[brianhn1]

Originally Posted by hotrod182

That is correct, heat of evaporation is what brings benefits with H20. What you are saying is true if air is saturated with high RH at 95F ambient, and discussed engine operating temps remain at those temps. But what happens to that saturation level when temps are raised considerably? (such as in your intake tract at temps approaching 170F after the intercooler!). The air at those temps is no longer saturated with water vapor, and there would be benefit from the evaporation effect. Warmer temps will support more evaporation.

You're right. But it is more obvious than that. As you increase the pressure of water vapor very quickly as in does in the engine, it turns into superheated liquid water. Refer to the phase diagram of water. As the ignition occurs, the liquid water turns back into vapor thereby decreasing the overall combustion temperature, exhaust gas temperature, reduce tendency to knock, etc. However, humidity/water generally reduces power because it does decreases the temperature away from which optimal combustion (most efficient) occurs and does not have a cooling effect to reduce the temperature of air coming into the engine (unable to increase density of air).