A cat-back system refers to the portion of the exhaust system from the outlet of the catalytic converter to the final vent to open air. This generally includes the pipe from the converter to the muffler, the muffler, and the final length of pipe to open air.

Cat-back exhaust systems are a very popular aftermarket performance enhancement. They generally use larger diameter pipe than the stock system. Good systems will have mandrel-bent turns that allow the exhaust gas to exit with as little back pressure as possible. The mufflers included in these kits are often chambered or strait through designs using stainless steel wool packing to reduce back pressure and soften exhaust pulses. If the system is engineered more for show than functionality, it may be tuned to enhance the lower sounds that are lacking from high-RPM low-displacement engines.

As a general rule of thumb, 4 cylinder engines with low horsepower/torque output require the low backpressure higher flow characteristics of a strait through pack type muffler (Magnaflow type) and higher horsepower/torque V8 engines work better with a chambered muffler (Bassani, Flowmaster type) that provides the right amount of backpressure.

If you have a racecar it is common practice to have an oversized exhaust system with a race muffler that offers little restriction or an exhaust system that uses no muffler at all, as a higher flowing system is essential to allow the engine to produce maximum horsepower.