Thread: vacuum question

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Assuming your vacuum gauge is correct, 13" sounds about right with the cam you have.

It's common for big cams to run 9" - 11" of vacuum at idle.

Spin the engine up to 1500 rpm or so in park or neutral if it's a stick car and if it's well sealed the vacuum will register 17" - 19".

The assumption is, you're reading manifold vacuum and not ported vacuum.

Originally Posted by DennyW

The cam in my Merc is 224º duration, and 514 lift. My vacuum reads 18.5". His duration is 222, and a smaller lift. Should read 18-19".

Interesting . . . and I'm not saying you're wrong.

The big cam I used to run in my 462" Buick has .488 lift with 230 degrees duration @ .050 on the intake - exhaust side is a little stronger.
Advertised duration is 284 degrees which is a fairly large street cam.
It idles @ 600 rpm and 11" - 12" vacuum.

In a way, vacuum level vs cam timing seems backwards compared to lit549's engine.
I know that bigger engines "calm down" a cam so along with that vein of thinking it seems the 350 engine should have an even lower vacuum level than the 462" Buick.

The Buick engine is pretty well sealed and with the present cam - intake side - .430 lift with 204 degrees duration @ .050 and 260 degrees advertised duration it idles @ 18.5" - 19" on a 2 5/8" 250 degree sweep S-W vacuum gauge which compares favorably to a shop pressure/vacuum test gauge.
Idle with this cam is also 600 rpm.

I do note that the bigger cam would smooth out and register a higher vacuum with an increase of a few hundred rpm.

Denny, what RPM level are you taking your vacuum readings at?
That could account for the disparity in vacuum levels between your engine and mine.