Carb CFM Calculator

How a carb CFM calculator works

An engine is an air pump. Every cylinder draws in up to its own displacement on each intake stroke, and in a four-stroke engine each cylinder gets one intake stroke every two crank revolutions. Multiply displacement by rpm, halve it, convert cubic inches to cubic feet, and you have the airflow the engine would move if it filled its cylinders completely. Real engines fill them only partly, and that fraction is the volumetric efficiency, or VE.

A carburetor is chosen to supply that airflow at the highest rpm the engine will use. This carburetor size calculator works out the figure, shows the common four-barrel sizes on either side of it and, if you enter your own carburetor, the rpm at which the engine would draw its full rating. As an engine cfm calculator it also works the other way: the Volumetric efficiency tab turns a measured airflow back into VE.

What to enter and where to find it

• Displacement in cubic inches, or liters with the unit switch. Not sure of it? Work it out from bore, stroke and cylinder count with the engine displacement calculator.
• Maximum engine rpm. The highest speed the engine will see in use: the shift point for a drag car, the redline for a cruiser. It is usually above the rpm of peak power.
• Volumetric efficiency. The honest guess matters more than any other input. Use the typical ranges below unless you have dyno or airflow data.
• Your carburetor (optional). The cfm rating from the carburetor's tag or specification sheet. Convert a two-barrel rating to the four-barrel basis first.
• Measured airflow (Volumetric efficiency tab). Wide-open-throttle airflow at a known rpm, from a dyno's air meter or a mass airflow log converted to cfm at standard air density.

Volumetric efficiency calculator and typical VE by engine type

Volumetric efficiency compares the air an engine actually draws in with the air its displacement could hold at standard conditions. It is not one number for an engine: it peaks around the torque peak and falls away at high rpm. Carburetor sizing guides apply the typical figures below at maximum rpm. VE there is usually a little below its peak, so this errs slightly on the generous side. The calculator starts at 85%, the typical figure for a performance build, and the Volumetric efficiency tab works the formula backwards from a measured airflow.

Typical peak VE of naturally aspirated engines

• Stock and mildly rebuilt engines of the carburetor era: typically 75–85%.
• Performance builds with better heads, cam, intake and headers: typically 80–90%.
• Highly developed race engines with tuned intake and exhaust: typically 95–110%. Filling the cylinders beyond their static displacement is possible because pressure waves in tuned runners pack in extra air, but only the most developed engines get past about 110%.

How carburetors are rated: two-barrel and four-barrel cfm

A carburetor's cfm rating is the airflow it passes on a flow bench at a fixed pressure drop across it, not the most air it could ever flow. By long-standing convention, four-barrel carburetors are rated at 1.5 inHg and one- and two-barrel carburetors at 3.0 inHg, so the two kinds of figure cannot be compared directly.

Airflow through a restriction rises with the square root of the pressure drop. A two-barrel rating therefore converts to the four-barrel basis when multiplied by 0.707, the square root of 1.5 ÷ 3.0, and back when multiplied by 1.414. Treat the result as an estimate: published ratings are rounded, and older ones were measured on different test setups.

The carb CFM formula

The formula itself is exact arithmetic. All the uncertainty sits in the volumetric efficiency and in the rpm you choose.

1. Airflow (cfm) = displacement (ci) × rpm × VE ÷ 3,456
2. VE = measured cfm × 3,456 ÷ (displacement × rpm)
3. 3,456 = 1,728 in³ per ft³ × 2 revolutions per intake stroke
4. Four-barrel basis = two-barrel rating × √(1.5 ÷ 3.0)

Displacement × rpm is the volume the pistons sweep each minute, in cubic inches. Each cylinder fills on every second revolution only, so the engine inhales half of that, and dividing by 1,728 turns cubic inches into cubic feet. Those two steps together are the 3,456 that appears in every carburetor formula. VE goes in as a fraction: 85% is a multiplier of 0.85.

Carburetors are usually chosen close to the result. One far larger than the engine needs only reaches its rated flow at an rpm the engine never sees, so the air moves slowly through the venturis at the speeds where the engine does run, which blunts throttle response; one far smaller costs power at the top.

Worked example

Three typical questions, worked out with the same functions that run the calculator.

Typical carburetor cfm by displacement and rpm

The airflow common engine sizes need at a typical performance-build volumetric efficiency of 85%, computed with the formula above. For another VE, multiply a figure by your VE and divide by 85.

Airflow rises in a straight line with both displacement and rpm, so an engine that revs higher needs more carburetor just as surely as a bigger engine does.

Common mistakes

• Using a dream redline. The formula wants the highest rpm the engine will really use. Every 10% too high adds 10% to the answer.
• Optimistic VE. Most road engines never fill their cylinders completely. Assuming race-engine VE for a mild build oversizes the carburetor.
• Comparing two-barrel and four-barrel ratings. They are measured at different pressure drops. Convert on the 2-bbl vs 4-bbl tab first.
• Applying it to a boosted engine. The formula is for naturally aspirated engines. Boost pushes airflow well beyond it; the turbo size calculator works with pressure ratio instead.
• Thinking bigger is always better. A carburetor far above the engine's need flows its rating only at an rpm the engine never reaches, and below that the air speed through the venturis is low.