FOS Pipe Calc

Two-stroke expansion-chamber designer

Engine & pipe parameters

× mm
degrees
1.0 = narrow power band, 2.0 = wide
r/min
HP
m/s
mm
updates live

Result

Exhaust port area
mm²
Exhaust port Ø
mm
Displacement
cc
BMEP
bar

Drawing shows the actual pipe only — Lcyl is not included. Angles are cone half-angles.

Cone layout — segment & flat-pattern development
Section Small Ø Large Ø Length Half-angle Flat inner R Flat outer R Sweep

Tailpipe / stinger. Tailpipe diameter is the minimum value it should be for its length to be unimportant — i.e. it can be however long you want it to be without affecting the pipe's performance.

Restrictor. The restrictor is right before the stinger and its diameter is a bit of an educated guess. If you select air-cooled you may want to make the restrictor adjustable and see what works best for you.

Cone development. The original site exported .con files for the third-party Cone Layout program. Since that format wasn't archived, the table above instead gives the flat-pattern development directly: for each cone, mark two arcs at inner R and outer R spanning the sweep angle, then roll. Straight sections roll from a rectangle length × π·Ø.

Wave speed

Wave speed is based on mean exhaust temperature. The hotter your pipe is, the higher the wave speed. This is a very important factor as it determines the pipe's length.

The default value of 550 m/s is said to be a good starting point, but if you find that the pipe gives power at a too low or high rpm you should adjust it. Too high rpm means your pipe is colder than expected — use a higher speed. Too low rpm means your pipe is hotter — use a lower speed.

The formula for calculating wave speed is
m/s = √402 × (TempC + 273)
where TempC is in degrees Celsius and the result is in meters/second. For Fahrenheit:
m/s = √223 × (TempF + 459)

Temp (°C)Temp (°F)Speed (m/s)
400752520
420788527
440824535
460860542
480896550
500932557
520968564
5401004571
5601040578
5801076585
6001112592