Originally Posted by
JaegerWrenching
40 PSI is at seal level and 40 psi at elevation are two different things. If our starting absolute is 11.9PSI at elevation 51.9psi absolute would be 40 psi above it. But at sea level if our starting was 14.7 obviously we'd have 54.7psi absolute. This is why trucks and cars make more or less boost on the same static waste-gate settings when changing elevation. A 10lbs waste-gate spring will open when it reaches that threshold, but as we change elevation atmospheric pressure on the backside of the waste-gate changes as well. So you will make the atmospheric difference in boost pressure with the same waste-gate spring on the same setting. This only truly matters if you're goal is X amount of absolute boost or X amount of Actual HP. You will have to spin the turbo faster at elevation to produce equivalent power. It may start pushing your turbo outside it's efficiency range or compressor island, producing more heat than the added boost is worth.