Originally Posted by Benton

What, pray tell, did you feed Google? I tried "engine power altitude" and got back 300k hits, with this calculator as the first -- http://wahiduddin.net/calc/calc_hp_dp.htm which also corrects for temperature and humidity. 3% reduction in power per 1000 feet of altitude is a pretty standard approximation.

I think carburetted engines of old lost power due to both the thinning air as well as the fuel/air mixture going wrong, whereas modern automotive FI does a better job of compensating the mixture with altitude. They still lose power due to altitude.

Benton 3may08