Author Topic: Open Plenum vs closed  (Read 338 times)

Offline JohnnyPopper

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Open Plenum vs closed
« on: June 21, 2024, 12:28:18 PM »
I cracked a Qjet off a 60's GM motor, and found it is not an open plenum.

The primaries are isolated, the secondaries, have a common space in the gasket.

There appears to be a notch in each primary toward the front of the carb, it joins the two via the space between the primaries.

Problem is the closed plenum gasket is not to be found locally.

Question: Does it matter?
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline JohnnyPopper

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Re: Open Plenum vs closed
« Reply #1 on: June 26, 2024, 06:52:06 AM »
I was hoping for a historical observation, as the industry did a lot of things back then for various reasons.

Must be a stupid question. Thanks.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction

Offline bd

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Re: Open Plenum vs closed
« Reply #2 on: June 26, 2024, 04:35:48 PM »
 :P

The OEM gasket is probably half the thickness of the newer generation gaskets, too.  A proper kit should come with a new base gasket.  As long as manifold vacuum isn't ported to atmosphere because of base gasket differences, I doubt you will noticeably forfeit any performance.  Why don't you post some pics of the carburetor inverted with the old gasket in position? 
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)