How to use this calculator
- Enter displacement and RPM. Use total displacement and target engine speed.
- Set volumetric efficiency. Enter the VE estimate for the operating point.
- Add pressure data. Set ambient pressure, boost gauge pressure and air density.
- Read airflow. Review volume flow, mass flow and pressure ratio.
How it works
A four-stroke engine fills each cylinder once every two crank revolutions. The calculator uses flow = displacement x RPM / 2 x VE x pressure ratio, then multiplies by air density for mass flow.
Use this with the BMEP calculator when comparing airflow, torque and normalized engine output.
Worked example
Verified against the live calculator
A 5.7 L four-stroke engine at 6,000 rpm and 90% VE needs about 543 CFM naturally aspirated at sea-level pressure, before inlet, filter and manifold losses.
Frequently asked questions
How do you calculate engine airflow CFM?
For a four-stroke engine, airflow is displacement times RPM divided by two, adjusted by volumetric efficiency and pressure ratio.
Why divide RPM by two?
A four-stroke cylinder fills once every two crankshaft revolutions.
Does boost increase airflow?
Boost increases pressure ratio, so the calculator multiplies airflow by (ambient pressure + boost gauge pressure) / ambient pressure.
Is this the same as compressor-map mass flow?
No. It is a sizing screen. Compressor maps need temperature, pressure losses, intercooler behavior and corrected mass flow.
Method & assumptions
- Uses a four-stroke airflow screen with volumetric efficiency and pressure ratio.
- Real airflow depends on temperature, humidity, pressure losses, cam timing, compressor map and manifold behavior.