Cylinder Head Flow Calculator

Cylinder head airflow is one of the biggest limiting factors in engine power — the Cylinder Head Flow Calculator tells you exactly how much airflow (in CFM) your heads need to support your power goals. Enter your cubic inches, number of cylinders, target RPM, volumetric efficiency, bore diameter, stroke, and intake valve diameter to get the required CFM per cylinder, total engine CFM, predicted horsepower, intake valve-to-bore ratio, and flow per square inch. You can also enter your current head CFM from a flow bench test to see how your existing heads compare.

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RPM
%
in
in
in
CFM

Optional: Current flow bench results

Results

Required CFM per Cylinder

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Total Engine CFM

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Predicted Horsepower

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Intake Valve to Bore Ratio

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Flow per Sq. Inch

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Frequently Asked Questions

What is the ideal flow ratio for cylinder heads?

The maximum ideal flow ratio is 146 CFM per square inch for a frictionless orifice. A good practical ratio is 133 CFM/sq.in for single intake valve heads, and 137 CFM/sq.in for dual intake valve setups.

How much CFM do I need per horsepower?

At peak power, a full-race engine consumes approximately 1.67 CFM of air per horsepower. The Air Flow Research formula suggests 0.25714 CFM per cylinder for each horsepower on a normally aspirated engine.

What's the optimal intake valve to bore ratio?

For wedge heads, aim for 52-52.5% of the bore diameter. For canted valve and hemi heads, target 53-53.5%. Exceeding these ratios can hurt performance by creating flow restrictions.

How does volumetric efficiency affect flow requirements?

Higher volumetric efficiency means the engine breathes more effectively. At 100% VE, you'll see close to maximum RPM at peak HP. At 127.5% VE (supercharged), you'll reach minimum RPM at peak HP for your CFM rating.

Why is flow measured at 28 inches of water?

28 inches of water (H2O) is the industry standard test pressure for flow benches. This standardized pressure allows for consistent comparison between different cylinder heads and manufacturers.

Can cylinder heads alone determine engine power?

No, reaching predicted power requires maximum compression, proper camshaft timing, tuned exhaust system, and non-restrictive intake manifold and carburetor. Heads are just one component of the complete package.

What happens if my flow ratio is below 133 CFM/sq.in?

A flow ratio below 133 CFM/sq.in indicates you're losing power potential. Consider working with a head porter to improve flow numbers through better port work and valve job techniques.