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Originally Posted by knucklesplitter
True that as altitude goes up, your turbo efficiency goes down, and the maximum potential power goes down. I agree with this totally and have argued till blue in the face with the "turbos make the same power at altitude" crowd.
What I'm talking about is something else I think. Take a turbo that has excess capacity - say the VF39 in stock configuration. The boost is not limited by the turbo at say 4000 rpm, it is limited by the wastegate opening and bleeding off the exhaust pressure as modulated by the ECU-controlled boost solenoid. If programmed to, the turbo is loafing along making 14 or 15psi at sea level and wastegating like crazy. This configuration will make 1-2psi less boost (guage) at 5000'. This doesn't change as the ECU "learns" either. That's not a turbo efficiency issue, it's a wastegate regulation issue. It's like the control system for the boost needs different parameters at elevation. This is a mystery to me and I wish somebody could explain it.
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Basically, Subaru got to 300/300 on the EJ257 and said "okay, that's plenty." And since boost is not part of the learning parameters of the ECU, it doesn't change. Which is by design- can you imagine the number of people going to the service department freaking out because their STi makes 1 or 2 pounds more or less boost today than it did yesterday?