Carburetor CFM Formula

Carburetor CFM Formula
  • Item #: Carburetor CFM Formula
  • Condition: New
CFM, Cubic Feet per Minute of air flow. This is the number that determines how much air your engine flows or requires at maximum operating RPM. CFM is used to determine the size of the carburetor you need. It is calculated using the following formula. It's important to get your carburetor the correct size for several reasons. By over carbureting you reduce or lose low-end performance, economy, vacuum signal and throttle response. Under carbureting will limit your upper end horsepower and usable RPM range. The variable in the calculation is Volumetric Efficiency (VE). This number takes into account the type of cam, heads, intake manifold, etc. that you are running. VE is a fancy way of saying how much air actually gets into the engine while the engine is running versus how much air can fit into the engine when it's sitting still. When the piston is a BDC or Bottom Dead Center and the engine isn't running the amount of air that gets into the cylinder is determined by the outside air pressure, around 15 psi. When the engine is running, the air meets lots of restrictions to getting into the cylinder. These restrictions start at your air cleaner, then your carburetor, your intake manifold runners, cylinder head ports and finally valves. All of these things create a restriction for air to enter into your cylinder. VE is the percentage of air that could get into your engine versus the air that actually gets into your engine. On a typical stock motor that number is around 80-85%, on a street performance motor that number can increase from 85-90%. On a very hot street motor that number can go as high as 95%. On all out race motors that number can exceed 100%, but only for a very small RPM band. For the sake of building a street motor, we'll use 90% VE when sizing a carburetor, just to be on the safe side. We'd rather be a little over carbureted than under carbureted.
  • CFM = Displacement X RPM X VE / 3456
  • Example: CFM = 350 CI X 6000 RPM X 90% Volumetric Efficiency / 3456
  • CFM = 350 X 6000 X .90 / 3456 = 546.87
  • CFM = 546.87
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