ignition timing
Of a spark ignition internal combustion engine, the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.
ignition timing: when the spark fires relative to piston position
In a spark ignition engine, the spark plug must fire before the piston reaches top dead center (TDC) on the compression stroke, not at TDC itself. Ignition timing describes this advance in crankshaft degrees: how many degrees before TDC the spark ignites the air-fuel mixture. A typical gasoline engine fires the spark 10 to 40 degrees before TDC, depending on engine speed, load, and fuel octane rating. This window exists because combustion takes time, about 30 to 50 milliseconds from spark to peak pressure, and the flame front must reach maximum pressure just after the piston passes TDC on the power stroke for efficient expansion and power output.
Ignition timing advances and retards with operating conditions. At idle and low load, timing is typically retarded (closer to TDC) to reduce engine knock and improve emissions. As engine speed increases, timing advances further before TDC because the piston moves faster and combustion requires more lead time to complete before power delivery occurs. Modern engines use electronic control: engine computers calculate optimal timing by reading inputs from sensors that measure intake manifold pressure, air temperature, coolant temperature, throttle position, and sometimes exhaust oxygen content. Mechanical ignition systems in older vehicles used centrifugal weights and vacuum diaphragms to adjust timing automatically.
Knock and pre-ignition are the primary failure modes. If timing is too far advanced, the flame front ignites and pressurizes the mixture before the piston finishes compressing it. This rapid pressure rise creates an audible metallic pinging or knocking sound and generates shock waves that damage piston rings, bearings, and cylinder walls. Conversely, if timing is too retarded, combustion completes too late in the power stroke, wasting fuel and producing high exhaust temperatures. Some knock is borderline acceptable in modern engines because knock sensors detect it and command the computer to pull timing back by fractions of a degree.
The term "ignition timing" comes directly from the physical action: you are controlling when ignition occurs relative to mechanical position. Degrees of crankshaft rotation serve as the measurement because they are absolute and repeatable across all engine speeds. A crankshaft that rotates 360 degrees per two full cycles defines the timeline; firing 20 degrees before TDC means the spark occurs when the crankshaft is 20 degrees away from its topmost position on compression.
Timing is not adjustable by the operator in modern fuel-injected vehicles; it is computed in real time by the engine control module. Older carbureted and mechanical-ignition vehicles had a timing adjustment bolt on the distributor, and mechanics would use a timing light, a stroboscopic instrument that flashes in sync with spark pulses, to align the timing mark on the crankshaft pulley with a mark on the engine block. Even small misadjustments of 3 to 5 degrees noticeably affect driveability, fuel economy, and emissions output.