I'd beg to differ on the ignition timing thing a little Jaba. Whilst there may be variation between dynamic ignition timing and static depending on engine speed (i.e. torsional vibration effects), at a constant speed the ignition timing is pretty consistent. Certainly less than a degree variation - you can see this clearly with a timing strobe as I have done on many test engines.
What does change from cycle to cycle is the ignition delay, which is conventionally defined as the time between the initiation of the spark and when the mass-fraction burned reaches 10% - that is 10% of the total heat energy has been released. This usually takes a very long and variable time compared to say the 10-90% burn time which is usually relatively more consistent. All of this is calculated from indicator diagrams - these days in real time with all the fancy DSP available to us.
The ignition delay is a function of how fast the flame kernel grows from the spark and depends on mixture ratio, preparation, micro turbulence, combustion chamber and gas temperature and probably also the phase of the moon.