Ignition & Spark Timing

On a petrol engine, the single most powerful — and most dangerous — number a tuner touches is ignition timing. To understand why, start with the cycle the engine repeats thousands of times a minute.

01The four strokes

Every power stroke is the payoff of three preparation strokes. Watch one cylinder run through the cycle:

02What the spark does

At the top of the compression stroke the mixture is tightly packed and ready. The spark plug throws a high-voltage arc across its gap, lighting the charge and starting a controlled flame front. Tap the plug to fire it:

03Timing and advance

Burning takes time. If you lit the mixture exactly at top dead centre (TDC), peak pressure would arrive too late to push effectively. So the spark is fired a little before TDC — measured in crank degrees BTDC — so that peak cylinder pressure lands just after the piston starts down.

• More advance (earlier spark) gives the flame more time to develop — often more power, up to a point.
• Too much advance makes pressure peak too early, fighting the rising piston — and inviting knock.

04Knock — the limit

Knock (detonation) is uncontrolled combustion: pockets of mixture ignite on their own ahead of the flame front, slamming the piston with sharp pressure spikes. Left unchecked it destroys pistons and rings quickly. The ECU listens with a knock sensor and pulls timing back when it hears it. Higher-octane fuel resists knock, which is what lets you safely run more advance.