Removing a vehicle's catalytic converter for any other reason except its replacement is illegal in the United States. Removal of the catalytic converter in modern vehicles results in little to no improvement in the vehicle's performance.
The Clean Air Act of 1990 made the removal of catalytic converters and other emissions control equipment from cars a federal crime, in addition to making it illegal to sell or install parts intended to bypass or deactivate the catalytic converter. While some people do remove the catalytic converters from their cars in favor of a straight pipe between engine and exhaust, such cars almost always fail required emission testing.
Removing a car's catalytic converter was popular in older vehicles with less-advanced converters, especially if the engine also used a carburetor for mixing fuel and air. Older catalytic converters often restricted the free flow of exhaust, reducing the performance of cars fitted with them. In rare cases, older catalytic converters could overheat and start fires in flammable material underneath a car.
Modern engines are tuned with the assumption that a catalytic converter is present on the vehicle, so no improvement in performance is obtained from removing the converter unless the engine is re-tuned as well. In addition, fuel-injection technology and improved airflow in current converters helps minimize the impact of catalytic converters on performance and eliminate the risk of overheating.