Engine Airflow Calculator (CFM for Carburettor, Throttle Body or Turbo Sizing)
How much air a four-stroke engine swallows at a given speed, in CFM, litres per second and grams per second, from its displacement, volumetric efficiency and boost.
Results
The dashed line marks the value you entered.
What this tool does
A four-stroke engine fills its cylinders once every two revolutions, so the volume of air per minute is displacement × rpm ÷ 2 × volumetric efficiency; in the hot-rodding form, cubic inches × rpm × VE ÷ 3456. A road engine runs at 80 to 90 % volumetric efficiency, a tuned racing engine at 100 % or more, and boost multiplies the mass of air by the pressure ratio. It is the starting point for sizing a carburettor, throttle body or turbocharger, where airflow is quoted at one condition and the real engine rarely sees it.
Formula
CFM = CID × RPM × VE ÷ 3456 (= litres × 61.02 × RPM × VE ÷ 3456) · × boost pressure ratio · mass = volume × ρ
Variables
| Symbol | Meaning | Unit |
|---|---|---|
dsp | Engine displacement (four-stroke) | L |
rpm | Engine speed | rpm |
ve | Volumetric efficiency | % |
pr | Boost as a pressure ratio (1 = naturally aspirated) | — |
ta | Intake air temperature | °C |
CF | Airflow | CFM |
LS | Airflow in litres per second | L/s |
M3 | Airflow in cubic metres per minute | m³/min |
GS | Mass of air per second | g/s |
Worked example
- Engine displacement (four-stroke)2 L
- Engine speed6000 rpm
- Volumetric efficiency90 %
- Boost as a pressure ratio (1 = naturally aspirated)1
- Intake air temperature25 °C
- Airflow191 CFM
- Airflow in litres per second90.0 L/s
- Airflow in cubic metres per minute5.40 m³/min
- Mass of air per second107 g/s
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.