[V8] HP loss at altitude?
ekellock at adelphia.net
Wed Apr 21 13:30:45 EDT 2004
I have heard this about turbos in airplanes. It was referred to as "normalizing".
If I could do even just that on my v8 here at 6000 feet, I'd love it. But then if one were to accomplish that, why be so frugal with the power increase? ;-)
Of course it's a whole different story with an earth-bound vehicle...
From: Mike Arman <armanmik at earthlink.net>
Date: 2004/04/21 Wed AM 10:52:34 MDT
To: v8 at audifans.com
CC: Anthony.Hoffman at tinker.af.mil
Subject: [V8] HP loss at altitude?
><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
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 . . .
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