396 Intake Manif. 3866963 Carb Mount Plenum/Gasket Details

[Wayne M.]

I believe the 3-hole plenum configuration is typical factory for 1965 396'ers. But the horizontal surface cast (not machined) about 0.11" deep, joins the 3 holes below the carb base plate. This would make a 3-hole (manifold match) or 4-hole (carb base match) gasket useless for sealing purposes between the carb bores.

So my question is whether the square plenum gasket shown would be factory correct type for this service.

[Stuart F.]

Re: 396 Intake Manif. 3866963 Carb Mount Plenum/Gasket Details

Wayne;

Believe all your assumptions are correct. My preferred source for specific carburetor base gaskets has been "Dr. Rebuild" as they go to great lengths to get theirs exactly like original, whereas most of the others handle individuals and/or kits with generic gaskets - some of which are not correct. If you've ever had an unusual vacuum leak that you finally traced to an improperly fit base gasket, you'll know what I mean. My 63 L-76 is a case in point as it has a unique gasket that only ol Doc has correct. For the 1965 425 hp Gasket, he lists: "also for 65 pass with Sp HiPer. No individual bores, instead one large rectangular bore that matches the intake. No heat slot used."

Not having been an engineer on air/fuel flow, I can't begin to guess why your manifold was designed as it is. my 63 has the same 3 hole configuration, but every gasket for it has 4 holes. It probably has something to do with the exact vacuum signal they were looking for. Back in the day, with dual quad Vette engines, the 57 and later manifolds had a cross cut between one primary bore and one secondary, with the front carb base opposite that of the rear carb base. I was running my engine w/o exhaust manifolds one night (kid stuff) and noticed the flame color indicated that two cylinders were running leaner than the rest. In studying the manifold design I concluded that there was a deficiency in it that was causing that. My solution was to use five 1/4" fiber gaskets together under each carb with these same cross cuts which, in effect, packed up more manifold area feeding these two cylinders. That did the trick, and that is one reason why my little 57 post Bel Air could turn near 13 flat @ 106 mph with street tires. Several months after making this modification, I read about it in Hot Rod magazine where an Engineer analized the same problem and came to the same conclusion as I had - five 1/4" spacers w/cross cuts.

Stu Fox

[Clem Z.]

Re: 396 Intake Manif. 3866963 Carb Mount Plenum/Gasket Details

it has been a long time but i don't remember my new 65 396 corvette manifold cut like that. you will need to use a single "open hole" gasket with that manifold or you will suck gasket parts into the engine. holley part # 108-10

[Joe L.]

Re: 396 Intake Manif. 3866963 Carb Mount Plenum/Gasket Details

I believe the 3-hole plenum configuration is typical factory for 1965 396'ers. But the horizontal surface cast (not machined) about 0.11" deep, joins the 3 holes below the carb base plate. This would make a 3-hole (manifold match) or 4-hole (carb base match) gasket useless for sealing purposes between the carb bores.

So my question is whether the square plenum gasket shown would be factory correct type for this service.

Wayne-----


Yes, of the general configuration you show and of GM #3868827. The latter was discontinued in February, 1968 and replaced by GM #3881847. GM #3881847 was then discontinued in December, 1973 and replaced by GM #682914. Finally, GM #682914 was discontinued without supercession in September, 1984. How were the supercessive gaskets different than the 3868827? I don't know.