Arrvaxx's quest for the 10s.

N8GTOL

That peak hp calculator is a gross over simplification of reality as it is based only on standard atmosphere with elevation. Also, that calculator does not really apply to your car since you have turbos which will just work harder (up to a point) to make up the pressure loss from increased altitude and temperature (aka density altitude). It may take your turbos a little longer to get peak boost and peak torque and therefore it will feel less responsive in the low and mid range rpms, but generally speaking your peak HP numbers will be much less affected by changes in altitude when compared to us lesser NA folks (and even supercharged vehicles to lesser degree). Superchargers are mechanically driven so they will only increase manifold pressure by a fixed amount relative to the outside air pressure. Your turbos will keep working until they hit the wastegate spring pressure (regardless of the outside air pressure).

As I alluded to, density altitude is the more relevant factor in assessing power loss of an engine as it accounts for changes in air density from both altitude and temperature. Density altitude is not linear but its kinda close. At 1000ft elevation and a temp of 70F, the density altitude is more like 2000ft. Humidity will also rob power as the air mass is replaced with water vapor at the same pressure.