How does a boost gauge work?

[nbnguy98]

Does anyone know? So it's tapped into the vaccuum line. When the engine is off, it goes to zero. But when the engine is running it actually goes negative (like a vaccuum). Not only that. it's measured in "inches of Mercury". Then when I start boosting it goes positive, gets to 0 and then peaks to about 12 PSI (which is a different unit of measure). So the question is why two different measurement of pressure? and why does it start in the negatives, passes 0 and then finally goes positive? Read the forms, it does seem that the stock turbo gets about 12 PSI, so I assume it's reading correctly.

 

[Quasimondo]

PSI = Pounds per square inch. That is a standard issue of measurement for any pressure gauge that uses SAE (standard american english) measurements.

in/hg (inches of mercury) = The standard issue of measurement for vacum gauges. This refers to the air pressure below ambient pressure.

Both measurements are levels relative to atmospheric pressure, so when the gauge reads 15 psi, the air pressure inside of the manifold is 15 psi above atmospheric levels. When it reads 15 in/hg, then it's reading 15 inches of mercury below ambient air pressure.

 

[nbnguy98]

What's causing the vaccuum?

 

[luv2rallye]

What's causing the vaccuum?

On the intake stroke the piston goes down and so increases the volume of the camber which causes a much lower pressure than outside the camber (thus a vacuum). Think of it like what happens to a syringe when you pull the piston back.