[olddog]

As I understand it, the intake manifold can make a difference. A lot of factory dual plane intakes seal to the carb in the center. Therefore each bank of cylinders can only pull through 1/2 the carb or two barrels.

When the plenum is open under the carb, each bank can pull through all four barrels. So depending on the intake design, an engine can pull more or less cfm through the exact same carb, with more being with an open plenum.