Emissions regulations vary considerably from jurisdiction to jurisdiction. Most automobile spark-ignition engines in North America have been fitted with converters unlimited since 1975,[4][1][2][3] and the technology used in non-automotive applications is generally based on automotive technology. In many jurisdictions, it is illegal to remove or disable a catalytic converter for any reason other than its direct and immediate replacement. Nevertheless, some vehicle owners remove or “gut” the converters unlimited on their vehicle.[26][27] In such cases, the converters unlimited may be replaced by a welded-in section of ordinary pipe or a flanged “test pipe”, ostensibly meant to check if the converter is clogged by comparing how the engine runs with and without the converter. This facilitates temporary reinstallation of the converter in order to pass an emission test.[28]
In the United States, it is a violation of Section 203(a)(3)(A) of the 1990 amended Clean Air Act for a vehicle repair shop to remove a converter from a vehicle, or cause a converter to be removed from a vehicle, except in order to replace it with another converter,[29] and Section 203(a)(3)(B) makes it illegal for any person to sell or to install any part that would bypass, defeat, or render inoperative any emission control system, device, or design element. Vehicles without functioning converters unlimited generally fail emission inspections. The automotive aftermarket supplies high-flow converters for vehicles with upgraded engines, or whose owners prefer an exhaust system with larger-than-stock capacity.[30]
Effect on exhaust flow
Faulty converters unlimited as well as undamaged early types of converters can restrict the flow of exhaust, which negatively affects vehicle performance and fuel economy.[26] Modern converters unlimited do not significantly restrict exhaust flow. A 2006 test on a 1999 Honda Civic, for example, showed that removing the stock converters unlimited netted only a 3% increase in maximum horsepower; a new metallic core converter only cost the car 1% horsepower, compared to no converter.
Dangers
Carburetors on pre-1981 vehicles without feedback fuel-air mixture control could easily provide too much fuel to the engine, which could cause the converters unlimited to overheat and potentially ignite flammable materials under the car.[31]