I don't know how this carries across to small engines, but on large supercharged engines, as you approach max power, you have to richen the mixture beyond the stoichiometric ideal in order to provide sufficient cooling to prevent detonation. There is a practical limit on how much you can richen the mixture without starting to loose power and this becomes a limiting factor on the top end on some engines. Using water injection allows you to keep adding power beyond the "dry" limit as it uses the water to prevent detonation rather than additional fuel. That's my understanding anyway but this was on 2800 cubic inch engines in the 2400-2500 HP range.
-RP
Assent that...
"to provide sufficient cooling to prevent detonation", is what I'am looking for.
Just for protection...
That