CFM ratings are more accurate than physical carburetor sizes, as the CFM rating takes into account the venturi size of the carburetor. It is not uncommon for a given physical size (see Carburetor sizes paragraphs) to have many different internal venturi sizes. Early Stromberg and Zenith carbs could have as many as 9 different venturi sizes for a given physical size. Carburetor CFM ratings have been around since at least the 1920’s; however many O.E. (original equipment) carburetors never had published CFM ratings. I have not seen any actual agreement that stated that it had to be this way, but the early published ratings that I have seen for 1-barrel and 2-barrel carburetors were measured at 3 inches of mercury. This rating was about the amount of vacuum available on engines of the period under wide-open throttle conditions.
Sometime during the 1950’s, engineers found that a passenger engine with a four-barrel carburetor would not maintain a vacuum of 3 inches of mercury at wide-open throttle; and by some convention 1 ½ inches of mercury was chosen for rating 4-barrel carburetors. The ratings for 1-barrel and 2-barrel carburetors were left unchanged.
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.