Aftermarket Exhaust

Aftermarket Exhaust

by Alex

Aftermarket exhaust parts are intended to replace the factory fitted exhaust components in order to gain a performance increase. Some aftermarket parts are designed to improve the look and sound of the car, rather than it's performance.

Headers

Switching from an exhaust manifold to exhaust headers (also known as extractors in Australia) will decrease the amount of back pressure and in turn increase the high-end power of the engine. Manifolds are usually made of cast iron, and can be restrictive. Headers are designed for better flow and generally use a series of tuned pipes to merge the exhaust from each cylinder into a larger pipe (down-pipe), just before the catalytic converter. Headers do not usually increase the dB range of the exhaust system but do make the sound deeper.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:

* 4-2-1 Headers where 4 pipes merge into 2 which merge into one

* 4-1 Headers where 4 pipes merge directly into one

Catalytic converters

Catalytic converters create back pressure due to the exhaust gases beiing forced through a catalyst , and therefore decrease high end engine power. They are required to neutralize dangerous chemicals and many modern catalytic converters only produce 1-3 psi of back pressure. 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.

Mid-pipe

* Equalization

* Crossovers

Glasspacks

Glasspacks 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

Silencers also use two tubes, but the smaller tube extends towards the engine INSIDE the larger tube. When the sound wave hits the barrier between the smaller tube and the larger one, it bounces back and cancels out (a little bit). The exhaust exits through the smaller pipe. See image on external site... Silencers decrease the volume of the exhaust, but also decrease high end power (but they don't hurt low end torque).

Resonators

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.

Stock mufflers

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, a larger diameter exhaust pipe is not always better. If the the pipe gets too large, the exhaust gases will cool down, and become more dense and therefore require more force to propel them out of the pipe. Too much cooling of the gas can lead to an increase in back pressure. And the risk that the larger pipe might not fit under the car as intended (you could break/damage it in a pothole).

Imagine the expanding exhaust gasses as energy. You can either disperse that energy directly into the atmosphere by having no header or exhaust pipes, or you can contain that energy. A good example is a megaphone. If you talk to someone 50 feet away they might have a hard time hearing you. But if you use a megaphone you can direct more of the energy used to create the sound in one direction. You want to contain the energy of the expanding gas to keep up the exhaust velocity, but not restrict the gas too much that it causes it to build up too much pressure, as increased pressure means more work (Pumping Loss) for your pistions. The momentum or velocity of the gas will help create a vacuum behind it by dropping the pressure in the pipes, this scavenging effect will help draw out exhaust gasses and can even help draw in fresh air fuel depending on the valve ovelap and timing. Sucking out exhaust gas can help reduce the work (Pumping Loss) of the piston as it travels up on the exhaust stoke.

Bottom line for performance

If you want top-end power and have no restriction on noise levels, don't use a muffler at all, just run the header(s). However for the rest of us that drive our cars on the roads it's possible to have a muffler that will flow enough CFM and is less restrictive then the exhaust pipe connected to the muffler. An extreme example is a muffler the size of your garage, it will not restrict the exhaust flow. But it's not that difficult to find a muffler that flows more CFM that your motor puts out will give you a low-no backpressure exhaust. Remember that the displacement of the motor at least for naturally aspirated vehicles is a way for you to determine how much exhaust gas will be created and how much CFM you'll need to flow so you don't create backpressure. If a single muffler cannot offer enough CFM you may want to consider a dual exhaust and dividing the CFM between two mufflers to get the required CFM. An example would be that you need to flow 400 CFM and install two mufflers that flow 200 CFM. Depending on the motor and car you may already have dual exhaust. CFM = Cubic Feet Per Minute.

Advantages

* It can provide an increase in engine power

Disadvantages

* Most modifications increase the noise level.

* Aftermarket systems can have lower clearence and can "bottom out" resulting in damage to the car.

* Modifications of the exhaust system can interfere with pollution laws in some countries.