Ignition timing in an internal combustion engine is the process of setting the time that a spark will occur in the combustion chamber (during the power stroke) relative to piston position and crankshaft
angular velocity. Setting the correct ignition timing is crucial in the performance of an engine. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled by an engine control unit use a computer to control the timing throughout the engine's RPM range. Older engines that use mechanical spark
distributors rely on inertia (by using rotating weights and springs) and
manifold vacuumin order to set the ignition timing throughout the engine's
RPM range. There are many factors that influence ignition timing. These include which type of ignition system is used, engine speed and load, which components are used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.
“Timing advance” refers to the number of degrees before top dead center (
BTDC) that the spark will ignite the
air-fuel mixture in the combustion chamber during the
compression stroke. “Timing retard” refers to the number of degrees that ignition is delayed after the point that would have resulted in generating maximum power.
Timing advance is required because it takes time to burn the air-fuel mixture. Igniting the mixture before the piston reaches top dead center (
TDC) will allow the mixture to become fully burnt soon after the piston reaches TDC. If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur sometime after the piston reaches TDC allowing the ignited mixture to push the piston down the cylinder. Ideally, the time at which the mixture should be fully burnt is about 20 degrees ATDC. This will utilize the engine's power producing potential. If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston causing
detonation and lost power. If the spark occurs too retarded relative to the piston position, maximum cylinder pressure will occur after the piston is already traveling too far down the cylinder. This results in lost power, high
emissions, and unburned fuel.
The ignition timing will need to become increasingly advanced (relative to TDC) as the engine speed increases so that the air-fuel mixture has the correct amount of time to fully burn. Another reason for advancing the timing is because as the engine speed increases, fuel consumption increases. Since more fuel is present in the cylinder, the time required to fully burn the air-fuel mixture will be longer. Poor
volumetric efficiency at lower engine speeds also requires increased advancement of ignition timing. The correct timing advance for a given engine speed will allow for maximum cylinder pressure to be achieved at the correct crankshaft angular position. When setting the timing for an automobile engine, the factory timing setting can usually be found on a sticker in the engine bay.
What is Timing? 
