Before Top-Dead-Center (BTDC)

Definition: A common term used to indicate the amount of ignition advance. For instance, 10 degrees BTDC indicates the ignition timing is set 10 degrees before top-dead-center.

Examples: Setting the ignition timing, so that the spark is initiated before top-dead-center, is necessary because of the time delay before the explosion reaches maximum force. The objective is to ensure the piston has begun its downward (power) stroke as the expanding gases reach their maximum pressure.