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 roughly quadratic in the timing error. In Engone:
| Error from MBT | Approx. torque loss |
|---|---|
| 5° | ≈ 4% |
| 10° | ≈ 16% |
| 15° | ≈ 36% |
Spark advance is bounded to a sensible window, roughly 5° to 40° BTDC.
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 1–2° on the table costs you a fraction of a percent and buys knock margin — usually a good trade.
Watch out
If torque is still climbing when knock appears, the knock limit is below MBT for that cell (common under boost or low octane). 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 holds your result.