https://www.ebay.com/itm/MAN-A-FRE-292-2...=item2f145f9430

Does anybody know anything about these?
The manifold is pretty self explanatory.

Mostly wondering about the modified carbs, and what was done to them.
Thanks

The carburetors are basically GM Rochester 2bbl units. I'm not sure what has been done to them, but probably the idle circuits have been disabled in the two end units. That was the usual procedure back when the "three deuce" setup was popular.

Except for the balance passage (visible as a small hole in the cross-over webbing) the carburetors are semi-IR like the original Corvette. This requires idling on all 3.
Typically the center carburetor feeding cylinders 3 & 4 needs slightly richer jetting due to the difference in firing intervals between its 360-360 and the end pairs 240-480 intervals (as Jaguar detected 60 years ago).
If those are common 2G carbs, they are simultaneous operation and will have all 6 throats open @ WOT.
Yes, this is considerably larger than he Corvette YHs but the needs of a pair of cylinders are not 1/3 of an engine unless there is a substantial plenum.
Anyone considering using 3 X 2G: although these were rated at 3" Hg I doubt it will pull that much. This means the high speed mixture will lean out as RPM rises, requiring a HSAB reduction (not just a larger main jet).

I know that is a lot of money for a manifold and 3 carbs that need to be rebuilt. You could do a new set of Webers for that.

IMHO it would also have better "manners" (tip-in response) using a set of progressive 2 bbls. like the 5200, so that the part throttle doesn't expose so much venturi area. The Harper is the same idea, but with progressive carbs - but the carbs are $$$.

I am somewhat certain that I have this right, that manifold was originally for a Pontiac but would fit the chevy. Notwithstanding, it was not a favorite because it did not work all that well. The carbs were too small as a result of the tiny tiny passage between the carbs. As noted above, turned the manifold into basically a independent runner manifold. So rare yes, but not all that good performance wise. The Dos Palmos intake being the better choice or a Clifford 3x2 ram if you needed one for race purposes.

I have about $900.00 into an offy, 2 weber 32/36 carbs, adapters and linkage. all brand new! Clifford has dual 38/38's with intake and linkage for $1500.00, again, brand new and ready to bolt on! unless it is the nostalga from a 'rare' intake, why bother? for what it's worth, that has been listed for at least a year and ebay prices on chevy 6 parts is usually high, I could probably sell mine at a profit if I waited long enough. auctioned 2 year old headers and nearly doubled what I paid new.

For someone trying to run 3 X 2 Rochester 2G: there are 2 different SAE bolt patterns and throttle plate sizes, sometimes referred to as small (1-7/16") = Chevy V8, large (1-11/16") = Oldsmobile.
In the larger 2G there are 4 venturi sizes 1-3/16" to 1-3/8" rated 352 to 435 CFM @ 3.0" Hg.

Rough approximation for a cylinder pair with 1 X 2G:
CFM = engine size ÷ 3 (78.5" for a 235 etc.) X VE (use 90% or less) X RPM (5,000 or less) X 3.3 ÷ 3,456
78.5 X .90 X 4,500 X 3.3 ÷ 3,456 = 304 CFM @ 1.5" Hg (430 CFM @ 3.0" Hg)
Hot 261: 87 X .95 X 5,000 X 3.3 ÷ 3,456 = 395 CFM @ 1.5" Hg (558 CFM @ 3.0" Hg)

Panic, This is somewhat OT but I'll ask anyway.
I'm building a 175 ci four cylinder. I want to run a pair of Zenith 28s on it. They have replaceable venturies that are available in 32mm, 30mm ,29mm, & 24mm. The pair I have were taken from a GMC small port Edumonds intake with linkage so I assume they were run as a pair likely on a 248 or 270. They currently have 30mm venturies. I'm trying to decide if they need to be changed. My use of you rough approximations come up with 224cfm to 265cfm @1.5" Hg based on your 235 & 261 calculations. Now I need to figure which venturies will do that.

You'll notice the "3.3" correction in my math. This estimates how much greater area is need for less than 7 cylinder fed by the same carburetor(s) in common. This is the maximum correction, it decays with runner volume. The correction for the common CFM formula by cylinders served by a single source (1 or more throats in common):
1 cylinder 6.6
2 cylinder 3.3
3 cylinder 2.2
4 cylinder 1.6
5 cylinder 1.3
6 cylinder 1.1
7 or more cylinders 1
If the carburetors are large enough that the WOT vacuum drops to 1.5" Hg multiple CFM by .707.

If you have paired intakes (even if not siamese), not a common plenum, the 3.3 factor is correct. If on the same plenum, use 1.6.
If this were a parallel twin motorcycle (giant Triumph) it would be 87.5" or 1434cc using a single carburetor. Even the 32mm is much smaller than optimum (some 650cc twins used 2 X 32mm).

Sorry, I can't guess at the Zenith CFM.

OK, Thanks. I have a few more of these carbs around, I'll look at what is in them. I have had a hard time finding venturies so if it turns out they are not too expensive I may get some of each size to play with once the engine and car are ready. It's not tough to change them and it's easy to make them bigger if need be the carb body being the limiting factor. These carbs were used on GMC trucks with 228 and 248 engines and on Internationals with 261s. I'll have to look at their flow rating.

What manifold? Have you measured the throat opening and bolt flange distance?

It is an Offenhauser 3X1 for 194-292 Chevy six with the back cut of to make it a 2X1 for a 153 Chevy four.

As panic has indicated the whole point of the design is individual runners with balance tubes. The first pieces were called intake plates - built for SBC and later for BBC. The name is supposedly derived from Manifold Free Port Induction
There are some that hail from Conoga Park, CA. And other pieces were built in Atlanta, GA from a different casting(perhaps several iterations) (image from Steve's Nova Site)

Here's the CA design from Jalopy Journal

Mentioned here at Curbside Classics

Some info regarding Robert Patrick's patent on the design can be found here:
The Limited Monopoly Patrick hailed from Atlanta.

IIRC the sales pitch was the very short distance from the throttle plates to the intake valve made response excellent and (claimed) better mixture distribution.
They sort of ignored the fact that one 2 bl. served both the #5 & 7 cylinders, which fire only 90° apart, so are worse than conventional manifolds in that respect.

To clarify: one reason why I think the response should really be better is not only that the distance is really short (like a Hilborn stack), but that the "buffer" (plenum) volume, which damps the vacuum pulse, is tiny.
When the intake valve cracks open, this pressure differential is always diluted by the empty space between the valve and the venturi. At WOT the vacuum is only 1.5-3.0" Hg, so any reduction at all slows fuel discharge from the main nozzle.

The 5-7 pairing also enriches mixture from the carburetor - a staggered sequence has proven to be stronger at the nozzle than more equal intervals. The carburetor intervals with conventional SBC V8 1 8 4 3 6 5 7 2 firing order:
1-3 (left front pair) 270° - 450° - 270°
2-4 (right front pair) 270° - 540° - 270°
5-7 (left rear pair) 90° - 630° - 90°
6-8 (right rear pair) 450° - 270° - 450°

{breathe}

A picture helps me to visualize this:

In comparison a dual plane (180 degree) manifold typically feeds 2 - 8 - 3 - 5 from passenger side plane and although 3 and 5 are adjacent in the layout they are at opposite ends of the upper plenum:


This is a great thought exercise around intake design principles and the role of firing order and intake dynamics.

But now lets pivot back to the I6 intake in question:

with the Chevy firing order:


Which helps to visualize the issue/difference with the center carb tuning: