Vacuum shortage

[camaroman1969]

Hi Guys...I have a BB 396/375 that has a "vacuum" issue.  Currently pulling about 9.0 inches.  I believe i should be creating around 11 - 12 inches.  It has a slightly  hotter cam than stock for this engine.  Before we dive into it, I just thought I'd ask the question of what might be the "root cause".  The cam maybe  a little.  We suspect may the intake manifold.  Could be slightly warped?not square???  What it does when it gets hot, idles down and stalls out.  Cold, it runs OK.  Anyhow, any input would be helpful before we go nuts.  Thanks guys!!

[BillOhio]

I would love my Z to have 9!  You can take some ether and spray around the intake and see if the idle picks up. The gasket could have a leak.  There are other areas that could do it to but might just be the cam

[jdv69z]

Bill, agreed! My Z idles at about 8 in-hg on a good day!

[X33RS]

What altitude are you at?  Automatic or 4-speed?

I run a "cheater" cam in our Z and it took some tuning to get it to idle around 9-10 inches at 1,000 rpm but that's way up here at 5,000 feet elevation.  It's just enough to run the power brakes without fuss.

When we drive the car down close to sea level it picks up about 3 inches of vacuum, which is normal, all our cars do that.

[jdv69z]

I'm idling at 900 rpm. Increasing the idle rpm will increase vacuum as well.

[X33RS]

Yep it does to a small degree with small changes.  Going from 900 to 1,000 on mine doesn't do much, maybe a 1/2 hg in. in vacuum so it's not detrimental.   We just like the idle a pinch higher for drivability reasons, like lukewarm restarts for instance where the choke doesn't activate but idle is generally lower until the engine gets back up to temp.  That way my wife doesn't have to fiddle with it.

Here's idle at 1,000

https://youtu.be/IZ0HlEY7b74 

[jdv69z]

Yes, I know what you mean. At 900 it sounds like it's going to stall. Increasing by a couple hundred rpm make sit sound much smoother.

[X33RS]

Yeah, it will idle at 900 fine, and even down to 700.  The issue I was having (not really an issue but...)  Is that when I had it set at 900-ish, and you shut the car down for 1/2 hour or an hour and come back out to start it.  Now the engine is cooled off a bit and idles slower and really doesn't want to quite idle on it's own until it builds a little heat back into the intake manifold.  So for a minute or two you might fiddle with the throttle until it settles on it's own.   It's typical with any car when you dial in a somewhat lean idle AFR like I do.  For me it's not a big deal but I didn't want my wife to have to deal with that though, so to make it easier for her to drive I just set the idle slightly higher so those periodic lukewarm starts don't give her any fuss with really low idle/stalling.