Calculating engine vacuum?

[Rimmo]

I'm looking at speccing a cam for a daily driver with auto trans.

Can anyone provide some input on the minimum vac I can get away with for the auto?

I want to run a tighter LSA on a mild cam, is there a way of calculating vac from overlap?

[user-23911]

Vac at what RPM?
As idle speed increases, you get more vac.

[mag2555]

As your combustion chamber gets smaller your vacuum leve goes up also!

[Geoff2]

If you want to run tight[er] LSA & want more vacuum, reduce the duration. No free lunches with vacuum. I assume we are talking about idle vacuum.

[PackardV8]

Too many variables; only a few are:

1. Displacement is the first and key. Not surprisingly, larger displacement pulls more vacuum with the same cam timing.
2. As mentioned, compression ratio.
3. Intake and carb or EFI throttle; smaller pulls more vacuum.
4. Head intake porting; again, less is more vacuum.
5. Headers or cast iron exhaust manifolds?
6. Converter stall speed

[groberts101]

You'd need to be MUCH more specific if you want a more definitive answer to such a broad ranging question. Generally speaking, higher overlap(gained from duration, LSA, and lift to a smaller degree(faster high lift lobe designs/higher rocker ratios than stock) increases internal EGR effect, mixture dilution(slightly erratic AFR), reduction of pressure differential or "draw" on the intake port and carb. I will go further out on the limb by saying that a smaller duration cams lobe design will likely not affect idle vacuum nearly as much as a larger duration cam will be when using tighter LSA because you're not adding even more insult to injury at lower piston speeds with the smaller cam. Plus, a smaller duration/lift lobe will usually not have a hugely fast opening/closing speed because there's just not enough lobe to keep things under control over the typically smaller nose.

Engine displacement is a MAJOR factor, nuetral idle speed preferences, manifold design(dual or single), valve sizing, static compression, carb sizing(mainly relating to primary throttle bore size), how aggressive the cam's I/E lobe designs are along with rocker ratios, tighter or looser torque converter stall speed(engine loading against the converter in drive), idle/off-idle ignition advance(vac advance hooked to ported(timed) or full manifold vac source), etc, etc.

When running a "smaller cam and tighter LSA"(which is often a completely subjective rating), I would expect to see a slightly more erratic idle(slight lope) and a little loss/very slight movement of the guage needle is all, maybe an inch or two loss at most unless LSA is taken to the extreme. Most of which issues can be tuned out with carb and/or ignition adjustments in many cases. To really make any measurable or perceptible differences in manifold vacuum at idle speeds you'd need to have a dedicated tighter LSA core custom cam ground to get that much further towards a higher overlaps negative idle effects.

And unless the trans has a vacuum style shift modulator.. the trans itself doesn't really care what manifold vacuum is either way.

[groberts101]

You'd need to be MUCH more specific if you want a more definitive answer to such a broad ranging question. Generally speaking, higher overlap(gained from duration, LSA, and lift to a smaller degree(faster high lift lobe designs/higher rocker ratios than stock) increases internal EGR effect, mixture dilution(slightly erratic AFR), reduction of pressure differential or "draw" on the intake port and carb. I will go further out on the limb by saying that a smaller duration cams lobe design will likely not affect idle vacuum nearly as much as a larger duration cam will be when using tighter LSA because you're not adding even more insult to injury at lower piston speeds with the smaller cam. Plus, a smaller duration/lift lobe will usually not have a hugely fast opening/closing speed because there's just not enough lobe to keep things under control over the typically smaller nose.

Engine displacement is a MAJOR factor, nuetral idle speed preferences, manifold design(dual or single), valve sizing, static compression, carb sizing(mainly relating to primary throttle bore size), how aggressive the cam's I/E lobe designs are along with rocker ratios, tighter or looser torque converter stall speed(engine loading against the converter in drive), idle/off-idle ignition advance(vac advance hooked to ported(timed) or full manifold vac source), etc, etc.

When running a "smaller cam and tighter LSA"(which is often a completely subjective rating), I would expect to see a slightly more erratic idle(slight lope) and a little loss/very slight movement of the guage needle is all, maybe an inch or two loss at most unless LSA is taken to the extreme. Most of which issues can be tuned out with carb and/or ignition adjustments in many cases. To really make any measurable or perceptible differences in manifold vacuum at idle speeds you'd need to have a dedicated tighter LSA core custom cam ground to get that much further towards a higher overlaps negative idle effects.

And unless the trans has a vacuum style shift modulator.. the trans itself doesn't really care what manifold vacuum is either way.

EDIT.. Jack beat me to the punch but hopefully I elaborated enough to help understand some of the "why's".

[77cruiser]

I'm looking at speccing a cam for a daily driver with auto trans.

Can anyone provide some input on the minimum vac I can get away with for the auto?

I want to run a tighter LSA on a mild cam, is there a way of calculating vac from overlap?

Transmission doesn't care so much but if you have power brakes it will. A minimum of 10" with power brakes 12" would be better.
What engine & what cam did you have in mind?

[Rimmo]

Thanks for all the detailed replies and yes, I'm aware a lot of the variables mentioned will effect idle vac.
I've seen engine software and cam recommendations come with an expected idle vac figure so I assumed there must be some underlying equations that will get you in the ball park. Giving me the option to play with a few variables and try different lobes on paper before trying a cam, wasn't expecting anyone to work it out for me.

std stall
750rpm idle
3.48"stroke, 355 cubes
small port heads, 1.94"intake, 1.6"exh
factory dual plane + Qjet
1 & 5/8" 4-2-1 headers
mild lobe, 268* adv, 106 LSA, 56* overlap, 6* @ 0.050"
9.5:1 comp subject to change to suit final spec

I suspect I'll be ok. Or at least happy to try it out.

[travis]

It should pull 13” or so at idle, so it should be fine

[groberts101]

Thanks for all the detailed replies and yes, I'm aware a lot of the variables mentioned will effect idle vac.
I've seen engine software and cam recommendations come with an expected idle vac figure so I assumed there must be some underlying equations that will get you in the ball park. Giving me the option to play with a few variables and try different lobes on paper before trying a cam, wasn't expecting anyone to work it out for me.

std stall
750rpm idle
3.48"stroke, 355 cubes
small port heads, 1.94"intake, 1.6"exh
factory dual plane + Qjet
1 & 5/8" 4-2-1 headers
mild lobe, 268* adv, 106 LSA, 56* overlap, 6* @ 0.050"
9.5:1 comp subject to change to suit final spec

I suspect I'll be ok. Or at least happy to try it out.

If you can't hit 17-18" of manifold vacuum on that type of deal then you just need to keep tuning. Probably settle down around 16" when in gear and up against the converter.

Key to getting there is bookoo manifold assisted vac advance being added via the distributor's adjustable vac pot. Probably also need to add some extra fuel into the idle circuit or it'll go lean and snap, crackel, pop,.. maybe even surge.

[Rimmo]

Sounds like I'll be good to go if I get on top of the tune.
Cheers for the feedback.

[rebelrouser]

Performance trends engine analyzer program calculates idle vacuum. I have used it for several years and is pretty accurate.