port timing reference with proper degree labels

 Here’s a detailed port timing reference with proper degree labels for your 125cc (59mm bore) two-stroke engine.

1. Target Port Timing Adjustments (Measured in Degrees)

Port Type Stock Timing (Estimated) Modified Timing (Target) Recommended Increase
Exhaust Port ~178–180° 188–192° Raise 1.0–1.5mm
Transfer Ports ~120° 126–130° Raise 0.5–1.0mm
Intake Port (Piston Port) ~140° 150–155° Lower skirt by 1.0mm
Intake Port (Rotary Valve) ~160° 170–190° Adjust rotary valve cut timing

📌 Blowdown Time (Exhaust to Transfer Delay):

  • Safe range: 28–32° before transfers open.
  • More blowdown = higher RPM power, but too much weakens midrange torque.

2. How to Convert Timing (Degrees) to Millimeters of Port Height

To calculate how much to grind (mm) per degree change, use this formula:

Port Height Change (mm)=Stroke×π360×Degrees Changed\text{Port Height Change (mm)} = \frac{\text{Stroke} \times \pi}{360} \times \text{Degrees Changed}

For your 59mm bore, ~50mm stroke engine:

  • 1° of timing change ≈ 0.14mm in height.
  • Example: 10° exhaust timing increase → Raise port by ~1.4mm.

3. Safe Limits for Porting

🚨 DO NOT exceed these limits unless testing carefully:

  • Max Exhaust Width: 72% of bore (42.5mm for a 59mm bore).
  • Max Exhaust Timing: 195° (beyond this, low-end power is lost).
  • Max Transfer Timing: 132° (beyond this, scavenging becomes unstable).
  • Intake Duration: Too much can cause poor idle & throttle response.

4. Expected Gains

✅ Higher top-end speed (better power beyond 180 km/h).
✅ More peak RPM (~500–1000 RPM increase).
✅ Improved scavenging = smoother high-RPM acceleration.

 

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