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Dyno correction factors, which some guys are quoting, do not take whether the motor is NA or supercharged or turbo into account. This is a flaw which should not be overlooked. Another thing, the dyno correction factors were originally meant to account for different days in the Detroit area, like plus or minus 3% or so, and therefore they are not really in their intended range any longer when they are spitting out 20% corrections.
Here is another clue about NA vs supercharged at higher elevation, dragstrip ET correction factors for 5800 foot elevation (like the Bandimere track near Denver):
modded NA elapsed time correction factor is .9405
modded SC elapsed time correction factor is .9702
This smaller handicap for the supercharged motor shows us that the supercharged motor is not expected to lose as much power from the thinner air as a normally-aspirated motor is. Lose something, yes, just not as much.
If you REALLY wanted to know how much hp you've lost, you'd need to modify synolimit's dyno idea, by using a dyno on a trailer up there, printing both corrected and uncorrected sheets, then drive both that dyno and your car to low altitude and repeat the test. Same dyno, same car, high vs low altitude in other words. Look to see how much the actual hp at the 2 altitudes changes, and also what happens with the corrected numbers. Based on supercharged dyno sheets from high altitude that I've seen posted, I think you'd find that you did not really lose quite as much hp up there (actual vs actual) as the dyno CF seems to indicate.
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