Density or "performance" altitude is the subject. Well described in any private pilot manual. Watch out for the 3 H's - High, Hot, Humid. Basically 5 or 6 thousand foot increase kills about 20% of the power. 30 degree temp rise above standard kills another 5 or 10%. Humidity robs power too but not as much as the other 2. Thinner air is less air to burn. Exactly the same effect as throttling back. Only cure is larger displacement or turbo/supercharging.

Multiflyer.