Boost (Hurricane 1 and Merlin II/III)

[Fareastdriver]

I agree, my friend. At 1013.2 at sea level. OR 1013.2 ambient pressure at ANY height.
I can still remember my checks from 1960 and checking that the boost gauge agreed with the QFE was one of them.

Anyway they don't agree, if the boost gauge is at Zero then the MAP should be at 29.9 ins Hg.

Increasing altitude will cause both instruments to reduce. That is the whole reason for a supercharger; to maintain inlet manifold pressure at that altitude.