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"Air/Fuel Ratio has a majoir impact on engine octane number requirement(ONR),increasing octane requirements by +12 per one increase in ratio(say from 9:1 to 10:1).Ideally, air/fuel ratio should vary not only according to loading but also according to the amount of air preasent in a particular cylinder at a particular time(cylinder VE).Richer air/fuel ratios combat knock by the intercooling effect of the cooling heat of vaporization of liquid fuels and a set of related factors.The volatility of fuels affects not only octane number requirement but drivability in general.The chemically ideal air/fuel mixture (by weight), at whitch all air and gasoline are consumed in combustion occors with 14.68 parts air and 1 part fuel,which is usually rounded to 14.7.This ratio is referred to as stoichiometric.Stoichiometric mixtures vary according to fuel,from a low of nitromethane at 1.7:1,to methanol's 6.45:1,ethanol's 9:1,up to gasoline at 14.6:1 and beyond to natural gas and propane, which are in the range of 15.5- 16.5:1.Mixtures,by the way,are often expressed as a percentage of stoichiometric, usually referred to as lambda.Thefore, the stoichiometric AFR is 1.0 lambda;the best -power ratio for gasoline (12.2:1)is 0.83 lambda.At high loading and wide-open throttle,richer mixtures give better power by making sure that all air molecules in the combustion chamber have fuel present to burn.At wide-open throttle, where the objective is maximum power,all four-cycle gasoline engings require mixtures that fall between lean and rich best torque,in the 11.5 to 13.3 gasoline range.Since this best torque mixture spread narrowes at higher speeds, a good goal for naturally aspirated engines is 12.0 to 12.5, perhaps richer if fuel is being used for combustion cooling in a turbo/supercharged engine."

I GOT THIS INFO FROM THE EMS HANDBOOK BY JEFF HARTMAN

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