Author Topic: Open Plenum vs closed  (Read 380 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

Online 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)

Offline 78 Chevyrado

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Re: Open Plenum vs closed
« Reply #3 on: July 23, 2024, 01:18:58 AM »
by open plenum do you mean single plane?

by closed plane do you mean dual plane?

if so single plane is for higher rpm with higher flow but at the cost of worse off the line low rpm acceleration.

Dual plane is for higher fuel mixture velocity at lower rpms like off the line acceleration.

If you wanna see an interesting intake manifold from back in that time period check out the offenhauser "dual port sbc" intake.  read the ads so they can explain how it was supposed to work.  they never caught on but should explain a lot of the whats and why about it.   or lookup single plane vs dual plane intake manifolds.
Kenny

1978 C-20, 350/400, 3.73, Graystone Metallic, Raceline Renegade 8 Wheels - 18x8.5, 275/70R18 BFG KO's