There is a simple formula to determine the correct size of carburettor to use.
Engine Size (CID) X Maximum RPM / 3456 = CFM @ 100% Volumetric Efficiency.
Example: 215 CID (3500cc) X 6000 RPM = 1,290,000 / 3456 = 373 CFM
Approximately 373 CFM would be required for this engine.
However, most street engines are capable of achieving only about 80% VE;
A modified street engine with ported heads,good intake exhaust and carburettor can achieve about 85% VE;
A fully modified race engine can achieve 95% or greater VE.
Some examples at 6000 RPM:
Rover 3.5 @ 80% VE = 298 CFM @ 95% VE = 354 CFM
Rover 3.9 @ 80% VE = 333 CFM @ 95% VE = 395 CFM
Rover 4.2 @ 80% VE = 361 CFM @ 95% VE = 428 CFM
Rover 4.4 @ 80% VE = 373 CFM @ 95% VE = 443 CFM
Rover 4.6 @ 80% VE = 384 CFM @ 95% VE = 456 CFM
Rover 5.0 @ 80% VE = 423 CFM @ 95% VE = 503 CFM
Volumetric efficiency is an indicator of how well an engine can breathe. The better an engine's "breathing ability" the higher its volumetric efficiency.
It is expressed as the ratio of the actual mass (weight) of air taken into the engine compared to the mass which the engine displacement would theoretically take in if there were no losses. The ratio is expressed as a percentage. It is quite low at idle and low speeds and varies with engine speed.
Volumetric efficiency should be computed at the expected operating RPM.
