Understanding Engine Flow Rate (CFM)
An internal combustion engine acts essentially as an air pump. The more air it can pump, the more fuel it can burn, and consequently, the more power it can generate. The Engine Flow Rate Calculator helps you determine the volume of air your engine requires at a specific RPM, measured in Cubic Feet per Minute (CFM).
Knowing your engine's airflow requirement is critical for selecting the correct size carburetor, throttle body, fuel injectors, or turbocharger. Undersizing these components chokes the engine, while oversizing them can lead to poor low-end throttle response and driveability issues.
How the Formula Works
For a standard 4-stroke engine, the air flow is calculated using the following formula:
CFM = (CID × RPM × VE) / 3456
Where:
- CID (Cubic Inch Displacement): The total volume of all cylinders in the engine.
- RPM (Revolutions Per Minute): The engine speed at which you are calculating the flow.
- VE (Volumetric Efficiency): The ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure to the mass density of the same volume of air in the intake manifold.
- 3456: A mathematical constant derived to convert cubic inches and revolutions per minute into cubic feet per minute for a 4-stroke cycle engine (since intake happens only every 2 revolutions).
Estimating Volumetric Efficiency (VE)
Volumetric Efficiency is the wildcard in the equation. It represents how well the engine breathes. Use these guidelines to estimate your VE if you don't have dyno data:
- 75% – 80%: Standard factory stock engines with restrictive intakes and exhausts.
- 80% – 90%: High-performance street engines with aftermarket intakes, headers, and improved camshafts.
- 90% – 100%+: Dedicated race engines with highly ported heads and high compression.
- Above 100%: Forced induction engines (Turbocharged or Supercharged) forcibly push more air in than atmospheric pressure allows.
Example Calculation
Let's say you have a 350 CID (5.7L) engine that you plan to rev to 6,000 RPM. It is a street performance build, so we estimate a Volumetric Efficiency of 85%.
Calculation: (350 × 6000 × 0.85) / 3456 = 516.5 CFM
In this scenario, a 500-550 CFM carburetor would be ideal. If you chose a 750 CFM carburetor, the engine would likely suffer from poor throttle response at lower RPMs because the air velocity through the venturis would be too slow.