Some early converter designs created a great deal of restriction to the flow of exhaust, which negatively affected vehicle performance, driveability, and fuel economy.[6] Because they were used with carburetors incapable of precise fuel/air mixture control, they could overheat and set fire to inflammable materials under the car.[7] Removing a modern catalytic converter in new condition will only slightly increase vehicle performance without retuning,[8] but their removal or "gutting" continues.[6][9] The exhaust section where the converter was may be replaced with a welded-in section of straight pipe, or a flanged section of "test pipe" legal for off-road use that can then be replaced with a similarly fitted converter-choked section for legal on-road use, or emissions testing.[8] In the U.S. and many other jurisdictions, it is illegal to remove or disable a catalytic converter for any reason other than its immediate replacement; vehicles without functioning catalytic converters generally fail emission inspections. The aftermarket supplies high-flow converters for vehicles with upgraded engines, or whose owners prefer an exhaust system with larger-than-stock capacity.[
