[DarkHorse]

It depends what additive ur using. I know alot of people on this board love seafoam. u can use that or any isopropyl based additive and it will usually help cut emissions because it will help lower combustion temps just enough over straight fuel to help with nox, and usually lean the a/f enough to lower the co and hc readings which may inturn get u through a sniffer test. these valuse do not change by great amount just usually enough for a vehicle that is '"on the fence". There is alot more theory behind this but this is as simple as i can put it with out going into many different scenario's which affect the readings in different ways. Egr,fuel,timing etc... all play a major role in proper performance. Most of the time u can judge an engines efficiency by the CO2 output, higher the number the better the efficiency and vice versa for the later. These are just examples and i am not saying this is gospel for the subject.