Stoichiometric Ratio

The exact air-fuel mix for complete combustion with no leftover fuel or oxygen.

The stoichiometric ratio is the air-fuel mass ratio that burns all the fuel with all the oxygen, leaving no excess of either. For pump gasoline it is about 14.7:1; E85 is roughly 9.8:1, methanol near 6.4:1. It matters because it is the reference point for everything else: closed-loop fueling targets it for emissions and economy, fuel trims correct toward it, and lambda is defined against it. The catalytic converter also needs the engine cycling near stoich to work efficiently. In HP Tuners VCM Editor, the stoich value lives in the fuel system parameters (often labeled as the stoichiometric AFR or fuel ratio for the active fuel) and must be set correctly when you change fuels, or your AFR-to-lambda math and trims will be wrong. Under power you intentionally leave stoich for a richer mix; stoich is mainly a cruise and idle target.