Borla - After-Market Exhaust Parts

Manifolds

Switching from an exhaust manifold to exhaust headers will decrease the amount of back pressure and in turn increase the high-end power of the engine. This is because manifolds take the exhaust gas and shove it into the exhaust pipe as quickly and cheaply as possible. Manifolds are usually made of cast iron, and the exhaust needs to make an almost 90 degree turn after exiting the port then travel down a short pipe to the catalytic converter. Headers are more expensive than a manifold since they generally use a series of specially bent and sized pipes to gently merge the exhaust from each cylinder into a larger pipe, just before the catalytic converter.

Catalytic converters

Catalytic converters create back pressure since they need to force the exhaust through a catalyst in a honeycomb shape, and therefore decrease high end engine power. However, they are required by law, and many modern catalytic converters only produce 1-3 psi of back pressure. This amount of pressure will not decrease the high-end power of an engine much.


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 are basically sections of exhaust pipe that expand to a larger diameter and allow the sound waves to bounce off the walls and cancel out. 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).