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Four-Stroke Engine Airflow Demand

Estimate four-stroke engine airflow from displacement, RPM and volumetric efficiency, or recover a missing quantity in SI and customary units.

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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

A 350 in³ four-stroke engine at 6,000 RPM and 80% volumetric efficiency has estimated airflow 486.111111 ft³/min.

Airflow in ft³/min = displacement in in³ × RPM × VE% / 345600.

Worked example

Enter these known values and leave the other values blank.

Total engine displacement
5735.4724 cm³
Engine speed
6000 rpm
Volumetric efficiency
80 %
Estimated airflow
486.11 ft³/min

Assumptions and limitations

  • This is a four-stroke displacement-flow estimate: one intake cycle occurs per two crankshaft revolutions. A two-stroke engine requires a different cycle factor.
  • Volumetric efficiency compares inducted volume with displacement at the chosen conditions. It is entered as a percentage by default; the decimal option uses 0.8 for 80%. Values above 100% are permitted and require a justified engine estimate.
  • The result estimates airflow at the entered speed and efficiency. It does not select a carburetor model or predict horsepower, manifold pressure, boost, fuel mixture or driveability.
  • Use total engine displacement, rather than displacement of one cylinder. The RPM selector is revolutions per minute; the Hz option counts crankshaft revolutions per second.
  • All four quantities must be positive. All four controls remain editable, including inverse estimates for displacement, RPM or efficiency.
  • An international inch is exactly 0.0254 m and a US liquid gallon is exactly 231 cubic inches. Changing the flow unit preserves the same volume per time.

Common questions

Is 80% entered as 80 or 0.8?

Enter 80 with the percentage selector, or 0.8 with the decimal selector.

Can I use this for a two-stroke engine?

No. The four-stroke formula includes one intake cycle per two crankshaft revolutions.

Does this choose the correct carburetor automatically?

No. Treat the result as an airflow estimate; selection also depends on engine configuration and rating conditions.

References

Displacement, engine speed, volumetric efficiency and finite airflow must be strictly positive, using exact-sign comparisons. US liquid gallons per minute use the exact 231-cubic-inch gallon instead of the captured rounded factor.

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