With gasoline engines, maximum HP is made from about 14:1 to 12:1. At 12:1 it is using far more fuel, but it will reduce combustion temps. See Garrett turbochargers.

At 14:1 you will still get the same HP, but use less fuel. As long as the engine survives (doesn't overheat components) 14:1 is preferable to 12:1. It results in better fuel economy at MAX power and less wear to the engine from unburned fuel particle. With EFI it is easily controlled. With carburetors it requires careful jetting -- which is why carbs are often jetted far richer just to be on the safe side to prevent lean-out or detonation.

The "stoich" ratio is 14.7:1 and will result in the least average emissions, but will make less power than a slightly rich ratio (e.g. 14:1).