Idle Vacuum - How much?

[davepl]

With a good large 2" gauge I'm seeing 15inHg of vacuum on my stock rebuild 402 at idle around 600-700rpm.

Since its a stock cam, I was expecting more. It runs fairly smooth, not perfect. The needle is steady, doesn't move more than 0.5" and doesn't bounce around.

What would be normal for vacuum on one of these? Or is it?

Thanks,
Dave

[steveo3318]

That's pretty good. Best I can get is 10 to 12 in my 385 stroker with a 296/304. 510/510 cam. I would be ecstatic with 15

[davepl]

Well yeah, but I need to know what's normal for a stock cam. Not a 296/304 advertised duration cam!

The internet says 15-20. I'd rather have 20.

[71 super]

Both of my bb are at 15-18 in. Vac., it is more than plenty to run all the a/c vac and brakes as well as the cruise on my 71- it is at 18.

[davepl]

This page

http://www.gregsengine.com/using-a-vacuum-gauge.html

...says that a steady 14-16 inches of vacuum indicates "incorrect ignition timing". My guess at timing is really a crapshoot. I think I'm at about 10 degrees initial right now (vs 4-6 with the old original fuels).

How do I know what's the best initial timing? When I was a kid I'd just run as much advance as I could without pinging or making it hard to start, but there's no doubt a more scientific approach.

[jmorri34]

Best I can get on my stock 350 is 15 and Im not losing any from what I can tell, no leaks at least and Ive tried everything I know how to get more! not like I need it but thats what I have anyway!

[Lattimer]

I'm running a small block with a moderate street cam (lopey idle), and I get 15 inches at idle. Initial timing is 14 degrees for me, and it runs great with no ping. I do have my vacuum advance on the manifold, which makes a huge difference in my idle.

Probably doesn't help much with your big block, but with a stock cam it seems like you would have 18 inches easily. I'd push the timing up a little and see if it helps. Carb adjustments have an effect on idle vacuum as well.

['68OrangeSunshine]

It may vary by type of engine. I don't have a full-time Vacuum gauge on the 350 V8 in my '71 Jimmy, so I don't know what is normal for an SBC, let alone a BBC.
My ['68 C/10 Stepside] L6 292 with a 260H Crane Cam pushes 17 inches of mercury at normal operating temp with 850 RPM at idle. I recently had an exhaust valve spring break and after I replaced it with another ''used-good'' spring my Vac was reading 16"Hg. That's good enough until I can pull a major tune-up.