Vacuum Guage Readings

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PaulRM

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A previous,but related thread posted by Mullinax 95 on Compression test??? and Street Demon's reply about a leak-down-test got me thinking about another way of checking the health of an engine. I got out my Chilton's manual and looked up the section on vacuum guage readings. After hooking up a vacuum guage on the intake manifold beyond the throttle plate, note what the needle does on the running engine. Depending on its movements and what they mean (interpreted from the manual) you would know where to start looking for the trouble. A leak-down-test would surely pinpoint the problem and confirm the vacuum guage readings.
In addition to just using the vacuum guage as a problem diagnosing tool, one could have it to monitor an "engine's health" on an ongoing basis plus it can be used as a gas mileage monitor. This gas mileage monitor is something I want to do with my Dart. On some Mopars (somewhere around 1970) some were factory equipped with a vacuum guage. When you stepped on the throttle pedal too hard, a light would come on from a signal light mounted on the front left fender. If anyone knows of such a device I would be interested in getting one.
Great forum and by what I'm reading on various posts, very knowledgeable people here. Good to be part of the group.
 
I did a vacuum test and it was a steady 9.5 at idle (big cam). So I don't know what that meant far as to your Chilton's manual. I know what you are talking about however. I did my own little leak down test without the proper gauge however and knew then something was up. I am going to do a compression test today and check the vacuum to see where I am at now with the new heads.

My 67 cuda has guage in the middle of the dash that supposed to have a vacuum hose hooked to it that tells you "Performance or Economy".
 
Vacuum gauges were an option in the rally dashes in most early a bodies in the sixties. Then in the 70's and early 80's some cars were equipped with a light in the dash to remind you of fuel conservation.

I had a vacuum gauge in several cars and it is great to use for diagnosis and fuel economy purposes. Of course, common sense works for fuel economy, too, but a vacuum gauge is a great "trainer" for getting more MPG. Light throttle provides the highest vacuum readings, thus the best MPG. 2 inches of vacuum will show you that your foot is in the firewall...fun to watch.

A fluttering reading will tell you that you have a burnt valve. Many diagnostics with a vacuum gauge.

here is one website with info:
http://www.centuryperformance.com/vacuum.asp
 
Thanks 340 four speed for your link to the vacuum guage page. It basically tells what my Chilton's manual says. It also confirms that the guage is not the end all be all tool but in conjunction with some other fine diagnostic tools can find problem areas or help tune an engine.
 
Yep a vacuum gauge is a great tool. I got one hooked up that I ride with all the time.

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I took mine gauge off awhile back because I just have too many gauges to begin with. I have pretty high compression ratio and the gauge help me find the right power valve.

There was a station that had bad gas and I didn't know it and all of a sudden my motor started pinging under light throttle. Not knowing it was bad gas I wrote it off as not enough fuel and started watching my vacuum gauge real close and noticed that when my vacuum went below 6 no more pinging. I also noticed my driving habits in normal acceleration the vacuum went down to 7.5 so I bought an 8.5 power valve and the pinging went away. I went ahead and backed up my theory with an LM1 meter and I was right on the money.

They are cheap and great tools.
 
i chose to leave the vacuum gage in my 66 just for the fact of monitoring the engine. the formula s came with a tach instead of the "performance indicator". id rather have an aftermarket tach which should be a heck of a lot more accurate than factory even though the factory tach is big $$$

cuda gages1.jpg


dash 2.JPG
 
sweatybetty, Thats a really nice looking dash.

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