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Perspective, wording, blah, blah, etc, etc...
The efficiency change in the compressor that you see from the altitude change is going to happen on any and all compressors. (supercharger included)
The amount of that change is going to be dependent on how much of an altitude change you're talking about, what the compressor map looks like, where in that map you're usually operating, etc.
Obviously, driving from sea level to Denver is going to mean one hell of a change in compressor efficiency, it will also mean there is less dense air to remove heat from your intercooler and less total density in your intake manifold. Atmospheric pressure in Denver is roughly 12psi so you've lost about 2.5 psi right off the bat. Your boost target (10 psi example) is still 24.5 absolute. Your pressure ratio on the compressor map has gone up two ticks to hit 24.5. You're making more heat. You're rejecting less heat. You're going to lose power. How much depends on your mapping. If you're an OEM you may have spent hundreds of hours mapping an ECU to generate the same power at altitude that people get at sea level, and that's probably the real reason why there's not (much) of a change.
WSIR in California is roughly in the 2300' range, so the changes will be less drastic than the Denver comparison, but they will be there.
We can't say it's all in the turbos efficiency, or all in the atmospheric density because there are things that change at damn near ALL points in the system.
If you're losing power because you didn't have enough headroom in your turbo system, then your turbo system is just too damn small to account for the altitude change. 100% accurate. You'll lose power and no amount of mapping changes will bring it back unless you switch to a fuel that needs less air to burn. So if you're on the ragged edge of what you can get out of an AVO or Greddy kit at sea level, then you're kinda fucked when you decide to drive through colorado and get a dyno sheet at every dyno shop you pass...
But if we're being realistic, we'll admit that we've known for a hundred years that going up in altitude means less power. It's always been that way. Unless you spend the time adjusting your tune to make similar power at altitude the way Mercedes Benz and BMW apparently have, it's going to continue to be that way.
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