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Old 11-21-2018, 05:10 PM   #1
fordv8j
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Default Vacuum reading

I put a vacuum guage on our 38 coupe today with 59A engine,warmed up and idling, it read 20, is that good?
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Old 11-21-2018, 05:17 PM   #2
Kahuna
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Default Re: Vacuum reading

Ford, That's great
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Old 11-21-2018, 05:20 PM   #3
Ggmac
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Default Re: Vacuum reading

As long as it's a steady needle at idle . Any dip could be indicating a problem
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Old 11-21-2018, 06:38 PM   #4
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Default Re: Vacuum reading

Vacuum gauge is a very under rated tool .. 20Hg is great , 21 Hg at idle is even better ; ) [if you have a standard cam]
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Old 11-21-2018, 08:42 PM   #5
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Default Re: Vacuum reading

Most motor or Chilton manuals will have a page that will tell you what various vacuum guage readings indicate. Or google vacuum guage.
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Old 11-21-2018, 11:14 PM   #6
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Default Re: Vacuum reading

http://www.vanpeltsales.com/FH_web/F...auge-chart.jpg
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Old 11-22-2018, 04:23 PM   #7
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Default Re: Vacuum reading

20-inches is perfect. You can also try the throttle snap test like the first gauge describes. This cause the piston suck the air in faster. When the throttle slams shut the vacuum increase a little. This is an additional test to see how well the rings are sealing. If the vacuum goes up your rings are sealing as good as they were when they were new. This why in the old-days, before you bought a car, you took it to your mechanic to have it checked out first. That vacuum reading quickly told the mechanic if the engine was good or worn out.

You will not see 20-inches of vacuum with a high performance cam. The reading will be quite a bit lower. That changes when the power valve opens. So then you need to change the power valve to one that opens that matches your cam's vacuum reading.

I see nice vacuum gauges at estate sales for cheap. Nobody knows what they are used for anymore. Especially the "millennial mechanics". But you can't hook them to laptop so the new mechanics don't have a clue where to plug them into. They diagnose new vehicle engines just as good as they do flatheads.
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