Carb CFM Calculator

How the formula works

An engine is an air pump: CFM = cubic inches x rpm / 3456 x VE. The 3456 is not a fudge factor - it is 1728 cubic inches per cubic foot times two, because a four-stroke fills its cylinders once every two crank revolutions. Every carb calculator runs this same arithmetic; the whole difference between a useful answer and a wrong one is the volumetric efficiency you multiply by, which is why this one makes you pick a VE range instead of quietly assuming 100%. If you do not know your cubic inches, the engine displacement calculator computes them from bore and stroke, and the compression ratio calculator shares the same measurements.

Two-barrel and four-barrel ratings are different rulers

Four-barrel carbs are flow-rated at 1.5 inHg of depression, two-barrels at 3.0 inHg, and flow scales with the square root of the pressure drop. The same number on the box therefore means different airflow: a 500 CFM two-barrel actually flows about 354 CFM by the four-barrel standard (500 x 0.707). That is why this calculator switches the whole scale to the two-barrel rating when you select one, and why swapping a "500" two-barrel in place of a 600 four-barrel is a much bigger step down than the numbers suggest.

When in doubt, buy the smaller carb

Oversizing is the classic mistake. A carb meters fuel from the air velocity through its venturis; too much carb means lazy velocity at street rpm, weak booster signal, poor atomization, and an engine that stumbles everywhere except wide open near redline. A slightly small carb costs a few peak horsepower and gives back crisp response everywhere else. Race engines that live at high rpm are the exception, and vacuum secondaries forgive a size step up because they only open when airflow actually demands it. One more honesty note: this formula is for naturally aspirated engines - forced induction multiplies airflow by pressure ratio, which is the territory of the boost to horsepower calculator.