[Mike V. Florida]

Originally Posted by sphanna

Moist humid air is less dense than dry air. So the density of the atmosphere is less and this robs power from the engine. If the air has water vapor in it, it is already vaporized and will reduce the effective air charge density. So in order to see the effect of increase power, you must inject liquid water into the cylinder. This water charge then instantly turns to steam and increases the compression. It also will remove carbon as this steam cleans the cylinders. That is what I have been taught anyway and confirmed by operating aircraft engines for 50 years.

Mustangs in WWII used water injection for a power boost.