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Header primary tube calculator

Starting-point primary tube length and diameter for a set of headers, from your cam's exhaust opening point and the rpm you want peak power.

Free, no sign-upUpdated September 2026
Your numbers
Engine and cam
Use seat or advertised timing. The cam timing calculator can work it out.

Primary tube length

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Measured from the valve to the collector.
Primary inside diameter
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Nearest common tube size (OD)
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Volume per cylinder
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Treat it as a starting point

These rules of thumb point you toward a size. Street engines often run a step smaller than the result for better low-speed torque, and experienced builders adjust after dyno testing.

The formulas

Primary length (in) = 850 × (180 + exhaust opening BBDC) ÷ rpm − 3
Primary inside diameter (in) = √(cc per cylinder ÷ ((length + 3) × 25)) × 2.1

These are long-standing rules of thumb used by header builders. The length rule times the exhaust pressure wave so it helps pull the next charge through the cylinder at the target rpm. The diameter rule sizes the tube to the volume each cylinder pushes out.

Longer primaries tend to favor lower rpm and shorter ones higher rpm. Tubing is sold by outside diameter, so the common size shown adds roughly two wall thicknesses to the inside diameter.