Carburetor CFM Calculator

The calculator above returns the airflow your engine can actually swallow at peak rpm. Two things routinely make that number wrong in practice: the efficiency figure you feed it, and the rating standard stamped on the carburetor you then go and buy.

The formula, and where 3456 comes from

A four-stroke fills each cylinder once every two crank revolutions, so it consumes half its displacement per revolution:

cubic inches per minute = CID x RPM / 2

A cubic foot is 1,728 cubic inches, so dividing again converts it to CFM:

CFM = CID x RPM / (2 x 1728)
CFM = CID x RPM / 3456

3456 is not a magic constant. It is 1,728 x 2, and it is only correct for a four-stroke — a two-stroke fills every cylinder every revolution, so its divisor is 1,728. Add volumetric efficiency and you have the working form:

CFM = (CID x RPM x VE) / 3456

Volumetric efficiency is the guess that decides the answer

Displacement you know. Peak rpm you choose. VE you estimate — and because it multiplies the result, your estimate lands in the answer at full strength. Here is a 302 at 7,500 rpm with nothing changing but that one figure:

Same engine, same rpm: 459 to 721 CFM, a 57% spread decided entirely by a number most people copy off a forum post. If you are unsure, an honest 80% for a decent street engine beats an optimistic 100%, because the failure modes are not symmetrical — and the next section explains why erring high is more dangerous here than it is with injectors.

A 600 CFM two-barrel is not a 600 CFM four-barrel

This is the one that quietly ruins carburetor selection, and almost no sizing calculator mentions it.

CFM is measured on a flow bench at a set pressure drop, and the two carburetor types are not measured at the same one. Four-barrels are rated at 1.5 inches of mercury. Two-barrels are rated at 3.0 inches — double the depression, which produces a substantially bigger number for the same hardware.

Flow rises with the square root of the pressure ratio, the same relationship that governs injector flow against fuel pressure, so the two standards differ by the square root of 2:

flow at 3.0" Hg = 1.414 x flow at 1.5" Hg

to compare a two-barrel against four-barrel figures:
four-barrel equivalent = two-barrel rating / 1.414

Work the calculator’s own example through it. You need 524 CFM, which is a four-barrel figure because that is the convention this formula is quoted in. You buy a 600 CFM two-barrel, believing you have 15% of headroom. In four-barrel terms that carburetor is:

600 / 1.414 = 424 CFM

You are not 15% over. You are 19% under, and the engine will sign off at the top of the rev range while the box on the shelf says otherwise. Going the other way, a 500 CFM four-barrel would be advertised as roughly 707 CFM if anyone rated it the two-barrel way.

Rule: only compare carburetors of the same barrel count, or convert first.

Carburetor CFM chart by displacement

Four-barrel figures at 6,000 rpm, for a quick sanity check against the calculator:

Raise the rev limit and every figure scales with it in a straight line. A 350 that pulls to 7,000 rather than 6,000 needs 567 CFM at 80% VE, not 486.

Why the next size up is not automatically the right answer

Round up to the nearest carburetor you can buy, but do not round up twice for comfort. A carburetor is not a tap that simply passes more air when it is bigger — it depends on air velocity through the venturi to create the vacuum signal that draws fuel.

Fit venturis that are too large and velocity falls at low and mid rpm, the signal weakens, and fuel atomisation suffers. The symptoms are soft throttle response, hesitation, and a bog when you open the throttle quickly. The engine keeps its peak power and loses everything you actually drive on.

This is where carburetors differ from injectors. An oversized injector costs you idle resolution and can often be tuned around. An oversized carburetor has no tune — the physics of the venturi is the physics of the venturi.

FAQ

What does CFM mean on a carburetor?

Cubic feet of air per minute — the maximum volume the carburetor can flow in sixty seconds on a flow bench, measured at a defined pressure drop.

Are two-barrel and four-barrel CFM ratings comparable?

No. Four-barrels are rated at 1.5″ Hg and two-barrels at 3.0″ Hg. Divide a two-barrel rating by 1.414 to express it in four-barrel terms.

What volumetric efficiency should I use?

Roughly 70-80% for a stock or mild street engine, 85-95% for a developed performance engine, and above 100% only for a race engine with intake and exhaust tuned to take advantage of it.

Is a bigger carburetor always better?

No. Too much venturi area drops air velocity, weakens the fuel signal and produces hesitation and bogging off idle, while adding nothing you can use below peak rpm.

Does this formula work for a two-stroke?

Not as written. The 3456 divisor contains a factor of 2 for the four-stroke cycle. A two-stroke induces on every revolution, so the divisor is 1,728.

Sources and assumptions

This calculates the theoretical airflow demand of a naturally aspirated four-stroke at the rpm you enter. It does not model intake or exhaust tuning, carburetor spacing, altitude, air temperature, or forced induction, and it assumes the engine reaches the volumetric efficiency you specify at peak rpm rather than averaged across the range. Results are a starting point for selection, not a substitute for testing on the engine.

All CFM figures on this page are stated at the four-barrel standard of 1.5″ Hg unless labelled otherwise.

• Raso Enterprises, Carburetion for the 3.0″ Hg / 1.5″ Hg rating standards, the 1.414 conversion between them, and the CID x RPM / 3456 formula
• EngineerFix, What Is CFM on a Carburetor for the CFM definition and the venturi velocity and signal effects of oversizing