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Engine condition, whether simply checking your engine or looking for a problem, can be done with a vacuum gauge.

The vacuum gauge can also be used to find your best timing and idle mixtures BUT there are many variables that go into using it in this manner and may not be the way to go with the modified engines most of us use. So the vacuum gauge is better used as a tool to diagnose an engine problem or simply get a fix on your vacuum reading for things that require vacuum such as power brakes, AC/heating, the power valve/springs found in most carburetors, or the vacuum canister used for engine timing.

Here is a sheet that I got with a testing gauge many eons ago. It can be printed out if needed. It shows needle movement and what it may mean as to what is going on within your engine.

The vacuum gauge can be used with a cylinder compression gauge to follow up and confirm some of those things that the vacuum gauge is telling you.

The vacuum gauge is used with the engine running, the compression gauge is not - it is used at engine cranking speeds to test cylinder pressure.
 

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I always used a vacuum gauge for tune ups - and in smaller displacement cars under prolonged WOT if the needle comes off of Zero, it was an indication of needing a larger flow rate carburetor. Sorry for the large image..

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