When I was cleaning out my LT1 I noticed a smallish kind of slit in the bottom of the intake manifold. It looked like a casting imperfection and I used some hard set quick steel epoxy to seal it up, thinking it would cause a vaccum leak. Upon researching something else, I found several pics of different manifolds with this same weird hole.

You can see it in the pic here, just under the edge of the turkey tray thing removed in the pic. What is this thing and what is it for? I've already installed the manifold, but will this cause some adverse problem, and should I go back and remove the seal I used for it. Any help would be appreciated.

 

After checking each of the outer vacuum ports, i noticed that the lower passenger side vacuum port that goes to the fuel pressure regulator is either plugged or clogged. Is this where the other side of that strange hole in the bottom of the manifold goes?

 

Alright, so after some further work I've found that the fuel pressure regulator port was just blocked by some dirt, totally unrelated. I pulled the manifold and decided to look at it again. In the pics you can see where it is located, and that it comes through to the rubber pcv port on the bottom driver side. I'm still a little confused about what this is for, but in the other pics ive seen, this hole is either just like mine, or someone had taken a dremmel tool and cleaned it up a bit, made it square.

Still confused as to it's purpose. Surely some intellectual and skilled mechanic can clear this up. I don't like leaving topics unresolved so that the next guy that comes along will be able to find some info regarding this, and not be like me hah.

 

Here is another pic of the hole I'm talking about on a different manifold. Looks like somebody cleaned it up a bit.

 

You found the answer yourself... PCV = Positive Crankcase Ventilation. The typical SBC solution is drawing fresh air into one valve cover, across the block to the other head/valve cover before sucked into the intake to be combusted. This is like the Buick V6 (Turbo GN and TTAs), which has the PCV in the intake, too.

PCV has to draw from inside the block/manifold/heads. Why not through the intake manifold?

 

It just seems like if this was supposed to be there it wouldn't look so jagged and wrong. I've read other instances where someone had overfilled the oil and it was getting into the manifold, probably through this port. I took off the epoxy steel and reinstalled the manifold like it's supposed to be. Just wanted to set the record straight for anyone else who may notice this.

 

it's just a rough cast hole, that's why it looks so jagged... it's actually a pretty clever place to put it so they didn't need to spend a lot of time developing baffles to keep oil out of the pcv valve..

 

The intake that I bought to modify had this hole welded shut.