My guess is that you don't need to change it from standard. The turbo will push just as much air into the engine at high altitude as you get at low altitude, so the lack of air pressure becomes irrelevant. Normally, gasoline becomes more volatile at higher altitude as well, but again I think this is due to the lack of atmospheric pressure which is mitigated by the turbo adding pressure. I'm pretty sure that 10 psi of boost at high altitude is exactly the same at 10psi of boost at low altitude with only the larger change from local atmospheric pressure to that level of boost being the difference (which has no real impact on the engine itself). The only thing I can think of is that a turbo at altitude may run out of boost sooner than the same turbo at sea level since there simply is less air at high altitude to push into the engine at the same turbo RPMs.Hello - anyone out there have any suggestions for setting the initial timing on a 1990 900 turbo operating at 7800 feet (above sea level) and above? I'm trying to get as much boost as possible with a swedish dynamics apc and 3.0 bar fuel reg. Thanks!