Vacuum readings
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
GENERALLY, want to see 18" to 20" steady needle at a factory idle RPM of 500 to 600. This is the standard for a stock engine, in good mechanical condition i.e. compression rings, valves etc., at sea level.
If living at altitude, 15" or 16" steady might be the best that can be achieved. Once a performance type camshaft enters the mix it gets tricky. The long duration overlap means a lower overall manifold vacuum. Might see something around 8" Hg.
The mechanic's vacuum gauge is practically an indispensable tool, all kinds of engine derangements can be diagnosed with a little user experience, maybe 20 to 25 different defects or tuning faults from late ignition timing, to valve problems, carburetor misadjustment, exhaust restriction and so on. Also measures mechanical type fuel pump pressure output, another important tune-up or replacement check.
If living at altitude, 15" or 16" steady might be the best that can be achieved. Once a performance type camshaft enters the mix it gets tricky. The long duration overlap means a lower overall manifold vacuum. Might see something around 8" Hg.
The mechanic's vacuum gauge is practically an indispensable tool, all kinds of engine derangements can be diagnosed with a little user experience, maybe 20 to 25 different defects or tuning faults from late ignition timing, to valve problems, carburetor misadjustment, exhaust restriction and so on. Also measures mechanical type fuel pump pressure output, another important tune-up or replacement check.
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