Exhaust Manifold Mod

[slantsixdan]

I've read all along that "supposedly" the Super Six two barrel option got a larger outlet exhaust manifold

Nope, that didn't happen. There was never a 1bbl/2bbl difference in exhaust manifolds in the US and Canada.

I've seen people say that some Super Six packages got a larger 2.250" head pipe

Supposedly all Slant-6s got a 2-1/4" headpipe starting in '77. Many did; some didn't.

makes zero sense to put a bigger head pipe on and not have a larger outlet manifold.

Makes a lot more sense, from an engine-breathing perspective, to put a 2-1/4" headpipe on a 2"-outlet manifold than to put a 1.75" headpipe on a 2"-outlet manifold. The factory did both, because the latter makes more sense from a pinch-every-cent-til-it-screams perspective.

 

[RustyRatRod]

Nope, that didn't happen. There was never a 1bbl/2bbl difference in exhaust manifolds in the US and Canada.




Supposedly all Slant-6s got a 2-1/4" headpipe starting in '77. Many did; some didn't.




Makes a lot more sense, from an engine-breathing perspective, to put a 2-1/4" headpipe on a 2"-outlet manifold than to put a 1.75" headpipe on a 2"-outlet manifold. The factory did both, because the latter makes more sense from a pinch-every-cent-til-it-screams perspective.

And yet, the largest slant 6 manifold outlet I've measured is 1 7/8. But yeah the factory pinched and pinched.

 

[slantsixdan]

I never measured one sub-2", back when I was measuring. ¯\_(ツ)_/¯

 

[RustyRatRod]

I never measured one sub-2", back when I was measuring. ¯\_(ツ)_/¯

I have maybe ten here and none measure 2". But then really, it's kinda splittin hairs. lol

 

[slantsixdan]

Y'all reminding me of the "extrude hone" process that had a moment in the sunshine of various magazines thirty years ago. Looks like it's still a thing. But boy, it'd make a hassle with the heat control valve: dismantle, which requires destruction…protect bushings…then reassemble with new parts which would require hunting down expensive/scarce NOS parts. This same hassle would exist with "acid porting"

…but now I think on it some more, seems to me it ought to be possible to do an extrude-hone on a Slant-6 exhaust manifold without having to dismantle or otherwise futz with the heat riser. All it would take is a way to block off the central collector/heat riser area of the manifold so the honing putty wouldn't reach/circulate in that area. Probably wouldn't even have to be any kind of super-strong material, as long as the top (intake manifold bolt-to surface) were blocked off, because then that would make the upper part of the centre area a dead-end zone, so there would be minimal putty movement there in the central area.

So: what might work? Maybe it could be as simple as stuffing the hotbox with aluminum foil from above, then bolting on the blockoff plate where the intake manifold would usually go, then flipping the manifold over and stuffing the hotbox with foil from below and ramming it firmly. Then the mind-movie gets a little fuzzy, but maybe…spray some truck bed liner material or other tough coating over the aluminum foil (again, going through the headpipe bolt-up) to seal off the hotbox reasonably well. After the extrude-hone process is done, then the foil could be removed, most of the bedliner chipped/scraped away, and the remainder would just burn off.