This is achieved by burning off the excess hydrocarbons which result from the extra-rich mixture required for a cold start. Slightly lean of stoichiometric, the exhaust gases from the engine contain excess oxygen, the production of NOx by the engine increases, and the efficiency of the catalyst at reducing NOx falls off rapidly. However, the conversion of HC and CO is very efficient due to the available oxygen, oxidizing to H2O and CO2. However, the efficiency of the catalyst at reducing NOx is good, and the production of NOx by the engine decreases.
By running pipes from the catalytic converter to the muffler and from the muffler to the exhaust tip, cat back exhaust systems improve exhaust gas flow, providing small performance boosts. However, your performance gains may be limited if your catalytic converter restricts gas flow, as a catback exhaust system is installed ‘downstream’ of the converter. 240sx header-back systems are increasingly rare, namely because of modern emissions systems. Since removing your vehicle’s factory emissions components is highly illegal, the previous two options are more common. However, header-back systems are still available for some older, pre-emissions applications. In these cases, header-back systems offer more performance and volume than cat-back or axle-back systems