In warm temps....

[HummerMann]

Being a Northerner I don't notice this much, BUT when I was down in Langtry, TX last month in the middle of nowhere in 95F temperatures, I DEFINITELY noticed this phenomenon. It reminded me of being in the mountains - I agree that it all has to do with air density.
Hot air is less dense, so you produce less horsepower. When I used to drag race we'd place ice on the intake manifold and in a screen by the air cleaner to make the air more dense. If you use the standard Morton/McFaren horsepower/air density chart - 95 degrees gives you 12% less horsepower than on a standard day which is used by Detroit to measure engine horsepower. A standard day is sea level, 59F and a barometer of 29.92. Yep I have a graduate degree in mechanical engineering. SO 12% is a loss of about 36 horsepower.
Then you can complicate it even more using the Military Standard Hot Day which is used in certifying Humvee/H1 engines.
http://www.eng-tips.com/viewthread.c...=231931&page=2