The pressure difference part of the measurement of CFM used in carb sizing is based on an assumption that when a 4 bbl is fully open, then the secondaries are suddenly closed, the level of vacuum "pull" would be doubled at that point, which could be true, but as time went forward, that "pull" will be somewhat lessened as the airflow thru the open throats increases as the engine continues to run. What any carb actually flows is purely dependent on what's sucking the air (engine displacement and rpms, the speed of the pistons moving down) these are all variables that affect how much air gets moved thru any carb. The cfm rating is only a rating, that's meant to be used for comparing different carbs, not the actual amount of air that's going to be moving thru it. The center carb on my 331's 3x2 setup flows about 350 cfm's at around 4500 rpms when the secondaries start to open, if you accellerate gradually. I based this on "doing the math", even that could be wrong too if it was ever bolted on a dyno where the airflow into it could be measured.
