* The car itself: A naturally aspirated engine will see a much smaller gain (if any) than a turbocharged or supercharged engine. The existing exhaust system's restrictions also play a large role. A car with a restrictive factory system will see a larger improvement than one already with a relatively free-flowing exhaust.
* The cat-back system itself: A higher-quality system, often made from better materials and with a more carefully designed flow path, will yield better results than a cheap, poorly-designed system. The diameter of the piping, the muffler design, and the overall design of the exhaust significantly impact performance.
* Other modifications: If the car has other performance modifications (intake, tune, etc.), the cat-back might contribute more significantly to overall gains. Conversely, if other parts are restrictive, the cat-back's effect may be limited.
Typical Gains:
Realistically, on a naturally aspirated car, you might see anywhere from 0 to 10 horsepower with a cat-back. It's quite possible to see *no noticeable gain* at all, or even a slight loss if the system is poorly designed. On a forced induction car, gains could be higher, potentially in the 10-20 horsepower range, but again, this is highly variable.
It's crucial to understand that the horsepower gains are often exaggerated by manufacturers and sellers. Dyno testing on the specific car and the specific exhaust system is the only way to accurately measure the increase. Many gains are felt more subjectively as improved throttle response and sound rather than significant raw horsepower increases.
