Ignition Timing: The timing of the spark relative to the operation of the valves and the placement of the piston in the cylinder. BTDC stands for “before top dead center” a reference to a mark on the main pulley on the front of the engine. For most engines, when the timing is set to 0 degrees TDC, the number one piston will fire at the very top of the stroke. For vehicles where the manufacturer specifies a timing setting, the Smog Check inspection allows plus or minus three degrees from that setting. For vehicles where the manufacturer specifies a timing range, the vehicle must be within that timing range. Advanced timing is when the spark fires before the piston has reached the top of the stroke. Retarded timing is when the spark fires after the piston has reached the top.