I'm reading Proficient Motorcycling on the suggestion of McMike, and I came across this curious quote regarding the effects of water in an engine intake during rain:
Don't worry about your engine drowning from too much water being thrown around in the airstream. Water is vaporized during intake and turns to steam during the heat of combustion, and steam can actually help increase horsepower.
I'd never heard this before, so I tried to imagine how it might work. My first reaction was that the water would just be taking up space, reducing the concentration of the fuel air mixture. Then I thought maybe it has to do with cooling the intake charge, like an intercooler? Maybe if the change would be registered by an intake sensor a computer controlled engine could change some parameter like the fuel mixture? I really don't know.