I have a Prosport Digital gauge. Just wondering how to read vacuum. My idle is -8 to -9.

If the gauge is reading "negative" PSI then:

1 PSI equals roughly 2 inHg.

So my vacuum is about 16 - 18inHg.

Seems correct. My question is, if you have a digital gauge is it reading negative PSI and not Hg right??

8-9 inHg is not good for vacuum. Car runs fine, but I'm running lean on my tune. Just wondering if its an air leak somewhere.

Thanks

negative psi technically doesn't exist, it can't...

anyway, check your documentation... doesn't take much searching to see that gauge only goes down to 14 in Hg, meaning it probably isn't accurate at the bottom end of the range anyway.

I know there's no such thing is negative PSI, but I was just wondering if the Digital number is in the PSI scale. Even though its negative.

With regular gauges with needles, when the needle goes under 0, there's a labels to help the user tell how much vacuum in Hg.

You should look up what the gauge reads in in terms of units.

If it is 8-9 inches of mercury that is around 4 psi. What are you using the pressure gauge to measure?