In a gasoline-powered internal combustion engine, an air/fuel mixture is drawn into the cylinder through the valves by the motion of the piston, and ignited by the spark plug. The easy assumption is that the mixture should be ignited when the cylinder reaches the top of its travel so that the resulting expansion of gases can force the cylinder down. However, it takes some time for ignition to occur. You want the reaction to be well underway when the piston reaches the top of its travel, with the connecting rod directly upright (or at least parallel to the cylinder wall), known as top dead center.
The solution is to advance the timing, so that ignition occurs before the piston reaches top dead center. The number of degrees to go before you reach that point is known as the timing advance. It is given in degrees before top dead center, or °BTDC.
As the number of revolutions per minute (or RPMs) increase, the amount of time between a given advance and top dead center decreases. To solve this problem, cars have an automatic timing advance which increases the timing advance as RPMs increase.
Most vehicles advance timing between 10 and 20 °BTDC. To find out what timing advance your car requires, refer to your service manual or call your dealer. Timing is adjusted with the aid of a timing light, which generally costs between US$40 and $150 depending on quality features.