I'll pipe in with my chem ideas: Water does two things: increases the thermal mass which lowers overall temp which would prevent pre-ignition. (someone noted this before) 2nd (this is where Mike is right): Since H2O (and EGR gases) don't burn, they interfere with collisions between O2 and gasoline molecules which inhibits combustion. (good ole Kinetic Theory) Pat 92 Turbo