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.
To maintain catalyst efficiency, the air–fuel ratio must stay close to stoichiometric and not remain rich or lean for too long. All of Milltek's performance exhaust systems are manufactured from T304L stainless-steel