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I'll copy and paste a segment of a post I wrote before about turbo engines and altitude: Turbocharged/intercooled engines do not lose as much HP due to the altitude as naturally aspirated engines do. All will lose some HP, but on average a NA car will lose about 3% of engine output per 1000 feet, so at 6000 feet you've lost about 18% of your output on a NA engine. But on a turbo engine the turbos' wastegates will try to maintain a pre-set amount of pressure *above* the atmospheric pressure. For example, if you're running 10 psi boost at sea level, you're running 10 psi on top of 14.7 psi for a total of 24.7 psi, and a naturally aspirated car will be running 14.7 psi. But if you go where the ambient pressure is only 12.3 psi, such as is the case in Denver, the air is only 83% as dense as it is at sea level. The naturally aspirated car is breathing the atmospheric air at 12.3 psi, which is 83% of the pressure it had at sea level. The turbo car running 10 lbs of boost on the other hand will have 12.3 psi+10psi=22.3 psi, which is 90% of the pressure it had at sea level. So as you can see the turbo engine is not losing power with altitude at the same rate as a NA engine is.
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