Aftermarket exhaust parts

Aftermarket exhaust parts are intended to replace the factory fitted exhaust components of a car, motorcycle, or other motor vehicle in order to improve the performance, visual appeal, or sound of the vehicle. Generally, performance enhancements are achieved by reducing the back pressure of the factory exhaust system. Frequently a side effect of a free-flowing exhaust system is either a "different" sound, a higher noise level, or both, which is desirable to some people. Aftermarket exhaust parts can also be a styling upgrade by changes to the visible parts of the exhaust like the exhaust tips.

The intent of an aftermarket component is not necessarily to modify the appearance, sound, or performance of the vehicle in question. While that is almost always the goal if the aftermarket component is replacing a fully working component, it is frequently the case that when the stock or factory component has worn out that an aftermarket component is either more widely available or less expensive than a factory or OEM replacement.

Switching from an exhaust manifold to exhaust headers (also known as extractors in Australia) will optimize the exhaust gas flow speed and in turn increase the high-end power of the engine. This is done by using an individual pipe for each exhaust port that has smoother bends with a larger turn radius, decreasing airflow resistance, as well as a calculated pipe diameter to obtain a good scavenging effect for the specific engine capacity. The pipes then merge into a collector and then flow into a larger pipe (down pipe), just before the catalytic converter. With equal length headers, as each exhaust valve in the head of the engine is opened and exhaust gas is forced out, it passes down one of the header pipes and through to the down pipe, where the exhaust gas velocity causes a slight vacuum in another header pipe. This has an effect of scavenging exhaust gas from the cylinder, which is beneficial for performance. In performance engines, which have cam overlap, the scavenging effect will pull air through from the intake manifold as well, further increasing performance. The length of exhaust headers can be tuned to perform best at specific RPM ranges. Stock exhaust manifolds are usually made of cast iron and can be restrictive. Headers may or may not increase the dB level of the exhaust, depending on the original exhaust manifold it replaces and the engine. However, the pipes do "ring" with the exhaust pulses and increase underhood noise. Headers can be ceramic coated to reduce the heat radiated in the engine compartment, and to increase the temperature of the pipes which increases the velocity of the exhaust gases.

Terminology:

Catalytic converters are necessary to reduce emissions but create back pressure due to the exhaust gases being forced through a catalyst, and therefore decrease high-end engine power. Many modern catalytic converters only produce 1-3 psi of back pressure, though this restriction worsens further with use. Hi-flow catalytic converters can replace the standard units in order to provide lower backpressure. Installing aftermarket catalytic converters is restricted by law in some countries,[1] with bolt-on straight 'test pipes' available to test whether a clogged catalytic converter is causing problems, which can be easily swapped out for on-road use or scheduled emissions testing.[2] Hollowing out a catalytic converter is unlikely to give power gains unless a pipe is placed through the converter to give a clear path for exhaust gasses.

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Wikipedia