Cat-back exhaust systems replace the stock exhaust components from the catalytic converter to the exhaust tips, improving flow and sound without altering emissions. Axle-back exhaust systems replace parts from the rear axle to the exhaust tips, offering a more modest upgrade primarily affecting sound. Turbo-back exhaust systems (for turbocharged engines) replace everything from the turbo's outlet to the exhaust tips, offering the most significant performance improvements. Note: these types of exhaust systems often require additional tuning for both performance and emissions reasons.