And our trucks are designed to give similar performance from sea level to around 10000 feet. So I would say the critical altitude for our trucks is 10000 feet.

 

There's more to this than just simple boost and air density to figure out. What we dont know first hand is the programming in the engine ecm. My truck does the same thing, what I dont know is whether the boost and overall power is lowered because of altitude or engine temp or maybe both. I cant isolate the two because both are happening at the same time when I tow up into the mountains. Boost is controlled by the engine ecm on the later trucks, maybe as a turbo saving function because turbo's have a habit of blowing up when the spin too fast, which can happen at higher altitudes. I know Detroit and probably Cat and cummins as well spec a different turbo when used on engines running above a set altitude.