Quote:
Originally Posted by Old Tele man
Uh, 29.92-inches Hg is associated with BAROMETRIC pressure, not MANIFOLD pressure. Typically, MANIFOLD vacuum (normal aspiration)/pressure (turbo or supercharge) are measured in XX-0-14.7 PSI or ±0-30 "Hg...without decimal places.
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and when you subtract the difference between barometric and manifold pressure you end up with how much suction your engine is pulling. In a plane it is used for determining a power setting. if you are running the engine at 2300rpm and at 5000 feet and the manifold is showing 23" you are making the same amount of power as if you were at 10,000 feet and showing 23" at the lower altitude you would have less throttle.
For a car it is more difficult to get the relevant data as you are really after vac so you know when you are accelerating efficiently.
If your car has a MAP sensor and is 96 or newer an ultra/scan gauge will give you manifold pressure if you desire it. it will also show psi boost or suction which I found much more useful.
Ebay can be a good source for used gauges and such.
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