a four-cylinder engine, a 55mm exhaust pipe is ideal but, for a six-cylinder engine, a 65mm pipe is better, though a 2000cc four-cylinder race engine could do with a 75mm exhaust pipe. The size of the exhaust header primary pipes also influences back pressure and flow velocity, while the length of the primary pipes affects 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 revs.
When designing the exhaust header, remember that a 1600cc four-cylinder or 2400cc six-cylinder normally aspirated engine with maximum usable revs of 5500 should have a header with a primary pipe diameter of about 40mm and a primary pipe length of about 910mm. While a 2000cc four-cylinder aspirated race engine should have a header with a primary pipe diameter of about 45mm and a primary pipe length of about 810mm that feeds into a 65mm collector.
The primary pipe lengths should be within 50mm of each other and all four primary pipes on a four-cylinder should join together in a single collector before feeding into the exhaust pipe. A six-cylinder engine should have two collectors with cylinders one, two and three joining into one collector and cylinders four, five and six joining into the other collector. A Y-pipe could be used to join the two collectors before feeding into the exhaust pipe.
Each primary pipe should match the exhaust port diameter or be slightly larger. A primary pipe that is slightly larger is better as it inhibits reversion: the flow of exhaust gases back into the combustion chamber when the downward movement of the piston creates a vacuum in the cylinder. The exhaust valve is still open when the intake stroke begins.
Preventing reversion will reduce the contamination of the air/fuel mixture by exhaust fumes. An anti-reversion (AR) header that is designed to inhibit reversion would be your best choice.
Ultimately, determining the correct primary pipe diameter and primary length will require that you have your car dyno-tuned.
* Additional information: www.customcar.us
