Exhaust Diameter Calculator

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The HP method is fast for known peak power. The displacement method uses engine volumetric flow and is useful when you know the engine build but not yet the final power figure.
Enter wheel or flywheel horsepower (HP) for imperial, or kilowatts (kW) for metric.
HP
Turbocharged engines benefit from a slightly larger downpipe to minimise backpressure on the turbine.
True dual: each cylinder bank has its own pipe from header to tip, so total flow splits across both pipes.
Recommended pipe diameterGood velocity range
3

Next standard size at or above the calculated minimum

Calculated minimum diameter3
Exhaust gas velocity51
Peak exhaust flow150
Capacity buffer0
51 ft/s
Too slow - pipe oversized<50Optimal range50-150Acceptable - near limit150-250Too fast - pipe undersized250+

Your ideal exhaust pipe is 3.00" (76 mm).

  • The recommended pipe diameter is 3.00 inches (76 mm), which is the next standard stock size above the 3.00-inch calculated minimum.
  • Exhaust gas velocity is 51 ft/s, well within the 80-150 ft/s sweet spot. This balance keeps backpressure low while preserving the pulse energy that scavenges combustion gases from the cylinder.
  • The standard pipe gives you 0.0% more flow area than the bare minimum, providing a comfortable buffer for engine-to-engine variation and future modifications.

Next stepEnsure your chosen diameter runs all the way from the header collector to the muffler inlet - necking down even briefly at a flange undoes the gains from correct sizing.

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