Another site states only the 340 6 pak had a 355 CFM centre carby the 440 was 1350 in total using the below method from The Carburettor shop the four barrel equiv comes in at 954 CFM which is in the region of what some folks use recommending.
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.
This worked very well up through the mid-1960’s, when carburetor comparison tests became popular in car magazines. One carburetor company determined that the results could be skewed by rating their carburetors “dry” (air only), instead of the conventional “wet” (a non-flamable liquid with the density property of gasoline and air mixed). Rating the carburetor dry would add approximately 8 percent to the rating (example – a carburetor rated on the four-barrel rating scale at 500 CFM would now amazingly flow 540 CFM).
As the general public was unaware of the “wet” versus “dry”, this system worked fairly well until the mid-1980’s when it seems that other scales were “needed”. No attempt will be made to explain ratings of carburetors produced after 1980. The best way to compare these units would be throttle area (which was a measurement used back in the 1950’s and 1960’s); or more accurately, the comparitive area of the main venturi. On the later carburetors, best to write to the company, and ask at what vacuum the carburetor was tested, and whether it was tested wet or dry. If this information cannot be obtained, then the rating is suspect.
Info found at The Carbeuretor Shop LLC.
