09-15-2009, 03:02 PM
Join Date: Jul 2006
Total Cats: 72
Bah. I just answered this.
At your altitude, your barometric pressure is approximately 20 kPa lower than sea level, or about 3psi lower.
Assume you would make the same amount of "boost" at sea level or at 6000' ft elevation. This is not necessarily a fair assumption as the s/c will be more efficient at lower altitude/higher atmospheric pressure, but we can ignore this truth to uncomplicate things.
The boost is difference between manifold absolute pressure and atmospheric.
However at 6000' elevation, your manifold absolute pressure at peak boost is 20kPa lower because atmospheric pressure is 20kPa lower.
The correction factor is intended to "add in" the missing 20kPa/3psi so that you can compare to what the car would pull at sea level.
Let's go the other way; up the mountain instead of down. At sea level, you pull 290 whp. You go up the mountain to 6000' and pull 260. Everything else being equal (which it never is), the 260 figure could be corrected up 11% to 290. I just pulled those numbers outta my ***, so use them only as abstract and not absolute.
But this whole thing is again very messy. As atmo pressure increases, your blower is able to work more efficiently. So a correction factor based on atmo pressure alone might not tell the whole story. Truth is at sea level, you would see several psi greater manifold absolute pressure than just the difference in atmospheric pressure alone.
Let's go down the mountain again. Assume you make 290 whp uncorrected at 6000'. We correct it up 11% to 320 at sea level. You have plenty of fuel and are well below knock threshold. You go to sea level and blow up your motor because you didn't have fuel and/or charge cooling sufficient for the higher atmospheric pressure, thus the higher absolute manifold pressure. You never make 320 hp to the wheels. Instead, you dine on metal salad while awaiting AAA to tow your busted *** automobile.
For this reason, IMO, best thing to do is ignore correction factors and always look at absolute manifold pressure on dyno runs.