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CFM demand
How much air does your engine want? The air demand drives the choice of intake, throttle body and how much porting can give. Enter the displacement, rpm and volumetric efficiency, and you get the demand in CFM.
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Air demand
543 CFM
Theoretical air demand at the given volumetric efficiency. Real flow is measured on a flow bench.
How the calculation works
The air demand is the displacement times the rpm, divided by 3456 for a four-stroke, adjusted for the volumetric efficiency. Volumetric efficiency is how well the cylinder is actually filled compared with its theoretical volume. A normal street engine sits around 80 to 90 percent, while a well-ported race engine can exceed 100 percent thanks to the intake's inertia.
This is a theoretical value and a starting point, not a measurement. Real airflow is measured on the flow bench, port by port, both before and after work. Porting is about shaping the channel right, not just making it bigger.
Example
A 5,7-litre V8 at 6000 rpm and 90 percent volumetric efficiency needs around 545 CFM, and theoretically 605 CFM at full filling.
Typical air demand by engine type
| Engine type | Typical air demand |
|---|---|
| Small V8 (5,7 litre / 350 CID) | 220-300 CFM |
| Race-prepped small block (7,0 litres) | 300-400 CFM |
| Large V8 (7,4-9,0 litres) | 400-500 CFM |
| Race engine (10 litres and up) | 500-600+ CFM |
Guide values at typical volumetric efficiency. Real flow varies with the cylinder head, cam and intake.
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Common questions about cfm demand
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