Well, Jyrki, the intake manifold is a intresting problem. I'm about 99.9% sure that everybody who has tryed to do anything with the intake has got it wrong, including me. The str8 is basicly two 4 cylinder engines back to back, which makes manifolding it properly, really hard. I'm sure that good manifold would make major power. The factory duals are very good, remarkably good considering when they were made, which makes me think that the boys in buicks engine r&d had something very different in mind than the engines that we got. The edmunds and other multi carb systems, like that 5-carb stewart(?) are going in the wrong direction. The engine, when properly built, like we've been talking about, will handle a lot more carb than 1 little 2bbl.
I've tryed everything with carter wcd 2bbl's. A very tuneable carb, looks correct, takes stock (looking) air cleaners, and when its all under the hood, most would bet it came factory that way. Not! The 263 I'm now building will take way more than 2bbl's can provide. 4bbl's are the only solution.
I've machined up finned side covers, finned valve covers and a host of other billet bits, so a intake did'nt seem that difficult. Hmmm...well now. Prehaps I was a bit optomistic. A 180 degree manifold seems to be the answer here, running a wcfb carter, looks period, and I had a few, including a very early matched 2-4bbl set. I want dual quads. A single 4bbl 180 would be easer to design and possiably work as good perhaps better but for major impact...well...
The manifold designs I came up with looked very much like the factory duals. This was machineable, weld the parts togather, presto!, dual quad intake. Cool, but not totaly there. The cross over tubes between the carbs are not there for mixture distribution but to smooth out pulsing in the system. This can be used as a advantage. When the intake valve closes a pressure wave is sent back up the intake, if it can be made to arrive in front of the next opening valve it will push the mixture into the cylinder. I happened to pull out a old dual quad manifold for a oldsmobile, the answer.!!! Like, in the last 2 days! I can not machine this design, it must be cast. My effort on the dual quad manifold was , I guess, practice. Back to the drawing board.
So what is needed in a manifold here is: carb heat, the engine will be no fun without it. Small plenum. No square corners. Smallish runners,1" X 1 1/4" apx. with no taper. Runners that start larger and get smaller act like a manifold with a big plenum and short runners. The opposite of what we need. Balance tubes between plenums placed in such a way that when the pulse is returning back up the runner it is routed to the next to fill cylinder and not the base of the carb. Long runners. That should get us 98% fo the way there. alleycat