cam size = x amount of vaccum

Is there a rule of how much vaccum a motor will have with a given size/duration cam? How much cam can you have and still pull 15" of vaccum?

Well, I know a 292h cam pulls about 9lbs of vacuum in my Mustang.

I'd guess ~270*.

I believe mine pulls about 10-11 at 1100rpms. (XE284H..240/246 @ .050)
I used to have the 270H(224/224 @.050) at it had around 17-18 I believe.

My 286r solid roller pulls around 10-11 also.

So nobody thinks things like cubic inches might play a role? I wouldn't put any faith in such a calculation unless it took a BUNCH of things into consideration. Bore, stroke, just about every aspect of the valve events, true compression, heads, intake, exhaust, etc, etc, etc...

Well sure cubes make a difference, I just assumed he was referring to a 347.

Originally Posted by 289nate

So nobody thinks things like cubic inches might play a role? I wouldn't put any faith in such a calculation unless it took a BUNCH of things into consideration. Bore, stroke, just about every aspect of the valve events, true compression, heads, intake, exhaust, etc, etc, etc...

very well stated. Too many variables left on the table.

The amount of overlap has as much to do with idle vacuum as the duration does. So does compression, valve size and lift.

so no, there isn't any direct correlation between just the duration and the idle vacuum you can expect.

Compression has absolutely zero to do with vacuum... It's what occurs AFTER compression that creates the vacuum... or am I wrong, and someone can show me how...?

The more compression you have the more vacuum the cylinder will pull -
If you start out with less volume as you draw the vacuum it will be higher vacuum at the bottom of the stroke. The same mechanism works for pressure and vacuum.

1" of mercury is about the same as -2psi