Not sure where this question belongs, so mods feel free to move it...

I was wondering why most boost gauges generally read in psi for boost and inhg for vacuum? I was using evoscan's gauges as a boost gauge and noticed that it reads both boost and vacuum in psi which was confusing at first because I thought I had really low vacuum. Then I started thinking that this would've seemed like a more natural thing to do...but usually physical gauges read in 2 different units...is there reason why?

I know I can easily convert the units after the fact, but I kind of want to be able to display (in evoscan's gauge layout) boost in psi and vacuum in inhg (just because that's what I'm used to looking at). Is there a way to do that with evoscan...can I have a formula that outputs to different units depending on if it's in boost or vacuum? I assume I would need 2 formulas no?

 

The reason the car reads in psi is because it takes manifold pressure in absolute pressure and subtracts atmospheric pressure (typically 14.5 psi at sea level).

Physics has to read vacuum as in of mercury because its very hard to read psi without a pressure differential. Like when you read the pressure of a tire, there is a difference between the tire pressure and the air pressure out side. So the gauge can read the pressure difference.

Reading atmospheric pressure is hard to do via a pressure differential. As in, there is no vacuum reference. They invented the silly mercury tube thing first. So it has stuck.


EvoScan isn't going to change units of measure on you depending on if the value read is positive or negative. So psi is the final answer.

 

Thanks for the reply. I've actually gotten used to it after a few days already and to be honest I like the scaling better since my virtual gauge only has to go down to -15 instead of -30.