Manifold Vacuum

[Thomas F.]

Can anyone tell me what the factory manifold vacuum should be on a 69 390 horse Vette?

[Terry M.]

Re: Manifold Vacuum

Can anyone tell me what the factory manifold vacuum should be on a 69 390 horse Vette?

Well Thomas at this time 34 of us have looked at your post and no one has responded.

I don't recall seeing engine manifold vacuum specifications for Chevrolet engines, but there may be some someplace. Have you tried the AMA Specifications which are available from the GM Heritage Center web site?

https://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html

I did a Google search and found this: "Normal manifold vacuum at idle for an engine in good condition is about 18 to 22 in.-Hg. Manufacturers used to publish vacuum specs in service manuals, but this isn't as common as it was years ago. Still, the physics of internal combustion haven't changed in a hundred years, so the guidelines given here are a good starting point for vacuum gauge troubleshooting. Your best analysis based on vacuum readings will come from your own experience, however. As you use a vacuum gauge on different engines, you'll learn what's typical for one model compared to another. Some engines have reputations as low-vacuum motors; others are unusually higher than average. Experience is your best teacher. " at http://www.motor.com/article.asp?article_ID=457

That Motor article also points out that engine manifold vacuum is dependent on altitude, and has a chart to allow one to compensate for the altitude at which the test is made.

That Google search also revealed a YouTube video of an engine manifold vacuum test, and many other articles about the process. There is far more information available on the net than I could begin to summarize here. Let your mouse do the walking.

[Duke W.]

Re: Manifold Vacuum

Chevrolet never specified idle manifold vacuum anywhere to my knowledge.

Manifold vacuum must include actual idle speed or the data is MEANINGLESS, because idle vacuum a function of idle speed. The higher the idle speed, the higher the vacuum. GM is vague about some idle speeds and some implied idle speeds are way too low. My rule of thumb is to set the idle speed as low as possible commensurate with acceptable idle quality.

The biggest factor in manifold vacuum is valve overlap, so any "guidelines" you see are basically worthless. High overlap cams have lower vacuum, even at higher idle speeds, than low overlap cams.

For manual transmissions measure in neutral and in Drive on automatics.

I know all the numbers for various small blocks with OE cams, and SHP big blocks (with the OE cam) are about 14" @ 900. I have asked many times for data on hydraulic lifter big blocks both manual an auto, but never received a response.

I guess nobody who owns a hydraulic lifter big block has a vacuum gage.

Do you? If so, measure and let us know. My guess is about 15-16" at 600 in neutral with a manual trans, assuming the OE cam. And whenever hooking up a vacuum gage it's a good idea to go through the simple idle speed/mixture adjustment procedure in the CSM to optimize idle behavior.

Duke

[Thomas F.]

Re: Manifold Vacuum

Thanks to Terry and Duke for your response. This is all good information to know. The reason I was interested in knowing this is because I am trouble shooting the vacuum system on my 69 that operates the headlight doors and windshield wiper door. I am trying to get everything to work as it was when the car was new. A previous owner had taken some short cuts by bypassing the vacuum reserve tank and some of the actuators and the wiper door switch. I had to use the over-ride valve switch under the steering wheel to open the wiper door before I could turn on the windshield wipers. I have hooked everything up with new hoses and other components but have been unable to get my headlight doors to open. They were working before. My engine is not correct for my car. It is a 454 with a performance cam of some kind. I do have a vacuum gauge and will let you know when I check it out. Now that warmer weather is coming I will be able to get back to work. It was 65 degrees in my garage today here in Tennessee.