An engine is nothing but an air pump. Every two turns of the crankshaft, in theory, it flows the volume of air the pistons displace. In practice, that figure is impossible, due to various fudge factors such as turbulence, the shape and size of the intake and exhaust passages, AND- - - -the efficiency and size of the carburetor. To figure the absolute maximum "cubic feet per minute" (CFM) an engine can flow, multiply half the cubic inch displacement by the RPM you want to run, which gives you "cubic inches per minute". Divide that figure by 1728 (cubic inches in a cubic foot) and you get "theoretical CFM" For a more realistic figure, use 90% of that number as a general rule of thumb. For instance:
half of 261 cubic inches = 130.5 X 4000 RPM = 522,000 cubic inches per minute
Divide by 1728 = 302 cubic feet per minute 90% of 302 = 272 CFM
Ideally, a carburetor capable of 300 CFM would be all a 261 engine at 4K RPM would ever need. "Bigger" is definitely not "better" where carburetors are concerned!
Instead of building a carburetor with one big barrel, putting two smaller ones side by side will probably give better throttle response, and better fuel mixing, since the velocity through each venturi will be higher, and carburetors mix fuel and air by creating a vacuum in the venturi that pulls fuel through the jets and mixes it with the air stream. To fit a 2 barrel carb to a stovebolt single barrel intake requires some creative milling and welding, since an adapter from a 2 barrel carb to a 1 barrel manifold creates all sorts of problems.
Now- - - - -how about using two small carburetors on a custom manifold? That opens up a Pandora's box of unintended consequences. An inline six needs either one carburetor- - - -or three- - - -one for each pair of cylinders- - - -particularly if it only has three intake ports like a stovebolt. The biggest mistake most engine builders make is "too much camshaft" and/or "too much carburetor". Doing race engine stuff on a street engine is beyond stupid!
Jerry
