I didn't know where else to post this....typically, how far above sea level does performance start to drop? Also is there anything I can do to the tune to compensate?...or should I just leave the tune alone? The area where live is at approx. 3500'. The motor is still strong, but I do notice it "struggling" a little unlike at sea level. Anyone have any input on this subject? Thanks!

 

It starts to drop immediately. It just isn't noticable at first. The mass of air in the cylinders is directly proportional to the barometric pressure. Go up 1,000-ft in altitude, and the "standard" barometric pressure drops from 29.92"Hg at sea level, to 28.86"HG. The loss of air mass = loss of power potential =

(28.86"Hg / 29.92"HG) - 1 = 0.963 - 1 = -.037 = 3.7% power loss

In an enigne making 300 flywheel HP, that's an 11 HP loss.

At 6,000-ft altitude (e.g. - Denver) the standard barometer is 23.98"Hg, which means you lose almost 20% of the HP. Your 300HP sea level engine now makes 240HP.
The "cure" is more compression ratio, a cam that produces a higher dynamic compression ratio, or forced induction. At altitude, an NA engine can normally handle a bit more timing advance... but they also don't sell pump gas with the same octane level at higher elevations.

 

wow...thanks for the info...I never realized it was that much

 

Interesting.. do they sell lower octane because of the lower o2? Im from Minneapolis 850-900ft above sea level.

 

Reducing the mass of air in the cylinder (in this case, due to less atmospheric pressure) has exactly the same impact as reducing the static compression ratio. Less SR requires less octane to prevent detonation (knock). I doubt they reduce the octane levels available at 1,000-ft.