High-performance mufflers are designed to allow as much of the car's exhaust to escape freely as possible in order to prevent backpressure on the exhaust system. These units are designed to balance noise reduction with engine performance, allowing the engine to draw in more fresh air and increase power and acceleration.
When an engine fires, it creates loud pressure waves that flow out of the car through the exhaust system. If left unimpeded, these waves produce a loud and distinctive noise that can disturb others. A muffler uses a series of baffles to reflect and break up these waves in order to reduce noise pollution, but the more baffles there are to halt these waves, the more pressure stays built up inside the exhaust system. This prevents fresh air from flowing freely into the engine's intakes and causes poor performance.
Since the exhaust system of an engine has an effect both on its noise profile and the amount of pollutants it releases, modifying a car's exhaust may be strictly regulated in some cities. For instance, a required catalytic converter adds backpressure on the system because it restricts airflow as it reacts with exhaust gases to reduce pollutants. Simply adding a high-performance muffler to a stock engine may not offer any significant performance benefits, but a carefully designed custom exhaust system can benefit from increased airflow.