Engine Air Flow Calculator
Calculate the required CFM (Cubic Feet per Minute) for your engine setup.
What is Engine Air Flow (CFM)?
Engine air flow, measured in Cubic Feet per Minute (CFM), represents the volume of air an internal combustion engine consumes at a specific RPM. Understanding this metric is critical for selecting the right intake components, such as carburetors, throttle bodies, air filters, and intake manifolds.
An engine acts as an air pump. The more air it can pump, the more fuel can be mixed with it, and consequently, the more power it can generate. However, installing a carburetor or throttle body that is too large can reduce intake velocity, resulting in poor low-end throttle response. Conversely, components that are too small will starve the engine of oxygen at high RPMs, limiting horsepower.
The Air Flow Calculation Formula
For a standard four-stroke engine, the formula to calculate the theoretical air flow requirement is:
CFM = (Displacement × RPM × VE) / 3456
Where:
- Displacement: The engine size in Cubic Inches (CID). If you have Liters, multiply by 61.02 to get CID.
- RPM: The revolutions per minute at which you want to calculate flow (usually maximum redline).
- VE (Volumetric Efficiency): A percentage representing how completely the cylinder fills with air during the intake stroke compared to its static volume.
- 3456: A mathematical constant derived from converting cubic inches to cubic feet and accounting for the fact that a 4-stroke engine only intakes air on every other revolution (RPM/2).
Understanding Volumetric Efficiency (VE)
Volumetric Efficiency is the variable that changes most based on the build of your engine. It measures the efficiency of the engine's ability to move the air/fuel charge into the cylinders.
| Engine Type | Typical VE Range |
|---|---|
| Standard Stock Engine | 70% – 80% |
| Mild Street Performance (Headers, Intake) | 80% – 85% |
| High Performance Street/Strip | 85% – 95% |
| Dedicated Race Engine | 95% – 110% |
| Forced Induction (Turbo/Supercharger) | 100% – 150%+ |
Sizing Your Carburetor or Throttle Body
While the calculation above gives you the actual air the engine consumes, selecting a carburetor usually requires a slight buffer. A carburetor is rated by how much air it flows at a specific pressure drop (typically 1.5 inches of mercury for 4-barrel carbs).
If you select a carburetor sized exactly to your engine's requirement, it might create a restriction at peak RPM. Therefore, it is common practice to select a carburetor rated approximately 10% to 15% higher than the engine's theoretical maximum airflow requirement. This calculator provides both the strict engine requirement and a recommended sizing range to ensure optimal peak power without sacrificing driveability.
Example Calculation
Let's assume you have a 350 cubic inch Small Block Chevy engine. You plan to rev it to 6,000 RPM, and it is a street performance build with headers and a nice intake manifold, giving it a VE of roughly 85%.
Calculation: (350 × 6000 × 0.85) / 3456 = 516.49 CFM
In this scenario, a 500 CFM carb might be slightly restrictive at redline, while a 600 CFM carburetor would be the ideal choice for street performance.