e.g. roughly speaking   boost PSI = (MAP - height thousands)-30/2
MAP = 42 @ SL = 6PSI
MAP = 48 @ 10,000 = 4PSI

Does this makes sense, or am I still missing something?

<p>

Your formula only works at sea level when ambient pressure is at 30"hg.  At altitude ambient pressure decreases about 1"/1kft.  (That number isn't linear, the decrease diminishes with each thousand feet, but it's ok to use that number for our purposes here...)  So, if your MAP at 10kft was 48" where ambient pressure is about 20"hg you'd have 28" or 14psi of boost, not 4. Ergo, your formula should actually be:  boost PSI = (MAP - ambient press)/2. Both MAP and ambient pressure are in inches of mercury.  If you run some other numbers you can quickly see how a manifold vacuum gauge works... let's say you're at 2000' msl where the ambient pressure is about 28"hg and you pull the power to initiate a descent... your MAP gauge might read 15"hg... (15-28)/2 says you have 6.5psi of suction going on inside the manifold and you're not making any boost at all.  I don't know if they even make a boost gauge that shows manifold vacuum (suction) as well as boost (positive pressure), but if they do the zero psi point would have to be referenced to ambient pressure in order for it to display properly.  BTW, in reality, 1psi = 14.695" hg, but that's close enough to 2psi = 30"hg for purposes of our discussion.

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