Negative boost numbers

[youngstroker]

then how dose a gas car build vacum at idle?

 

[Tom S]

If I remember right scan gauge boost pressure is Manifold Absolute Pressure minus normal atmospheric pressure 14.7. When you are at higher altitude the scan gauge will read off because the atmospheric pressure is less then 14.7

 

[Blowby]

PSIG + 1 atmosphere = PSIA
PSIA - 1 atmosphere = PSIG