Most engines in the emission-control era won't idle at a vacuum level much above about 18"Hg, mostly because of retarded ignition timing. But even with lower idling manifold vacuum, vacuum gauge indications are still valid and useful. A difficulty with both after-market and OEM dash-mounted gauges is that they are usually heavily damped, and as a result they don't show some of the useful information that a test instrument will show.
Adding some to previous information, "vacuum" as normally used refers to absolute pressure that is below atmospheric pressure and is measured in either inches of mercury (in hg, or "Hg) or millimeters of mercury (mm Hg). Standard atmospheric pressure at sea level in pounds per square inch is 14.7, which translates to 29.92 "Hg and 760 mm Hg. The full-scale vacuum gauge reading for those two measures in nice round numbers is 30 "Hg or 760 mm hg.
So when you open the throttle in a running engine, the manifold pressure rises (toward atmospheric pressure) and manifold vacuum lowers proportionally.
Harvester
