L98 Intake Vacuum Diagram
Thread Starter
Safety Car
Joined: Nov 2008
Posts: 3,726
Likes: 558
From: Seattle Washington
L98 Intake Vacuum Diagram
I'm chasing a whistling Vacuum issue. Usually easy to find but in this case the sound comes from everywhere and can't nail it down. In this case I can pressurize the Intake threw the Hose that sucks from the Left Valve Cover and I can get a reverse Whistle. Not that it makes much difference, but this is a Accell Superram. When I removed the Plenum I Duct Taped the Runners shut. When I pressure's it now it blows the Tape off the Runners threw the Intake Ports. I don't understand just how it could do this. I have a Stock L98 lower manifold to study but that's not telling me much. I can see that the Vacuum from the Port to the Valve Cover runs up the right Runner to the Plenum. But where else might it go ? The Interwebs are not producing a diagram of the internals of the L98 Intake.
Drifting
Joined: Jul 2022
Posts: 1,805
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From: Central Florida
Sorry if this doesn't help I am just trying, What year? apparently the 85 is diff than the cold start years.
Perhaps internally it shares the gasses from the PCV with the other runners. If I remember correctly just inside the runners there were small holes. I am not sure if these were EGR passages or PCV or both. Maybe all of those gases were considered the same (waste) and shared between cylinders to keep any one cylinder from bearing the brunt of all the crankcase gases. The below conversation even talks about the IAC being connected by passages.
this is an interesting read
https://www.corvetteforum.com/forums...e-runners.html
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.
Perhaps internally it shares the gasses from the PCV with the other runners. If I remember correctly just inside the runners there were small holes. I am not sure if these were EGR passages or PCV or both. Maybe all of those gases were considered the same (waste) and shared between cylinders to keep any one cylinder from bearing the brunt of all the crankcase gases. The below conversation even talks about the IAC being connected by passages.
this is an interesting read
https://www.corvetteforum.com/forums...e-runners.html
.
.
Last edited by Vets-Vet; Sep 9, 2025 at 08:37 AM.
Thread Starter
Safety Car
Joined: Nov 2008
Posts: 3,726
Likes: 558
From: Seattle Washington
I'm glad that I'm not the only one confused. And your link provides lots of info and yes, more confusion. I have two Superram intakes the one that is on the Car is using the Accel base manifold and my spare one is using a factory base that somebody has ported to match. Not sure of either, year but odds are its not an 85.
When I blow air into the PCV Port I'm getting Air coming up threw the small ports in the runners. My AFR Heads do not have EGR ports, and I have no EGR Valve its been blocked off. I think I will fill in the Small Ports in the Runners to stop any air movement in any direction as my engine combination shouldn't need any.
I was going to start pouring water down my factory manifold just like somebody else did and see what goes where.....
Thank you for the link to the thread.
When I blow air into the PCV Port I'm getting Air coming up threw the small ports in the runners. My AFR Heads do not have EGR ports, and I have no EGR Valve its been blocked off. I think I will fill in the Small Ports in the Runners to stop any air movement in any direction as my engine combination shouldn't need any.
I was going to start pouring water down my factory manifold just like somebody else did and see what goes where.....
Thank you for the link to the thread.
Thread Starter
Safety Car
Joined: Nov 2008
Posts: 3,726
Likes: 558
From: Seattle Washington
The vertical port on the right side of the Manifold is directly tied into the EGR port. It is then tied into the right cylinder head and not the left head. It is then tied into the small runner port and then into the Plenum.
Thread Starter
Safety Car
Joined: Nov 2008
Posts: 3,726
Likes: 558
From: Seattle Washington
The above picture is from the left side the angled port. This is tied into the PCV port and the small Runner Port. If you look in the right side of the Intake Port you will see a small hole. This is where the vacuum comes from all 8 of them have this small hole. What is confusing is that the Small Runner Port is also tied into this and it runs up to the Plenum. So, the Vacuum source is coming from both the 8 small holes and the Hole up in the Plenum? This would also mean that the related thread that describes that both the small ports in the Runners are EGR gas going up to the Plenum is wrong. The way I see the right one is EGR going up and the left is Vacuum going down. Why GM decided to get Vacume from both the Plenum and the 8 small ports is odd.
Thread Starter
Safety Car
Joined: Nov 2008
Posts: 3,726
Likes: 558
From: Seattle Washington
For those of Us with a Superam Manifold they didn't bother to use the left Small Port on the Runners. It just dead ends when it gets to the Plenum. I don't have a Factory Plenum to compare. Makes one wonder why they put the small port in the Runners? So the joint where the Runners meet the Lower Manifold is a known source of a vacuum leak all for no reason. ( at least for me ) Just block it off it serves no purpose. This should also confirm that the Left Runner small port is not EGR.
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