Yes traditional CFM ratings were based on higher 3" vacuum signal. When 4v carbs hit the market they were rated at 1.5" of vacuum - which is a less restricted flow rating and indicates the flow with secondaries open.
But one can translate between the two systems:
The Carburetor Shop "To convert from one system to another (with a very small percentage of error) is relatively simple. Simply use the square root of 2 (1.414). Thus to convert a two-barrel rating into a four-barrel rating, divide the two-barrel rating by 1.414. To convert the four-barrel rating to a two-barrel rating, multiply the four-barrel rating by 1.414"
