Vacuum reading

fordv8j

fordv8j

Senior Member
I put a vacuum guage on our 38 coupe today with 59A engine,warmed up and idling, it read 20, is that good?
 
Vacuum gauge is a very under rated tool .. 20Hg is great , 21 Hg at idle is even better ; ) [if you have a standard cam]
 
Most motor or Chilton manuals will have a page that will tell you what various vacuum guage readings indicate. Or google vacuum guage.
 
http://www.vanpeltsales.com/FH_web/FH_images/FH_other-pics/vacuumgauge-chart.jpg
vacuumgauge-chart.jpg
 
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.
 
My '46, with a stock 59AB, runs a steady 21" at idle, and 15" to 16" at 50 mph.
 

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That is a fine engine with reading like that .
Th3 temp and oil pressure are excellent as well .
It’s good to see a Flathead that is operating like it was designed.
Good job .
Tommy
 
The vacuum gauge is not simply a go or nogo gauge it should also be used to adjust the carburetor idle mixture adjustment screws. Under the same conditions you took your reading just slowly turn the mixture screws, one at a time, in or out until you get the highest vacuum reading.
 
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