The combustion process requires vaporized fuel. Most of the vaporization occurs after the air and fuel droplets have moved past the intake valve, but a large portion must occur before the intake valve open. In a cold engine the air, fuel and all the components contacted by the fuel are at temperatures that do not promote vaporization, so additional fuel must be added so that the percentage that does vaporize will support combustion. The amount of this additional fuel (cold enrichment) depends on the temperature. If the starting climate is very cold (-20f) the a/f ratio may need to be as rich as 4.0:1, as the engine warms up the a/f ratio must be leaned to normal values to balance the increase in vaporised fuel available in the intake.

Idle

The a/f ratio for a stable idle is determined primarily by the camshaft profile. A long duration camshaft with big valve overlap causes the inlet charge to become diluted by exhaust gases, this diluted charge burns very slowly and may require a lot of spark advance. Also the combustion becomes erratic, so a rich a/f ratio is required to reduce the cyclic variation (loping idle) when tuning. The a/f ratio may need to be 11.5:1 or richer with a really serious race camshaft profile. With a short duration camshaft profile the a/f ratio does not need to be as rich for a stable idle and may be as lean as 14.7:1 where emissions need to be a minimum.

Low speed and light throttle

The conditions that affect a/f ratios at idle also affect the fuel calibrations at off-idle operating conditions where engine rpm is low and the load is light (low inlet density and high manifold vacuum). Again the longer valve duration / overlap, the richer the a/f ratio will need to be for surge-free operation. In the immediate off-idle range the a/f ratio may need to be as rich as the idle, somewhere around 12.5:1 to 13.0:1 is common, then gradually becoming leaner with an increase in speed or load. With a very mild camshaft profile, the engine may tolerate a/f ratios in the 14.0 to 15.0:1 range for the same operating conditions.

Minimum speeds and loads

As engine rpm increases the throttle is opened, the effect of valve duration and overlap begin to diminish. There is less inlet charge dilution, so a leaner a/f ratio may be used without any surges or drivability problems. A/f ratios from 14.0 to 15.5:1 are the norm. Typically the best economy a/f ratio is from 15.5 to 16.5 with a "streetable" high performance engine combination, but will require additional spark advance to compensate for the slow burn rates of leaner mixtures.