I have worked this out to absolute 100% accuracy for my SC thread:

http://www.m3post.com/forums/showthr...=374270&page=3
Sea level pressure 1019hpa THIS IS CONSTANT

You work out your air pressure at your altitude. Check with a weather staion if you dont know how

At 4500 ft pressure its approx 865hpa (this can be worked out for any altitude)

The percentage of air pressure at 4500ft compared to sea level is *

derived by 865/1019 = 0,848

Therefore at 4500, you lose exactly 15,5%

Take the factory rated power of the M3 at sea level 320 whp * 0,848 = 271,36 whp

Crank HP 410 HP x 0,848 = 347HP