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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.

Derivation

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 equation 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.

Carburetor selection

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 airstream 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.

Examples

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