"Maximum" Allowable Vacuum

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Aug 9, 2019 | 06:29 AM

QuRace

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Joined: Oct 2010

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From: near Nuremberg Bavaria

"Maximum" Allowable Vacuum

I have a small, single stage rotary vane vacuum pump at my disposal.

I'm planning to use it for working on / debugging the vacuum system of my car (instead of the MityVac hand pump I also own - I'm no Popeye...).

BUT: The question I have is, is it safe to connect it to the vacuum system of my C3.
.

I know that a good / tight engine will make up to ~21" Hg at idle.
I assume that manifold vacuum will be even lower (more inches of Hg) in certain driving conditions (high RPM decel). But I don't know how much lower... And also, whether such peaks will actually reach the vacuum system, due to flow restricions like long lines and check valves, etc.
I'm confident that the rotary pump will eventually produce 30" Hg.
.
Which makes me ask whether such a low vacuum level might lead to damage in the vacuum system, mainly a crushed vacuum reservoir.

Thoughts / experience made the hard way?

.

Last edited by QuRace; Aug 9, 2019 at 06:35 AM. Reason: Typo.

Aug 9, 2019 | 07:24 AM

7T1vette

Team Owner

Joined: Jan 2006

Posts: 37,637

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From: Crossville TN

About 21" Hg (approximately -10 psi) is all that a good, tight stock engine will produce. There are portions of the stock vacuum system in a C3 that you COULD harm with nearly full vacuum (-14.7 psi): vacuum reservoir tank comes to mind. But most of the parts will not care about that difference. You might limit the vacuum when you test the reservoir (pump it down to 15" vacuum and then close it off to see if it leaks down at all. Shutting off the pump is not adequate, as that pump will have some leakage when not running and you wouldn't know if the leak is from the pump or the part. You need a shut-off valve in the system to isolate the pump.

Aug 9, 2019 | 08:04 AM

Jebbysan

Dr. Detroit

Joined: Mar 2012

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From: New Braunfels Texas

You better have good hoses if you pull more than 20" as they will collapse......old hoses do this on a stock engine.

Jebby

Aug 9, 2019 | 11:09 AM

REELAV8R

Le Mans Master

Joined: Apr 2011

Posts: 6,284

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From: Hermosa

Just put a vacuum gauge in line with your supplied vacuum line, then introduce a leak in your vacuum line, ie a small hole, monitor your vacuum level. It it’s too much then make your hole in the supply line bigger. Not enough, tape the hole over a little.

Last edited by REELAV8R; Aug 9, 2019 at 11:10 AM.