Part of a post by Jon [in CT]
in another thread:
For most fuels used in spark ignition, max torque is achieved with an air/fuel mixture that is about 15% richer than stoichiometric. For gasoline with a stoichiometric ratio of 14.7:1, that works out to 12.5:1. Running an even richer gasoline mixture is "safe" because the evaporation of the extra fuel cools the intake charge to stave off knock; but some torque is sacrificed in the process.