But a 2-barrel carb is rated at 3" of vacuum, 10% loss in manifold pressure, and roughly 10% loss in power.
4-barrel is rated at 1½" of vacuum.
Just because a carb has a rating on it doesn't mean it will flow that much. The typical 4-barrel will have a regulated secondary side. Weather it be a vacuum secondary, that only open when there is a strong enough vacuum in the venturi signal (not manifold vacuum). Or some other sort of velocity actuated.
Chrysler was selling a 340 with 1350CFM of rated carb in the showroom floor. You drove around on the 350 CFM center 2-barrel but if you got on it enough the vacuum signal would open the 2 outer 500 CFM carbs. Think of a Holley 1850 600CFM 4-barrel but with two sets of secondaries. The secondaries just happen to be in the form of other carburetor bodies.
This is the era of when the quadrajet came out. In a single carb you got the drivability of a small 2-barrel carb, but the airflow potential of a 6-pack with those huge secondaries. The secondaries were self regulating, only opening enough to match the demands of the engine. I once had one factory installed on a 3.8 liter chevy V6 boat motor. No way could that 1982 anchor of an engine ever thing about using that much airflow potential, but GM made it standard.
So yes, there is math that says what should happen. But reality doesn't mean that those numbers are absolutes.
Back to the carb airflow at vacuum ratings. If you get the big Holley book out and look up the specs on the 750 4-barrel double pumper, and the 500 CFM 2-barrel. Look at everything. Throttle bores, venturi size, booster size, everything that has anything to do with airflow. They are exactly the same. The difference in ratings is the pressure drop through them. Put a 750 double pumper on, run it hard enough to pull 3" of vacuum and you will be pulling 1,000 CFM of airflow through the carb. Ain't that a trip? The CFM rating isn't a block wall, it's just a rating at a given pressure drop. It can flow less, or more than it is rated for.
Now there is a thing with too large. For the venturi to work there has to be enough velocity across it. and to get the velocity there has to be a differential in air pressure. And the benifit of EFI, you can go oversize on the throttle body, have no real drop in air pressure as it enters the engine, fuel is squirted in under pressure instead of sucked in. And it doesn't care.
