Boost (Hurricane 1 and Merlin II/III)

[megan]

So it seems correct to think that a boost gauge will read zero when at rest, whereas manifold pressure, which is an absolute pressure, will be indicated as a positive value (which will vary according to local conditions, altitude, etc)

The boost gauge is an absolute reading also, it will read zero at sea level ambient pressure, where as a MAP gauge will read just under 30 inches, eg relationship is as follows,

Boost, Absolute, MAP

-4, 10.7, 21.8
0, 14.7, 29.9
+12, 26.7, 54.3

The boost gauge is merely telling you how much above or below sea level ambient you are. -4 boost = 14.7-4, +10 boost = 14.7+10

Spitfire boost gauge



Typical MAP gauge for a turbo/super charged engine