quick question about vacuum in manifold

[mspiegle]

If you have a cylinder not working (either no fuel, or no spark), will that affect your vacuum at idle... or is the action of the piston moving up and down what creates your vacuum?

[Ryan Stewart]

Its the action on the piston moving down with the intake valve open and the exahust valve at least mostly closed pulling against a partially closed throttlebody.

what is the problem you are having.

[mspiegle]

well, on my 928, i've removed and installed the custom intake manifold like 5 times, and cut a new gasket each time. I always seem to have this vacuum leak that makes my car have idle vacuum of 10psi. When it was working before I took it all apart, it was doing about 18psi of vacuum.

i'm kind of looking for another cause of my problems... but i probably just need to take it off again and try another gasket....

do you know of a good custom gasket material for intake manifold parts? I'm using a cork one right now which I swear worked before, but i'm gonna try using a paper one next.

[Ball325]

cork is the work gasket material I have ever seen. Im not sure what to use, but not cork. I think they sell gasket paper, its kinda thick like paperboard almost use that and a little Gasket Seal, you can probably pick that up at Sears or an automotive supply store, like pepboys or something. I dont know about Cork Gaskets on porsches and such but, a cork gasket was the cause for HP MGB to overheat, and burn up Cylinder 1 and 3. at 100mph when you overheat I guess your pretty much done.

[mspiegle]

i think i found out what the problem is...

while cork probably isn't a great gasket material, it did help me notice something. My manifold comes in 2 parts. There is a top part that fits the supercharger, and a bottom part the makes the seal against the intake ports. When I pulled everything apart, I noticed that the cork gasket was indented (from the intake ports) only near the places where the upper manifold was pushing down. The bottom part of the manifold was made from 1/8th in. aluminum, so I think it was able to flex when I bolted everything to the car, and thus not making a solid seal against the intake ports.

I got a 1/4th in. thick aluminum sheet now which is MUCH more rigid. Hopefully this will solve my problem...