Ignition timing is arguably the single most important tuning variable in a spark-ignition engine. It determines when the spark plug fires relative to the piston's position, and getting it right is the difference between peak power and a destroyed engine.

Timing is expressed in degrees of crankshaft rotation Before Top Dead Center (BTDC). Why fire before the piston reaches the top? Because combustion isn't instantaneous — the flame front takes roughly 2 milliseconds to propagate across the chamber. You need to light the charge early so peak cylinder pressure lands around 12–15° After TDC, which is where you get maximum mechanical leverage on the crankshaft.

Here's a practical rule of thumb: At 6,000 RPM, the crankshaft rotates 36° per millisecond. If your burn time is ~2 ms, you need to fire about 72° before you want peak pressure. Subtract the 15° ATDC target, and you're looking at roughly 57° BTDC total advance. That's why high-RPM engines need aggressive timing advance — there's simply less time for combustion.

The ECU builds ignition timing from three layers:

• Base timing table: A 3D map of RPM vs. load (manifold pressure or airflow). This is the starting point, calibrated on a dyno.
• Modifiers: Corrections for coolant temp, intake air temp, barometric pressure, and ethanol content.
• Knock control: A real-time safety net that pulls timing when detonation is detected.

Knock (detonation) occurs when the unburned air-fuel mixture ahead of the flame front auto-ignites from heat and pressure. This creates colliding pressure waves that produce the characteristic "ping" — and can hammer rod bearings, crack ring lands, or burn holes in pistons within seconds.

Knock sensors are piezoelectric accelerometers bolted to the block, typically tuned to resonate around 6–7 kHz, right in the frequency band where knock vibration lives. When the ECU detects knock, it retards timing — usually 1–3° per event — then slowly re-advances (around 0.5° per combustion cycle) to find the edge again. This is called closed-loop knock control.

A real-world example: GM's Gen V LT1 (C7 Corvette) uses two knock sensors and individual cylinder timing control. If cylinder #7 runs hotter due to poor cooling, the ECU pulls timing on that cylinder only, keeping the other seven at optimal advance. Earlier systems retarded all cylinders, leaving power on the table.

When tuning, you'll often hear "MBT timing" — Minimum advance for Best Torque. This is the sweet spot where advancing further yields no additional torque. Running past MBT wastes burn time pushing against the rising piston and increases knock risk for zero gain. On pump 93 octane, most modern engines hit the knock limit before MBT, meaning fuel quality is the constraint, not combustion physics.