How this calculator works
This estimate applies continuity to an assumed mean hot-gas flow. Four-stroke intake volume flow is displacement times RPM divided by 120, adjusted for volumetric efficiency. A user-entered absolute-temperature ratio approximates expansion at similar pressure, ignoring added fuel mass. Divide flow among cylinders for a single primary. Target gas speed is an assumption, not a universal optimum. Pulses, scavenging, bends, silencers and acoustic tuning require a separate design method.
Formula
Qhot = V RPM VE (Texhaust/Tintake)/(120 N), with N=1 for a collector or cylinder count for a primary. A=Qhot/u; ID=sqrt(4A/pi); OD=ID+2t.
Worked example
A 4 L four-stroke at 3000 rpm, 100% filling and temperature ratio 1 gives 0.1 m³/s collector flow. At assumed gas speed 10 m/s, ID is 112.838 mm; 1 mm walls give OD 114.838 mm. A 60 mm OD tube with 2 mm walls has 56 mm ID.
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Frequently asked questions
Is this a tuned-header formula?
No. It estimates mean flow area under explicit assumptions and cannot predict the pipe length or diameter that maximizes torque.
Should I enter Celsius in the temperature ratio?
No. Divide absolute temperatures in kelvins. A ratio of 2 means twice the absolute intake temperature.
Why distinguish primary and collector?
A primary serves one cylinder. A collector carries the combined flow, so their mean-flow areas differ.