Gofstbuick posted this link in another thread: http://franzh.home.texas.net/lean.html It gives a very nice chart showing what happens to the important gases (HC, CO, CO2, O2, NOx) as a function of air-fuel ratio. However, what about ignition timing? How does that affect things? My guess would be that more advance would give more complete combustion and a hotter chamber, so my guess is lower HC, CO, O2, and higher NOx and CO2. Anyone actually know the trends? In my experiments making an emissions chip for my car, it is amazing what a little load will do. Idling in park or neutral the best I could get the HC (without a cat) was about 400 ppm, and the CO I forget but maybe 0.2%. Drop the car in gear or rev it up to 2000 rpm and the HC fell to less than 100 ppm and the CO to less than 0.1%. Here in MD I face the IM240 dyno test so I really don't care about the no load idle, but friends in DE have a simple idle in park, then 2000 rpm in park, test and I want to know how to help them do better so I am hoping that the answer is in the timing map. Anyone?