Carburetor sizing and CFM

Carburetor sizes – 1 barrel | Carburetor sizes – 2 barrel | Carburetor sizes – 4 barrel

CFM ratings

Carburetor sizes – 1 barrel

The Society of Automotive Engineers developed standards for carburetor physical sizes. The more common sizes found on automobiles are listed below. Bore size is listed as approximate, as some carburetors were much more efficient than others.

Nominal ½ inch or 5/8 carburetor, center to center 1 13/16 on mounting bolts, bore size approximately 13/16 inch.

Nominal ¾ inch or 7/8 inch carburetor, center to center 2 ¼ on mounting bolts, bore size approximately 1 1/16 inch.

Nominal 1 inch carburetor, center to center on mounting bolts 2 3/8, bore size approximately 1 3/16 inch. (Often referred to as size 1).

Nominal 1 1/8 inch carburetor, center to center on mounting bolts 2 3/8, bore size approximately 1 5/16 inch.

Nominal 1 ¼ inch carburetor,center to center on mounting bolts 2 11/16, bore size approximately 1 7/16 inch. (Often referred to as size 2).

Nominal 1 3/8 inch carburetor, center to center on mounting bolts 2 11/16, bore size approximately 1 9/16 inch.

Nominal 1 ½ inch carburetor, center to center on mounting bolts 2 15/16, bore size approximately 1 11/16 inch. (Often referred to as size 3).

Nominal 1 ¾ inch carburetor, center to center on mounting bolts 3 5/16, bore size 1 15/16 inch. (Often referred to as size 4).

Nominal 2 inch carburetor, center to center on mounting bolts 3 9/16 inch, bore size approximately 2 3/16 inch. (Often referred to as size 5).

Carburetor sizes – 2 barrel

This information to be loaded at a later date.

Carburetor sizes – 4 barrel

This information to be loaded at a later date.

CFM ratings

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.

So, to give an example of the 4 ratings scales prior to the mid-1980’s:

Using a Carter 4-barrel AFB carburetor that is rated wet and rated 500 CFM for an example.

To convert this rating to the 4-barrel dry scale, add 8 percent (multiply by 1.08). 500 CFM multiplied by 1.08 is 540 CFM.

To convert this rating to the 2-barrel wet scale, multiply the rating by the square root of 2. 500 CFM multiplied by 1.414 is 707 CFM.

To convert this rating to the 2-barrel dry scale, first convert to the 2-barrel wet rating as above, then add 8 percent. 707 CFM multiplied by 1.08 is 764 CFM.

Going the other direction, if we take a Rochester 2-GV carburetor that is rated wet and rated 435 CFM.

To convert this rating to the 2-barrel dry scale, add 8 percent. 435 CFM multiplied by 1.08 is 470 CFM.

To convert the rating to the 4-barrel wet scale, divide the rating by the square root of 2. 435 CFM divided by 1.414 is 308 CFM

To convert the rating to the 4-barrel dry scale, first convert to the 4-barrel wet rating as above, then add 8 percent. 308 CFM multiplied by 1.08 is 333 CFM.

These conversions will give a fairly accurate number. If one must be exact on a given carburetor, then a “flow-test” on that carburetor will be required.