tomcat

Air temperature, atmospheric pressure and relative humidity all affect how many oxygen molecules are in a cubic foot of air. An engine will always draw in the same actual CFM of air, but the number of oxygen molecules in that air will vary, and therefore the amount of fuel that can be burned and the amount of power that can be made will also vary.

The way to answer your question is to calculate the density ratio.

D.R. = (T1 + 460)/(T2 + 460)

eg. if you reduce air temperature from 85F to 75F

D.R. = (85 + 460)/(75 + 460) = 545/535 = 1.0187

That's about a 2% increase in density, meaning each cubic foot of air drawn into the engine has almost 2% more oxygen molecules.

IF you add more fuel in proportion, or you were a bit too rich before, you will make about 2% more power.

This type of calculation is used to report engine HP at "standard conditions". For example, an engine might be tested on the dyno with 85F air on a day when the air is humid and the atmospheric pressure is low. The actual dyno reading will be converted to the HP that the engine would have achieved at say 65F, normal atmospheric pressure and no humidity. This will increase the reported dyno HP, but on a hot humid day in the summer, the engine will deliver the lower actual number.

Since altitude has a significant effect on atmospheric pressure (higher altitude = lower pressure = less oxygen molecules in a cubic foot), HP is reduced, and fuel flow must be reduced or the engine will run rich.