Originally Posted by Gaseous
Why is the ambient pressure affecting the allowable MAP for a given temperature?
Other Lycoming equipped aircraft do not have this limitation.
Well, actually the Hughes/Schweizer 300C and a few others do.
Fact is, that for a given MAP, a piston engine will make more and more power (assuming mixture is not excessively rich) as ambient pressure goes down, up to the point where ambient pressure prevents that given MAP from being made.
To be honest, I've never seen any textbook which explains this, but on the surface, the battle line between the expanding /cooling exhaust gases and the relative density of the outside air, combined with the fact that to achieve a given MAP, the throttle plate will be opened wider as altitude increases, would explain the increased power. It's not that the engine is making more power in the combustion process, it just expends less energy to sustain that process - getting the "stuff" in and out.