Ignition timing and MBT

Advance the spark until torque stops rising. That point is MBT; past it you add pressure and knock risk for nothing.

Combustion takes time. Firing the spark before top dead centre lets the pressure peak land just after TDC where it does the most work. Too little advance and the burn is late: torque falls, exhaust heats up. Too much and the pressure rises while the piston is still coming up: torque falls again, and the chamber is primed to knock.

MBT

Minimum advance for Best Torque is the point where more timing stops helping. It changes with RPM (faster engine, more lead needed), load (lighter load burns slower, wants more advance), mixture and charge temperature. Near MBT the torque curve is flat: the last few degrees are worth almost nothing and cost cylinder pressure.

Finding it on the dyno

• Fix the fuel first; timing tests on a wrong mixture are meaningless.
• Change one region at a time, 1–2° per pull.
• Compare torque in that region between pulls, with the same conditions. Up: keep going. Flat (inside the noise): you are there, back off a degree. Down: you went past MBT or knock control is pulling it.
• Watch the knock level the whole time. The knock limit, not MBT, is usually the ceiling at high load on pump fuel.