I posted this on the tech website also, I may be misreading something in FlyingStones's answer but he was assuming because the air is cooler at 12,000ft it is denser, but doesn't take into account increase in induction air temp due to max impeller speed. Therefor I'll have to go with the simple answer - backpressure. Reduction of this improves cylinder fill with fresh charge of air/fuel, 40" pushing against 18" rather the 30". More air/fuel more power.
Last edited by Lumps; 4th June 2012 at 22:11.