Aftermarket Exhaust Parts

in Exhaust
Common aftermarket exhaust parts


Headers on a 440 Max Wedge

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, 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 together 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. 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.


4-2-1 Headers where 4 pipes merge into 2 which merge into 1 (tri y's)

4-1 Headers where 4 pipes merge directly into 1

Shorty Headers where each exhaust pipe travels the shortest distance from the exhaust port to the collector cup

Equal Length Headers where each exhaust pipe is the same length from the exhaust port to the collector cup

Catalytic converters

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, 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. 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.


The section of tubing between the catalytic converter(s) and the rear muffler on cars that have two parallel exhaust pipes. Performance mid-pipes often have a perpendicular connecting pipe or the pipes temporarily merge. This is to equalize the pressure in both exhaust pipes and keep the engine back-pressure as low as possible since back-pressure is detrimental to high end power.

H-Style Pipe where there is a perpendicular connecting pipe, resembling the letter H

X-Style Pipe or X-Pipe where the exhaust pipes temporarily merge, resembling the letter X


Glasspacks (commonly called cannons or hotdogs) employ two tubes, an inner perforated one, and an outer solid one. Between these tubes, there is sound insulation. These mufflers decrease back pressure and don't decrease the decibel level much. Glasspacks can be used to give the engine a deeper "throaty" sound.


Silencers are a series of concentric pipes around the exhaust pipe. These concentric pipes allow sound to travel into them and cause the sound waves to bounce off the closed, flat, ends of the pipe. This reverses the direction of the sound waves making them collide with oncoming sound waves and cancel each other out. This design is usually very free-flowing but does not offer as much sound reduction as a muffler.


Resonators, also known as Helmholtz resonators are sections of exhaust pipe that expand to a larger diameter and allow the sound waves to reflect off the walls and cancel out. Resonators are mostly used to reduce raspiness and popping. Resonators are similar to an Expansion chamber, only for 4 stroke engines. They do not produce much back pressure.

Many North-American cars (and possibly cars in other parts of the world) made since the early to mid 1990's can have up to 3 distinct (but similar looking) exhaust components downstream from the catalytic converter(s). Each of these components may be called resonators or mufflers. Usually only the last component is the actual muffler, and the other components are the resonators.

Stock mufflers

Stock exhaust muffler of a car

Stock mufflers typically bounce sound waves off of the back, front, and sides to cancel out sound. They also increase back pressure, but are very effective at reducing the sound levels.

Exhaust piping

The piping that connects all of the individual components of the exhaust system is called the exhaust pipe. Contrary to popular belief, the largest diameter exhaust pipe is not always better. If the pipe is too large, the scavenging effect will suffer at low rpm, resulting in loss of torque and drivability . Running a pipe that is too large may also decrease a car's ground clearance, increasing the risk of the exhaust being damaged when the car moves over an uneven surface.


While it was common for the stock or factory automotive exhaust system to be made from ordinary (and sometimes "aluminized") steel, some manufactures (such as Chrysler) have been using stainless steel for the entire exhaust system (pipes, mufflers, resonators) since the mid to late 1990s. The use of stainless steel has made it possible for the exhaust system to last for practically the entire life of an automobile. This is a considerable improvement given that in some locations (such as the North-American rust-belt) it was quite common for ordinary steel exhaust components to fail after 3 years of service.


Free-flowing exhaust systems may maximize peak horsepower; this is typically accomplished by making the path as straight as possible, using a consistent diameter throughout the system and eliminating obstacles such as catalytic converters and pre-catalytic converters or by replacing the muffler with a less restrictive (and usually louder) one. In a normally-aspirated engine, this may result in the use of oversized piping which may negatively affect low-end torque (forced induction engines are not as susceptible to this side effect). Furthermore, despite the potential (albeit generally slight) performance increases possible by eliminating catalytic converters and other emissions control hardware, doing so will often render the vehicle illegal for street use in many jurisdictions. Other aftermarket modifications that bypass the catalytic converters and/or mufflers, such as electric or cable activated exhaust cutoutshich open and reroute the exhaust flow to bypass restrictions in the systeman similarly increase performance, though would likely be illegal in areas where tampering with emissions devices is prohibited, particularly when placed in front of the catalytic converters.


An increase in peak engine horsepower can be achieved

Gas mileage can be increased in a well designed aftermarket system

Aftermarket parts can be cost effective replacements for stock parts when OEM parts are not readily available (or are more expensive)

Aftermarket parts are often available in longer lasting stainless steel, whereas many OEM exhaust components are made from mild steel and have a limited lifespan


Removing or bypassing a catalytic converter may interfere with emission laws in some jurisdictions.

An increase in noticeable noxious fumes may result (particularly when the catalytic converter is removed or bypassed, and/or when exhaust outlets are rerouted to the side of the car as with side-exit exhausts)

Engine sound may be more noticeable to both driver and pedestrians, resulting in increased noise pollution

An incorrectly designed exhaust can cause loss of low-RPM torque, and a decrease in fuel economy

Some modifications can void factory warranties

See also

Expansion chamber

Exhaust system

British Leyland - litigation involving right to supply aftermarket exhaust systems

External links

The inside of a stock muffler.

Article discussing the validity of manufacturer's warranties with installation of an aftermarket exhaust in Europe.




Categories: Engine technology | Auto parts | Vehicle modification | Engine components | Exhaust systems
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Aftermarket Exhaust Parts

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This article was published on 2010/10/09