Ignition Timing, MBT and Combustion
Ignition or spark timing describes when the spark plug fires relative to the piston’s position in the cylinder. Because the air–fuel mixture does not burn instantly, the spark must occur before top dead center (TDC) to allow pressure to build so it can push effectively on the piston during the power stroke. Spark timing sets when combustion begins, combustion behavior determines how pressure rises, and MBT defines the ideal timing for maximum torque. Engines continuously adjust timing to stay near MBT while avoiding knock, balancing performance, efficiency, and durability.
Effects of Advancing and Retarding Timing
The ignition timing is the angle of the crankshaft relative to TDC when the air-fuel mixture is ignited. Positive ignition timing means that the spark will occur before TDC is reached (during the compression stroke) and negative timing means the spark is occurring after TDC is passed (during the power stroke). If the timing angle is increased, it is said to be advanced and if it is reduced, it is said to be retarded.
If spark timing is too retarded, combustion pressure rises late, after the piston has already begun moving down. This wastes potential energy and reduces torque and fuel efficiency. If timing is too advanced, pressure builds while the piston is still moving upward, which can reduce smoothness, increase mechanical stress, and raise the likelihood of knock. Proper timing balances these effects to maximize the reliability and torque the engine produces.
Combustion and Flame Speed
The spark plug does not control how fast the mixture burns; it only initiates combustion. Flame speed is influenced by air–fuel ratio, intake turbulence, pressure, temperature, and fuel characteristics. A highly turbulent, well-mixed air-fuel charge burns faster, while lean or uneven mixtures burn more slowly. As engine speed increases, the piston moves faster, so spark timing must be advanced to compensate for the limited time available for combustion.
Maximum Brake Torque (MBT)
MBT, or Maximum Brake Torque timing, is the spark advance that produces the greatest torque for a specific engine speed and load when knock is not a concern. At optimal MBT timing, peak cylinder pressure typically occurs around 15–20 degrees after TDC. This placement allows the expanding gases to push on the piston when the crankshaft has the best mechanical leverage. Advancing spark timing up to MBT increases torque, but advancing beyond MBT does not add power because pressure builds too early in the cycle.
MBT vs. Knock Limits
Knock occurs when unburned air-fuel mixture ignites before the spark due to excessive pressure and temperature, which can damage the engine if uncontrolled. Advancing the timing increases engine torque up until MBT is reached however the timing is often set slightly retarded from MBT at higher loads to avoid knock. In most real-world cases, an engine will knock under load before reaching MBT but this can vary largely on the engine configuration.
Timing vs. Boost
In a boosted engine, the timing can often be significantly retarded from MBT as a safety measure to ensure that the engine doesn't produce excessive low or mid-range torque. High torque at low engine speeds increases the force on the pistons, rods and bearings which can result in bent rods or failed bearings if tuned improperly. It is common to see an ignition map for a turbo car where the timing is most retarded at the point where maximum torque is reached and timing is advanced gradually as RPM increases.