Ignition Timing

The fuel and air mixture needs a short time to burn, so the spark fires slightly before top dead centre. The best moment depends on engine speed, load and temperature, and the control unit calculates it continuously. Firing too early causes knock; too late wastes energy and heats the exhaust. Modern engines have no manual timing adjustment: the control unit sets it from maps and corrects it with the knock sensor.

Timing is given in degrees before top dead centre (BTDC). The spark is fired before the piston reaches the top because the mixture needs time to burn.

The crankshaft position sensor gives the exact piston position. The control unit fires the coil at the calculated angle and retards it when the knock sensor detects knock. The control unit advances or retards timing based on engine speed, load, temperature and knock sensor signals. It cannot normally be adjusted by hand on modern engines.

Retarded timing from knock, poor fuel or carbon deposits costs power and fuel economy. Timing problems can also come from crankshaft sensor or timing chain faults. Too much advance can cause knock that damages pistons; too little costs power and economy and raises exhaust temperature. Using fuel with the octane rating the manufacturer specifies matters here.