The Carburetor Shop LLC
204 East 15th Street
Eldon, Missouri 65026
Aftermarket 4-barrel selection
The first criteria in selecting an aftermarket 4-barrel carburetor for the street is to determine one’s existing manifold type. Here is a link that explains 4-barrel manifold types: Manifold types
Once the type of manifold is known, one can then determine the size of carburetor necessary for the engine.
Be aware that, by the late 1960’s virtually ALL street engines of 350 CFM or more were equipped by the original manufacturer with the spread-bore design. This is because the spread-bore design is generally more efficient in the lower RPM ranges, resulting in MUCH BETTER fuel economy, as well as better lower RPM driveability.
Carburetor aftermarket size selection criteria – street
Most car enthusiasts are familiar with (or have at least heard of) the following equation:
CFM = (RPM times CID) / 3456
This equation will give the required CFM for a multi-cylinder four-stroke internal combustion engine consisting of at least four cylinders (it may be used for fewer cylinders, with modifications) and a common plenum area for the intake; with a volumetric efficiency of 100 percent.
Unfortunately, many do not understand either the derivation of the equation OR its correct usage.
CFM is airflow of the engine measured in cubic feet per minute.
RPM is the speed of the engine measured in revolutions per minute.
CID is the displacement of the engine measured in cubic inches.
The constant 3456 is more easily understood, and much more easily remembered if it is re-written (12 x 12 x 12 x 2). Note that the dimensions on the left side of the equation are in cubic FEET, and the dimensions on the right side of the equation are in cubic INCHES. As there are 12 inches in a foot, and we must then multiply 12 by 12 by 12 to keep the dimensions the same. The “2” is required because of the 4-stroke engine only drawing air on ever other revolution of the engine. The equation assumes 100 percent volumetric efficiency of the engine (more on this later).
Use/misuse of the equation
Many enthusiasts will take an arbitrary RPM that would be a “dream” RPM for their engine at WOT (wide open throttle), ignore the volumetric efficiency, and buy the next larger carburetor to the figure derived by plugging this dream RPM and the engine CID into the equation. This WILL yield less than desirable results for most street engines!!
One should be reasonable in the maximum RPM figure used. If one doesn’t know, then one may concern a “Motors” manual in the reference section of one’s local library for WOT rated RPM. Once a CFM figure is obtained at 100 percent volumetric efficiency, apply an estimate of the volumetric efficiency of your engine to this figure. Examples will follow later. Some sample estimates of volumetric efficiency of various engines:
Pontiac V-8 400 CID Ram Air IV approximately 90 percent
Pontiac V-8 400 CID Ram Air III approximately 85 percent
Pontiac V-8 400 standard 4-barrel engine approximately 80 percent
Most 2-barrel V-8s and most USA-built 6-cylinders 75 percent
Once the 100 percent figure is obtained, the actual need may be obtained by multiplying the 100 percent figure by the appropriate percentage.
Since the necessary CFM varies as the RPM varies, it should be obvious that the equation may be used for both cruise, and WOT to obtain CFM requirements for both conditions.
The figures obtained above will refer to CFM using “wet” ratings. Remember that there are several different rating systems – see CFM RATINGS
When selecting the carburetor, use the WOT rating to determine the carburetor’s maximum airflow, and the cruise rating to as much as possible maximize airflow in the primary side of the carburetor at cruise. Why is this important? The two major factors in atomizing the fuel in the air stream are heat and air velocity. Given the same airflow requirement for carburetors of different size, it should be obvious that air velocity would be less in the larger carburetor. This usually requires that if the primary side of a four-barrel is too large, that extra fuel must be provided to provide the proper mixture. This is simply a waste of fuel. The lower air velocity will also create a “sluggish” engine at lower RPM’s. Often this issue can be better solved by the use of a spread-bore carburetor rather than a square-bore carburetor for engines of wide RPM range.
While this article is meant to be “brand-independent”, since I am most familiar with Carter and Rochester 4-barrels, primary and secondary flow data for popular Carter aftermarket carburetors is provided in the CFM Ratings article mentioned above. However, UNLESS the aftermarket carburetor manufacturer specifically states that a specific carburetor is designed for replacement on a specific engine (example, the Carter 9400s was originally calibrated for a Chevrolet 283); EXPECT to re-calibrate the carburetor. Thus one’s familiarity (or willingness to learn) with a given carburetor manufacturer/design should be a factor in one’s decision.
Pontiac 400 CID basically stock with factory 2 barrel
CFM = CID * RPM / 3456 = 400 * 4600 / 3456 = 532 CFM at 100 percent VE
From estimate chart VE = 75 percent, or actual required CFM = 532 * 0.75 = 399 for a two-barrel
By adding a four-barrel and dual exhaust, the WOT RPM would increase to 5200, while the VE would improve to 80 percent
Thus for our modified 400, necessary WOT CFM = 400 * 5200 *.8 / 3456 = 481. Pontiac used a Carter 550 CFM as original equipment, which is entirely adequate for this application.
Calculating for cruise: CFM = 400 * 2200 * 0.75 / 3456 = 204 CFM on the primary side
Chevrolet 283 basically stock with factory 2 barrel
CFM = [CID * RPM / 3456] * 0.75 = [283 * 4600 / 3456] * 0.75 = 282 CFM for a two-barrel
By adding a four-barrel and dual exhaust, the WOT would increase to 5200, while the VE would improve to 80 percent
Thus for our modified 283, necessary WOT CFM = 283 * 5200 * 0.8 / 3456 = 341. The original Carter/Rochester four barrels flowed approximately 450, which was more than adequate.
Calculating for cruise CFM = 283 * 2200 *0.8 / 3456 = 144 CFM on the primary side
AGAIN, FOR BEST RESULTS, ONE SHOULD BE HONEST IN ONE’S ASSESSMENT OF THE ENGINE RPM AND VOLUMETRIC EFFICIENCY. ONE MAY LIE TO ONESELF, AND ONE’S BUDDIES; BUT ONE CANNOT LIE TO ONE’S ENGINE!
Finally, there is an old story here in the Missouri Ozarks which, unfortunately, is ignored by many enthusiasts when choosing the correct carburetor size. Seems like a young hillbilly reached his 16th birthday having never owned a real pair of shoes. His uncle told him to go into town, and come up with a new pair of shoes, and the uncle would buy. So, much later in the day, the youngster comes walking back from down, with the shine of new boots visible through the dust of the dirt road. His uncle asked what size he bought, and the youngster replied "wal, the salesman thot I oughter wear a 9, but 10's felt so good, I got 11's!".
If acquiring a carburetor for a street engine, give the above some thought!