RPM to Air Flow Rate Calculator
Calculate the intake air flow (CFM) for internal combustion engines based on RPM and displacement.
Understanding RPM and Air Flow Rate
The relationship between engine speed (RPM) and air flow rate is critical for automotive engineers, tuners, and hobbyists. It helps in selecting the right carburetor size, fuel injectors, or turbocharger for a specific engine build.
The Air Flow Formula
For a standard four-stroke internal combustion engine, the air flow rate in Cubic Feet per Minute (CFM) is calculated using the following physics-based formula:
CFM = (Displacement × RPM × Volumetric Efficiency) / 3456
Where 3456 is a constant derived from 1728 (cubic inches in a cubic foot) multiplied by 2 (since a 4-stroke engine only fills its cylinders every two revolutions).
If you have a 350 cubic inch (5.7L) V8 engine spinning at 6,000 RPM with a Volumetric Efficiency of 85%:
CFM = (350 × 6000 × 0.85) / 3456 = 516.5 CFM
Factors Influencing Air Flow
- Displacement: The total volume of all cylinders. Larger engines move more air per revolution.
- RPM: As the engine spins faster, the frequency of intake strokes increases, directly increasing air demand.
- Volumetric Efficiency (VE): This represents how well the engine "breathes." A VE of 100% means the cylinders are completely filled with air at atmospheric pressure. Natural restriction from air filters and intake manifolds usually keeps stock engines between 80-85%.
Why Calculate Air Flow?
Calculating the RPM to air flow rate is the first step in performance tuning. For example, if your engine requires 500 CFM at peak RPM, installing a 750 CFM carburetor provides plenty of overhead, while a 390 CFM carburetor would significantly restrict power at high speeds. Similarly, turbocharger compressor maps are plotted against air flow (usually in lbs/min or CFM) to ensure the turbo can efficiently provide the necessary volume.