Resolved How much vacuum (in hg)?

[Sbgriffin]

So I have a early 1g with a 6 bolt that's been rebuilt several years back. Then more recently when I bought it, I did a complete upper end overhaul. With all new gaskets throughout. So my question is how much vacuum (in hg) should I have at idle. My gauge usually shows between 15-16 in hg. Is this descent? I read conflicting info. Some say 17, others are saying 19-20. Thanks, and sorry if this has already been answered.

 

[boostdawd]

17-20 is the norm if your on stock cams and don't have any boost leaks. Elevation also plays a part.

 

[pauleyman]

Check your mechanical timing and make sure its not off. Look closely at the cams. Ive seem dozens of people who think thry have it right when they dont.

 

[motomattx]

16hg-18hg is normal.

 

[curt-s]

15-16 with stock cams is on the low side at elevation of Lewiston.
My 5 year old, 20k km, stock cammed 1g 6bolt pulls between 19 and 20 at 850rpm idle. I am only about 30ft higher in elevation.

i would be looking at intake manifold leaks but more closely at intake valve sealing and cylinder condition with a bore scope. also timing as said.

19/20/21 is what i would expect under 1000ft.

 

[Sbgriffin]

Yeah I've checked the timing twice now by bringing the 1 cylinder TDC, looking at the cam gears, and the crank pulley mark. It sure looks good. The crank pulley mark is off just a hair when at TDC, but I think it's from the pulley starting to separate a little.

I've done a compression test, 156-158 across. Done a few boost leak test to 20psi, it takes about 45sec to leak off to zero. Also the strange thing is, once in awhile my gauge will show 18in hg. I'm really wondering if my gauge is accurate. It's a Stewart Warner boost gauge, came with the car.

Am being over critical, or should I be concerned?

 

[pat white]

Gauges being gauges they can be off and your idles got to be within spec also for vacuum, also minor leaks such as in the throttle plate shaft can drag your reading down. I wouldn't worry too much if everything is running fine.

 

[RAW92Talon]

If your not running stock cams but instead running Kelford 272s how would this affect your vac? Not debating just being a noob and asking. Am I thinking of it right that it would be less...If so what would the normal range be?

 

[Sbgriffin]

From what I understand, mostly from the v8's I've built. The more lift and duration a cam has the less vacuum you will pull. Because the valves are open for more of the 4 strokes. Plus there's what's called lobe separation, where both intake and exhaust valves are open at the same time for a few degrees.

More so with hot cams. If I'm wrong, please someone jump in?

 

[RAW92Talon]

That's kind of what I was thinking that with the valves being open longer you would be running with a lower vac. My car has 272s and seems to run much lower that the 17-19 stated above. IIRC more like 12 or so at idle maybe less. Is this an issue.... The car runs fine.

 

[Brembo]

Mine is fairly stock and aem digital gauge reads between 18 and 20 at sea level. A/C on reads 16. As far as I know I have no leaks. Also when I did have leaks I was like at 14 if my memory serves me right.

 

[boostdawd]

I have crower 272 cams and my aem gauge reads 12-14 at idle.

 

[A Sharp]

I have webcam 276/274s and have around 7 at idle ... Not the best grind by any means. About crapped my pants when I first took the car out and didn't have any vac assist when coasting up to a stop sign.

 

[Sbgriffin]

So I've noticed that it's pretty inconsistent, and sometimes is at 17-18 at idle. Other times it's more like 15-16 at 750-800rpm. I've done several boost leak tests, and it all seems pretty decent. Last time I tested up to 25psi, and held for 90 sec before hitting zero. So I'm wondering what I should look at, Or what am I missing? Is there anything else that could cause this? Also I should state my last compression test was 156-158 across. Being that healthy I would think it would pull more vacuum?