[Dee Cab]

First of all, octane is a rating of compression. The higher the octane the greater the fuel/air mixture can be compressed before detonation, or "pinging" occurs. Octane is NOT a measure of power. However, a higher compression engine will produce more power and will require a higher octane fuel. But this extra power comes from the engine and not the fuel. Most all cars today will run fine on 87 octane. If pulling heavy loads or excessive hill climbing you may need 89. Corvettes, Vipers, etc usually have engines in the 11 to 1 compression and require 91. MOST piston aircraft engines require 100 octane fuel.

I believe ANDO140 has some training in my field and I have to go with him. An engine, any engine, will loose power as altitude increases. The exception being a super or turbo charged engine which will then lose power like any other once the critical altitude has been reached and the waste gate is fully closed. Any pilot can tell you this. The max performance out of an engine will occur at sea level. As altitude increases the pressure decreases (Standard is 1"/1000ft). The nitrogen and oxyen percentages remain the same but there are less molecules of each. For the optimum mixture, and power, you must reduce the fuel input to compensate for the "air" reduction. This is why all piston aircraft have mixture controls in the cockpit. You reduce your fuel with altitude to gain optimum power or fuel economy. This is done by leaning mixture with the Exhaust Gas Temperature gauge. The exception are newer aircraft with FADEC (full authority digital engine control) which does it automatically.

My Learjet has N1 DEECS (digital electronic engine control). Our max power is 3500 ft/lbs of thrust for each engine at sea level and 72 degrees F. This power decreases with altitude. We are not making 7000 lbs of trust at 49K feet. We have considerably longer runway requirements at Aspen then we do in Houston. (I'm ignoring the wing efficiency issue at altitude at this time.)

Now, we are also able to get the same Mach number, .78, at 20K feet that we can get at 49K feet. But we burn a whole lot less fuel at 49. 900 lbs and hour at 49 versus 3000 lbs an hour at 20. But, we are having to run a higher N1 (engine fan speed, similiar to RPM) to get the same speed at 49 as we do at 20.

So, at altitude, you will require more throttle to see the same speed, you will burn less fuel, and your power for a given RPM will be reduced. Don't worry about mixture, your computer will handle it for you. Unlike my old 79 Harley.

Rant over. Dee Cab