They are referring to the same thing. The spark has to occur before the piston gets to top dead center because the gas and air do not explode; they burn rapidly, and it takes some time for that to occur. The spark has to occur at varying times before top dead center depending on engine speed, load, temperature, and other factors.
Since the Engine Computer can't get a timing pulse from a sensor, then fire the spark plugs a measured amount of time BEFORE that pulse arrived, there is quite a bit of initial timing advance built in to that signal, then the computer varies how long of a delay is needed before the spark should occur. For example, the pulse might be designed to arrive at 30 degrees before top dead center. If the calculated desired spark timing is 10 degrees before TDC, the computer waits 20 degrees after the signal arrives to fire the ignition coil. The spark advance in this example is the desired 10 degrees before TDC.
Sunday, July 29th, 2012 AT 4:29 AM