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[Kevin Wood]

I always thought that damp air was better due to the water molecules expanding when the mixture ignites? And thus a bigger bang?

I would imagine there are beneficial effects, but, at the end of the day, the simplistic view is that, if you've got less oxygen going in (because the air is "diluted" with water vapour) there is less potential to burn fuel and therefore less power.

Water injection is used in some forced induction engines to keep the burning charge cool. It vapourises during the combustion process and takes heat out of the mixture, but also drops power a little. It does enable you to run more boost without blowing up the engine though.

Most of the references I've seen talk about aircraft piston engines and the need to derate them at higher humidity levels. We're only talking about 4% power loss at 100% RH relative to 0% RH so barely worth worrying about. If it was more of an issue more cars would sense the humidity of the intake charge and adjust fuelling and ignition as they do in response to intake air temperature changes.

Kevin

[zippo]

Driving back from Nottingahm in very misty weather, the thing was flying.  It loved the cool, damp air so much, I gave up trying to stick to speed limits

[/highlight][/highlight]i cant imagine even you having the nuts to say that to a traffic cop when hes just pulled you over  

[Entwood]

Cool/cold air is denser as stated, so more fuel is added to give the same fuel/air mix, more fuel = more power.

The damp side of this is a shade more esoteric .. this is part of the theory .. (comes from aircraft piston engine theory that I did some 38 years ago ......)  The water vapor in the air should cool down and slow the combustion process. The knock sensor, due to the slower burning mixture, will advance the timing, creating more power.

So cold/damp air leads to more fuel and advanced ignition ... and more power.

HTH