An aftermarket exhaust system replacing everything from the catalytic converter back. The most common exhaust upgrade.
Cat-back exhaust (or simply cat-back) is an aftermarket exhaust system that replaces all components from the catalytic converter to the exhaust tip. The system typically includes new mid-pipes, mufflers, and tips. Cat-back is the most common exhaust upgrade because it changes the sound and slightly improves flow without affecting the emissions system (the catalytic converters are retained).
Cat-back exhausts are popular because they are typically legal in most jurisdictions (the catalytic converters remain), they produce a more aggressive exhaust sound, and they are usually a straightforward installation. The power gain from a cat-back upgrade is typically modest (5-15 horsepower on naturally aspirated cars, less on turbocharged) compared to other modifications, but the sound change is significant.
Major cat-back exhaust manufacturers include Borla, Magnaflow, Akrapovic, ARK, Greddy, HKS, Tomei, and many others. Quality varies widely; cheap cat-backs can be drone-prone (constant boomy noise at cruise) and rust-prone. Premium systems use stainless steel construction, properly tuned mufflers, and often include integrated valves for adjustable noise levels. Akrapovic in particular is known for premium titanium construction at high price points.