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What is a muffler?

Also known as a "backbox" or "secondary muffler", a muffler is a part of an exhaust system, usually near the rear of the car, that is designed to decrease the volume of the exhaust.  There are many different technologies available in muffler design, some for different purposes.  The muffler is a key to the performance of the car because in general terms, the more the muffler decreases the sound, the more back pressure it creates on exhaust flow.  And increased back pressure means decreased performance.  This is only generally speaking, of course, as there are very advanced designs available that increase performance and decrease noise at the same time over the stock muffler design.  And, of course, there are very poor designs that increase noise and hinder performance.

Muffler Designs:

• A basic muffler consists of a series of metal plates (or tubes) with multiple holes.  The holes put pressure against the exhaust flow and decreases the noise.  Chambers exist between the metal plates to ensure the flow of the exhaust.
• A high-performance muffler contains a single, large pipe with holes along the side.  This reduces the sound a little, but it barely puts pressure on the exhaust and allows it to flow freely.

Where does the noise come from?

Most people incorrectly assume that engine noise comes from the small gas-ignited explosions within the cylinders of the engine.  While the combustion does create some noise, most of the sound actually comes from the opening and closing of the valves.  Each cylinder in an engine has valves--normally two or four--that allow air in and exhaust out.  As engine speed increases, the cylinders fire very rapidly and the valves open and close on each fire.  The valves open and close so fast, they combine with the exhaust to create sound waves that travel through the exhaust system.