Working on a car for a friend of mine. Just put together a forged LS3 with AFR Mongoose LS3 heads. Due to hood clearance issues, he decided to go with a rod modded LS3 intake. I just got the car running and it seems to have a pretty good vacuum leak. Car won't idle under 1500rpm. When I installed the intake, it took a little work to get it to sit down on the heads. The aluminum sleeves for the bolts weren't fitting into the provisions on the heads very well. I'm assuming this might have put the intake into sort of a bind and likely causing the leak.

Has anyone modified those sleeves on the intake side? Cut them flush? Any downsides to that? Or should I try to open the provisions on the head side to get it to sit better?

 

Jonathon, were the heads milled down considerably for compression reasons?

 

Off the top of my head, I don't think they were milled a whole lot. I'll have to look back at the conversation with the owner.

 

What head bolts/studs were used? I ran into issues on a build where the nuts were hitting the intake, not allowing it to sit down on the heads properly.

 

ARP head bolts. I will look to see if there are witness marks on the intake in that area. The valley cover plate has the OEM bolts.

 

Dang, good call. Looks like we milled .042” and then did .040” head gaskets.

 

Those numbers will make intake alignment tougher for sure, but maybe not the entire problem…the Oem valley plate is thick, and along with those valley cover bolts…if he’s trying to run heat masking material like in the pic, it adds to the issue. Could be 2 or 3 things fighting you here.

 

That was just a picture I pulled from the internet. I did look around the provisions for the bolt heads underneath the intake and didn't see any witness marks. The main issue was the sleeves for the manifold bolts, being extra tight on the head side.

I'll look over everything again and see if I missed anything. Is there a gasket that I can use along with the o-rings or even by itself that might help the alignment?

 

CID makes a .030 thick rubberized intake gasket for LS3. Maybe it would help???? Not sure man. As far as a paper gasket goes, I don’t know of any, but they are prolly out there.

 

Did a little more searching and found that people have cut off the ends that protrude to the mounting surface. Gonna try that and see if it helps.

 

Cutting down those things helped but still no good. Went from a minimum of 1500rpm idle to 1200ish. When looking at it straight on, the top of the manifold is flush with the heads and the bottoms are maybe 1/16th of an inch away.

 

Were the heads cut on the intake side to compensate for the deck cut?
LS head milling and plastic intakes - Don Terrill’s Speed-Talk

 

No, they were not

 

Ordered some thicker intake gaskets before I try to sand down the flanges of the intake.

 

It sounds like you’ve got a tricky situation with the intake installation. If the aluminum sleeves weren't fitting properly and caused the intake to sit unevenly, that’s definitely a potential source of the vacuum leak. Modifying the sleeves can be a viable solution