Ignition Timing & Knock Control

Concepts

Ignition

For more information on how fuel is delivered, see our documentation page on Fueling & Injection

Ignition and Degrees BTDC

Gasoline engines operate on the principle of fuel combustion. In a direct injection system, first, fuel is directly injected into a cylinder roughly during the time the piston is traveling downwards during the intake stroke. Then, it is compressed during the compression stroke as the piston travels upwards towards top dead center (TDC). Relative to the top dead center position, and with its precise timing varying based on the tuner's map, the spark plug is electrically ignited by a signal from the ECU to start the process of combustion.

To control this ignition timing, the ECU has a series of maps that define when the ignition spark should take place, relative to the top dead center position of the piston as it ends the compression stroke and is begins the power stroke. Typically, and in the Subaru DI logic, this timing is measured in degrees before that top dead center position relative to the crankshaft rotation, or BTDC. Here is a reference of BTDC values and the timing they are associated with relative to cylinder events:

Extreme values such as 45.00°, and 90.00°, and 180.00°, negative or otherwise, are only provided as an example reference to cylinder events; these values are entirely unrealistic for engine operation, and should not be used in real-world tuning.

Degrees Before Top Dead Center (BTDC)	Event	Relative to TDC
180.00°	End of intake stroke, beginning of compression stroke.	Before
90.00°	Half-way through compression stroke, piston travels upwards towards the valves.	Before
0.00°	Top dead center, power stroke begins.	TDC
-90.00°	Half-way through power stroke, piston travels downwards towards the crank-shaft.	After
-180.00°	End of power stroke, beginning of exhaust stroke.	After

 

Knock, Detonation, Pinging, etc.