Spark Fundamentals¶
There's a single best spark timing — MBT. Small errors barely cost anything; large errors cost a lot, because the penalty grows with the square of the error.
What it is¶
For any operating point there's one advance that extracts the most torque: MBT — Minimum advance for Best Torque. Fire earlier than MBT and peak pressure arrives too soon, fighting the rising piston; fire later and the burn finishes too late to push effectively. Either way torque falls off.
The penalty isn't linear — it's quadratic in the timing error. In Engone the loss is 0.0004 × (error in degrees)²:
| Error from MBT | Torque loss |
|---|---|
| 5° | ≈ 1 % |
| 10° | ≈ 4 % |
| 15° | ≈ 9 % |
| 20° | ≈ 16 % |
| 50° | total |
The table accepts −20° to +40° BTDC, but the ECU never fires later than 5° BTDC — see the Timing Table.
What drives MBT in Engone. At WOT it rises from about 22° at 800 RPM to about 34° at 6800 RPM. On top of that:
- up to +10° more as load falls;
- −1° per 1:1 of compression ratio above 9.5;
- +0.06° per mm of bore above 86 mm;
- a faster-burning head needs a little less;
- residual burnt gas (large cam overlap at low RPM) can add up to +18° — which is why a big-cam idle wants 30–35° where a stock idle wants about 15°.
Why it matters¶
The quadratic curve has a practical upshot: being a few degrees off MBT costs almost nothing, so you don't need to chase the last degree. But the curve gets steep fast, so a badly wrong table — or pushing far past MBT into knock — loses serious power and risks damage.
How to tune it¶
Add advance and watch torque (on the dyno this is easy). Torque rises, flattens at MBT, then falls. Settle slightly below MBT in risky cells for safety margin — the loss is tiny and the knock protection is worth it.
Rule of thumb
The torque peak is broad near MBT. Leaving 2° under MBT costs only about 0.16 % of torque and buys knock margin — usually a good trade. Real engines also run idle maps around 20° retarded of MBT and lose 15–20 % of their torque there; that is normal.
Watch out
If torque is still climbing when knock appears, the knock limit is below MBT for that cell (common on low octane or hot days). The limit wins — don't advance into it.
In the game¶
Use the dyno to find MBT cell by cell: sweep advance, watch the torque readout peak. The timing table on the IGNITION page holds your result.