Is it boosting combustion or just boosting compression like water
injection did in some WWII aircraft engines. It also has a cooling
effect as it boosts compression:
http://www.aviation-history.com/curtiss/p40.html

"Following experiments in cooling-drag reduction in 1943 with a
P-40K-10-CU which had its "beard" radiator removed to wing
installations, and in rear vision improvements by installing a "bubble"
canopy on a standard P-40L, a general " clean-up " programmed was
initiated, resulting in the sole XP-40Q. With a 1,425 hp  Allison
V-1710-121 engine, the XP-40Q was modified from the first P-40K-I to
have a "bubble" canopy and cut-down rear fuselage, wing radiators and,
eventually, clipped wing tips. A four-blade propeller was fitted, and
water injection installed. With a weight of only 9,000 lb, the XP40Q
attained a maximum speed of 422 mph. This was still less than the speed
attained by contemporary production  Mustangs  and  Thunderbolts ,
however, and the XP-40Q did not achieve production."

Here is what wikipedia has to say about it:
http://en.wikipedia.org/wiki/Water_injection_(engines)

And another article: http://www.rallycars.com/Cars/WaterInjection.html

Many many more. Just do a search for water injection engines.

Many years ago there was a device that promised more power and mileage
if it was installed. It consisted of a spacer block that fitted between
the carburetor and the intake manifold. Inside was two little propellers
that were supposed to atomize the fuel more efficiently. My grandfather
bought one out of curiosity and installed it on his '46 Ford sedan. It
made no difference whatsoever although there were many testimonies by so
called happy users that said it dramatically boosted the power and
economy of their cars.

 Snake oil !!  Save your money for gasoline.