Andy was exactly right.

I'm going to expand on Andy's answer. Two barrel carbs are rated at 3" depression and four barrel carbs at 1.5" depression. That makes any two barrel carb's rating 1.414 times the rating if it was a four barrel. That's the square root of the ratio of the depressions.

So that 500 Holley 2-barrel is actually 500 / 1.414 = 354 cfm rated as a four barrel.

Back in the day the "how big" question was answered by magazine writers as [(cubic inches X rpm) / 3456] X volumetric efficiency.

That effectively resulted in a carb that wasn't too big, but as we have figured out, it left quite a few horsepower on the table.

There is a lot of power to be had in getting manifold vacuum to be less than the rating point of 1.5". The problem is that as the carb gets bigger the air velocity gets smaller and it becomes harder to get enough signal to meter the fuel properly. The carb becomes too big when it can't meter properly.

Most carbs have some sort of mechanism that responds to engine demand, so in effect the carb is only as big as the engine needs. Vacuum secondaries and spring loaded air doors are some of the ways to do this. On the Mopar six-packs, the outboard carbs were opened by vacuum. On most OEM dual four setups the carbs are set up to open sequentially with a progressive linkage.

R.