[V8] HP loss at altitude?

[Mike Arman]

>
><Anthony.Hoffman at tinker.af.mil>
>Subject: [V8] HP loss at altitude?
>
>Does anyone know the calculation factor used in figuring hp loss for each
>1000ft elevation in a Naturally Aspirated engine? It seems I'v heard 3.7%,
>but I'm not sure this is accurate. Also, if anyone knows what it is for a
>Turbo, that would be helpful also.



Well I'm looking at a performance chart for a Cessna 150, which at full 
throttle (2,750 rpm) has a whole 100HP at sea level (not exactly nosebleed 
material . . . )

At 2,500 feet, it has 94 hp

At 5,000 feet, it has 87 hp

At 7,500 feet, it has 76 hp, but will only turn 2,700 rpm

At 10,000 feet, it has 71 hp, same 2,700 rpm

At 12,500 feet it has 63 hp, but will only turn 2,650 rpm.

Therefore, the number appears to be about 3% per 1,000 feet on a NA engine.

Turbo engines in *airplanes* are used to restore sea level performance at 
altitude - if you had a turbo 150, you could get 100 hp at 12,500 feet, 
instead of only 63. Generally, turbo pressures are limited to maintain or 
reproduce the 1.0 bar pressure at sea level, and are not used to boost 
beyond that.

The altitude where the turbo can no longer maintain sea level pressure is 
known as the "critical altitude", and above that, the pressure diminishes 
simply because there is less air for the turbo to work with.

With cars, we often run 1.5, 1.8 or 2 bars boost at sea level, so you'll 
probably lose the same 3% per 1,000 feet, but since you're starting with 
more, you won't notice the loss until you get WAY up into the mountains.

By the way, the altitude record for a piston engine is held by a NASA Rotax 
powered drone with three stages of turbocharging - a little over 83,000 
feet, 16 miles! There is just about no atmospheric pressure to speak of at 
that altitude. About one inch or so of mercury - practically nothing. Time 
of useful consciousness at this air pressure (or lack of pressure) is about 
five seconds, so the drivers of turbocharged SUVs will feel right at home . . .


Best Regards,

Mike Arman