The formula

First compute piston position term T:

T = R + L + C − E    where R = stroke ÷ 2

Then exhaust duration D in degrees:

D = (180 − cos⁻¹((T² + R² − L²) / (2RT))) × 2

Blowdown = D − 180° (time the exhaust port is open after BDC on the upward stroke).

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How to measure E

  1. Measure from the top edge of the cylinder down to the top edge of the exhaust port in the bore (mm).
  2. Use the same reference plane for deck height C — consistency matters more than guessing.
  3. Cross-check with a degree tape on the flywheel if you can.

Puch E50 reference (forum chart)

Port height (mm) Duration (°)
21.5 193.7
24.0 180.6
26.0 169.8
28.0 158.6

Next step — pipe design

Once you know exhaust duration, plug it into the expansion chamber designer for Graham Bell tuned length, cone angles, and a scaled pipe profile.

Before you cut metal

  • Opening exhaust without jetting and pipe match = seized pistons.
  • Automatic-clutch bikes (Puch, Tomos, ZA50) hate excessive blowdown — see blowdown notes on the wiki page.
  • Same math works for transfer/intake ports if you measure E to those port edges.