Enter your engine's CID, cylinder count, target RPM, volumetric efficiency, bore, stroke, intake valve diameter, and head CFM @ 28" H2O into the Cylinder Head Flow Calculator to find the required CFM per cylinder, total engine CFM, and flow efficiency.
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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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.
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.
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.
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.
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.
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.
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.