73-87 Chevy _ GMC Trucks > Fuel Systems and Drivability
Open Plenum vs closed
JohnnyPopper:
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?
JohnnyPopper:
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.
bd:
: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?
78 Chevyrado:
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.
JohnnyPopper:
Thanks for the reply!
Looks like I'll need my thinking cap for your questions-still have one around 'I think'. Offenhauser made some 'out there' stuff!
Recap- GM Qjet, split, isolated intake ports. Right side sitting on a higher 'plane' than the left IIRC.
L & R primaries and secondaries, determined by the intake manifold, are isolated from each other.
In later models of Qjets, the base gasket commons all the intake ports by virtue of its open design. They are also very thick, which allows for a slight swirly swirl cross contamination (sic) of fuel/air before being devoured by the maw of a huge V8.
This is an earlier model, '66, and my question centered on whether to go with the stock, thin, isolating gasket, over the common, open, thick base gasket. BTW, the two secondaries of this gasket are common, though the swirly action must be greatly reduced due to the skinny profile.
Wondering if there were any historical prospective on why they chose to isolate the primaries, rather then the more common, current approach?
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