I have an Offy Slant 6 4bbl Intake. Does anyone know if any of the other Slant 6 "off the shelf" intakes out there have a larger internal diamerter in the runner areas?

Reason being, I'm having a head ported and I see the intake as a restriction now. Especially with respect to the diamerter/area of the runners.

Thx.

All the aftermarket SL6 intakes have the same basic runner size.
The old Mopar Performance "M1" 2 bbl intake had slightly larger runner cross-section by about .040, which is not a lot.

The Clifford 4 bbl intake has the shortest runners and the largest plentum area so it is the best conventional carburator manifold for high RPM use.
The 3 side draft Weber set-up is the best flowing SL6 intake.
DD

Thanks Doc. I was afraid of that.

According to my measurements, the Clifford 4bbl intake has substantially larger runner cross section than the Offy. I think it's about 1mm larger in both directions.

Lou

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Home of Slant6-powered fun machines since 1988

Thanks Lou. Do you have those measurements handy so that I could compare them to my Offy? The runners on the Offy seem pretty thin, are the Cliffords any thicker for potential porting?

Was it Hanatayo who had an intake extrude honed? Would that open up the runners?

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Excuse me, does this smell like chloroform to you?

It would open up the runners but it is pretty expensive and there is not much material on the Offy intake.

These figures are for a stock head/port and a stock Offy intake. They represent on a percentage basis how much larger the heads intake port is vs the intake. It appears that our intakes are a restriction.

Cyl %
1 30%
2 26%
3 27%
4 30%
5 27%
6 32%

Did you measure the head at the port face or down the runner say 1/2". You might be surprised at the difference if you don't know already.

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Joshua

It was at the port face but either way only so much can flow through the intake before it gets there. Also, with a ported head the intake makes it even worse.

You only measured the port area at the head's port face where it is larger. Remeasure and recalculate and those area differences between the manifold and head ports will shrink. An inside caliper is handy for measuring the head ports since they do taper. I could do it since I have a bare head and an Offy intake on my work bench, but I'm stuck at work.

Not everyone that ports a head opens the port cross section at the manifold face. In fact that's probably one of the last places one would want to do port work.

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Joshua

I understand that but the intake manifold can only flow so much. It appears that the intake manifold has the same cross section for the entire runner length. Knowing that, head porting will make gains but now the intake is acting as the restriction.

Again, you are assuming that the intake port cross section is too small. Unless you flow test with and without the manifold you're just guessing.

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Joshua

If that was the case then a 318 intake (small port) would have worked fine on all of the other LA engines. Offy made large and small port intakes for those engines. My guess is the intake cross section is to small to feed the engine properly.

Exactly, your guess.