That would depend on the airframe it was fitted to the same as any other engine/ airframe combination.

Standard atmosphere using U.S. units This table give density in slugs per cubic foot because it uses the American system of altitude in feet, pressure in inches of mercury and temperature in degrees Fahrenheit. While people often use pounds per cubic foot as a measure of density in the U.S., pounds are really a measure of force, not mass. Slugs are the correct measure of mass. You can multiply slugs by 32.2 for a rough value in pounds. Altitude Pressure Temp. Density - (ft) (in. Hg) (F.) slugs per cubic foot 0 29.92 59.0 0.002378 1,000 28.86 55.4 0.002309 2,000 27.82 51.9 0.002242 3,000 26.82 48.3 0.002176 4,000 25.84 44.7 0.002112 5,000 24.89 41.2 0.002049 6,000 23.98 37.6 0.001988 7,000 23.09 34.0 0.001928 8,000 22.22 30.5 0.001869 9,000 21.38 26.9 0.001812 10,000 20.57 23.3 0.001756 11,000 19.79 19.8 0.001701 12,000 19.02 16.2 0.001648 13,000 18.29 12.6 0.001596 14,000 17.57 9.1 0.001545 15,000 16.88 5.5 0.001496 16,000 16.21 1.9 0.001448 17,000 15.56 -1.6 0.001401 18,000 14.94 -5.2 0.001355 19,000 14.33 -8.8 0.001310 20,000 13.74 -12.3 0.001267 25,000 11.10 -30.15 30,000 8.89 -47.98 35,000 7.04 -68.72 40,000 5.54 -69.70 45,000 4.35 -69.70 50,000 3.43 -69.70 55,000 2.69 -69.70 60,000 2.12 -69.70 65,000 1.67 -69.70 70,000 1.31 -69.70 75,000 1.03 -69.70 80,000 0.81 -69.70 85,000 0.64 -64.80 90,000 0.50 -56.57 95,000 0.40 -48.34 100,000 0.32 -40.11 Source: Aerodynamics for Naval Aviators Note: These are summary tables. For more detailed tables, consult an aerodynamics or meteorology textbook. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------- So at an altitude with 1/2 the air as at sea level you will only be able to make 1/2 the horsepower that you could at sea level, unless of course you supercharge by one means or another. Usually you are not going to cruise an airplane at much below 1/2 the naturally aspirated rated horsepower. There are exceptions of course.

WK95, what is stated is somewhat true, but a little short of detail. For instance if you a looking to fly a plane that would be sufficient with 100 hp. on a good day,( but a bit lethargic on a hot day at altitude) then it would need approx. 55% (just over 50%) for your cruise hp. 'at sea level' (what ever that may be - perhaps 75% power). If your using 120 hp. that gives you a little leeway. Lets first look at the weight of the airplane (in Lbs.) divided by 12 (formula provided by Mr. Hiscocks) to give the required hp. So for the Light Sports category a 1320 lb. airplane / 12 = 110 hp. (required for on a hot day at altitude). 120 hp. gives a little more power to work with, provided you achieve the rated hp. (advertised) at sea level. So verify your confirmed advertised hp. and the hp. required -and take off 3% for every 1,000 ft. (loss of hp. for both advertised and required hp. - if there is a difference), until you get to that 55% you need for cruise and that should be the max altitude for cruise. However getting there might be the problem as you have to climb to get there and as you get higher you climb rate falls off as altitude increases and power decreases. But you should be able to work that out yourself, with this rough guide. George

I get the sense that you are trying to compare the performance at altitude of a Corvair to the performance at altitude of a brand L or C. if you are wondering if the performance of a Corvair run at 15,000 ASL suffers (as compared to its performance at sea level) more, less, or essentially the same as a L or C lifted from sea level to 15,000 ASL, the answer is: 'essentially the same'.

Naturally aspirated engines will loose the same % of HP. Only so much fuel can be burned when you take O2 away. Turbo and you can make as much power as you can cool and how tough the internal parts are.