🔎 Should Your Timing Peak at 8,000 RPM or 6,000 RPM?

 

🔎 Should Your Timing Peak at 8,000 RPM or 6,000 RPM?

Your concern is completely valid, and this decision is crucial to balancing power, acceleration, and top speed. Let's analyze this carefully by comparing:

✅ Your Old Setup (Before 60mm Bore, Before Spacer Changes)
✅ Your New Setup (60mm Bore, 27mm Exhaust Port Height, and Revised Compression)

🔥 1. Why Did the Old Setup Peak at 6,000 RPM?

Previously, your engine had:

🔹 Smaller Bore (Stock 54mm or 57mm) → Smaller combustion chamber volume = Faster burn rate.
🔹 Shorter Exhaust Duration (~24mm port height) → Cylinder pressure stayed higher in the midrange, so peak advance earlier at 6,000 RPM maximized power.
🔹 Higher Low-End Pressure → Your squish clearance was tighter relative to combustion volume, increasing low-end cylinder pressure.

✅ Result: Advancing timing aggressively around 6,000 RPM helped compensate for the smaller bore and shorter exhaust duration.

🔥 2. Why Does the New Setup Peak at 8,000 RPM?

Your current engine setup now features:

🔹 Larger Bore (60mm) → Increased combustion chamber volume = Slightly slower burn rate.
🔹 Increased Exhaust Port Height (27mm) → Higher exhaust duration = Power shifted toward higher RPM.
🔹 Modified Squish Clearance (0.7mm) → Improved turbulence speeds up flame travel, but still requires a slightly later peak timing to balance combustion pressure.

✅ Result: Advancing peak timing at 8,000 RPM ensures optimal combustion at the point where your engine reaches peak volumetric efficiency.

🔎 3. Key Rule from the PDFs (A. Graham Bell & Jennings):

  • Larger bore engines + extended exhaust duration demand later peak advance to sustain power at higher RPM.
  • Engines designed for high top speed benefit from peak advance closer to peak horsepower RPM.

✅ Since your power curve is now stretched higher, a timing peak at 8,000 RPM aligns better with your engine’s revised airflow dynamics.

🔥 4. Potential Risks of Peaking at 6,000 RPM with Your Current Setup

❗ Advancing timing too early (e.g., peaking at 6,000 RPM) with your taller exhaust port can:

🔹 Overheat the piston crown at high RPM.
🔹 Cause detonation because combustion pressure would peak too early in the power stroke.
🔹 Flatten your top-end power, reducing your chances of reaching 200 km/h+.

⚙️ Final Recommendation:

✅ Peak advance at 8,000 RPM is the safer and more efficient choice for your big bore, increased exhaust duration, and high-RPM goal.
✅ Advancing too early (at 6,000 RPM) risks losing top-end power and may push combustion pressure too high for sustained high-speed runs.

🔧 Best Strategy for Your Goal:

  • Midrange Advance (~23° @ 6,500 RPM) → To compensate for reduced low-end torque.
  • Peak Advance (26° @ 8,500 RPM) → To maximize power in your revised top-end powerband.
  • Retard Progressively After 8,500 RPM → To prevent detonation and overheating during sustained top-speed runs.

🚨 Trust the Data

Your revised timing curve follows the best practices from the tuning PDFs combined with your engine's new characteristics. The goal is to create a safe, powerful, and high-RPM-focused curve that matches your revised engine behavior.

If you'd like a side-by-side comparison chart of the old timing map vs. new timing map, let me know! 🔧🔥

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