The poster child for the L6 with 2 X 1 or 3 X 1 is the Jaguar XK-E/Mk IX etc. 231" engine. The sedan engine uses 2 X 1-3/4" (or larger) S.U. carburetors, divided (as said) into separate banks, with 1 carburetor supplying the front 3 cylinders and the second the rear three. The L6 firing order (1-5-3-6-2-4) divided these groups evenly at 240 degrees. This is not possible with the stovebolt (or any siamese intake) engine because the 3-4 center cylinder pair shares a common port.
The performance car used 3 of the same size carburetors, but divided differently. 1 carburetor supplied each adjacent pair of cylinders: 1-2, 3-4 and 5-6. This didn't produce equal intervals, but each cylinder was nearly doing the opposite of its mate. It can be used on the stovebolt even though the ports are merged.
Stovebolt (and other L6 siamese) size selection:
2 X 1 or 2 X 2 must use a common plenum manifold, which means that all carburetors feed all cylinders. Easy to use too large carburetors since the engine sees only the total of both.
3 X 1 or 3 X 2 may use a common plenum manifold, with the same warning: easy to choose too big, progressive linkage helps.
3 X 1 or 3 X 2 may also use 3 individual manifolds like the Corvette (sometimes with a small vacuum connection to steady the idle), in which each carburetor only feeds 2 cylinders. Too small is possible here.
The famous CFM formula: CFM = D X VE X RPM / 3,456 is only nearly correct as the total for carburetors joined on the same manifold.
3 X 1 individual carburetors are NOT 1/3 of this total, but MUCH LARGER.
