What the inputs mean
- Displacement: the total swept volume of the engine. Enter cubic inches, liters or cc; the formula works in cubic inches.
- Max rpm: the highest engine speed you plan to use, since that is where airflow peaks.
- Volumetric efficiency (VE): how much air the cylinders actually take in, compared with their swept volume. 100% means they fill exactly. VE depends on the heads, cam, intake and exhaust, and a well-tuned engine can go past 100% at some rpm. It is an assumption you set; the default here is 85%.
The formula
cfm = displacement (ci) × rpm × VE ÷ 3456Where 3456 comes from: a four-stroke engine fills each cylinder once every two revolutions, so it takes in its displacement every 2 revolutions. There are 1,728 cubic inches in a cubic foot (12 × 12 × 12). So 1,728 × 2 = 3,456 turns cubic inches per revolution into cubic feet per minute.
Worked example
A 350 ci engine that you plan to rev to 6,000 rpm, at 85% volumetric efficiency:
350 × 6,000 × 0.85 ÷ 3,456 = 516 cfm. At 100% VE the same engine would pull 608 cfm.
In liters: 5.7 L is 347.8 ci, which gives 513 cfm at the same rpm and VE.
Reading the result
Carburetors are sold by their rated airflow in cfm. Four-barrel carbs are usually rated at a pressure drop of 1.5 inches of mercury and two-barrels at 3.0, so compare like with like. The table shows how much the answer moves with VE: from 75% to 110% the 350 ci example runs from 456 to 668 cfm. That spread is why the VE you enter matters more than any other input. The result is maths from your numbers; the right carburetor for your engine is a tuning choice.
Questions
What size carburetor do I need for a 350?
At 6,000 rpm and 85% volumetric efficiency, a 350 ci engine pulls about 516 cfm. At 100% VE it pulls 608 cfm. Enter your own rpm and VE for your engine.
What does 3456 mean in the carb cfm formula?
It is 1,728 cubic inches per cubic foot times 2, because a four-stroke engine takes in its full displacement once every two revolutions.
How do I calculate carb cfm from liters?
Multiply liters by 61.02 to get cubic inches, then use cfm = ci × rpm × VE ÷ 3456. A 5.7 L engine at 6,000 rpm and 85% VE comes to about 513 cfm.
What volumetric efficiency should I use?
It depends on the engine. 85% is the default here and is an assumption, not a measurement. The table shows every step from 75% to 110% so you can see the range.