Testing Engine Vacuum on the Simtester

John's at it again, this time he's explaining how engine vacuum is measured on his Simtester.
www.Engine-Machining.com

Category:

Autos & Vehicles

Tags:

• john
• edwards
• costa
• mesa
• r&d
• automotive
• machine
• vacuum
• test
• testing
• simtester
• fiat
• ferrari
• studebaker
• packard
• jaguar
• olds

[Qwertyuiop8504]

Hey I got a question, I put a vacuum gauge on the manifold of my motor and I got weird readings. The vacuum started low and as I raised the RPM the vacuum increased and it did this anywhere I would hook the gauge to. Do you know what this could mean?

[fiatnutz]

I could mean a bunch of things, try and get a copy of my engine rebuilding handbook, I have a detailed section on how to use a vacuum gage w/illustrations. There isn't enough room here to tell you everything so you are gonna have to do some leg work.

[puchdiesel]

How can you measure the vacuum with the sparkplugs out?

[fiatnutz]

By inserting a plug in the hole.

[poy41]

what if the engine is camed up? dont you loose some vacuum?

[fiatnutz]

Very true, this engine had a mild cam in it. I'm just trying to see that I do indeed have vacuum on the intake side.

[jacktheripped]

This hold true with the rating of carbs. 4 bbls are rated a 1.5 in vacuum, 2 bbls with 3 inches so a 500cfm 4 bbl will massively outflow a 500 cfm 2 bbl at the same vacuum.

[waynep712]

hi...i know this is old... but with the spark plug out.. the only vacuum signal you are getting is the restriction from the size of the opening..

even if you just stick a rubber tapered plug over the spark plug hole.. so you don't load the pistons building compression..

its the pistons moving down in relation to the cam closing the valves to create vacuum in the intake ports.. with the spark plugs out.. its just not an accurate test...

[turner593]

great stuff learning lots thanks

[Mr572u]

You didn't sayanything about plugging the spark plug hole before the test. It's running evenly. Did you plug all the holes?

[laansoo]

@Qwertyuiop8504 Vacuum should increase with RPM on a noramlly aspirated engine, this is normal. But on a turbo charged or a supercharged engine there should be a 0 or slight pressure that sould increase when you put load on a engine aka driving. But in can see what makes you doubt on your readings, you see he uses the vacuum gauge on only one inlet witch draws air in only when valve opens. On a inlet manifold there is at least one valve always open that draws constanlty air in.