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Let's list the factors affecting establishing a good spark:
1) The gap size: The bigger the gap, the more juice needed (I seem to remember some old rule of thumb relating that for every thousandth of an inch increase in gap, you needed five thousandth more volts to jump it...I cannot remember exactly).
2) The geometry of the gap: Small pointed electrodes require less voltage. Note that surface conditions makes a big difference.
3) The temperature of the electrodes and the incoming charge: High temperatures allow for low voltage (but increase probability of knock).
4) The density of the charge: High densities (like those under boost) require more voltage to spark.
5) Leakage resistance of the insulator (i.e., electricity always wants to take the easy way out): Maintenance can take care of this.
6) Rate of increase of the voltage at the gap: We are bound by design here, so there is no sense worrying about this.
7) Ionized gases: Their presence, if any, reduces the voltage needed. More of an issue for multi-spark systems than our long duration systems.
8) The air-fuel ratio of the charge: Why? Because it sets the electrical properties of the charge. Lean mixtures are harder on arc initiation than slightly richer ones.
9) Electrode material.
Having obtained a good spark, we still need to obtain ignition. And yes, it has its own list of issues:
1) A combustible mixture must be between the electrodes during arc.
2) Large gaps increase the probability of regular firing - except when #4 above, in which case misfiring can occur.
3) A high density mixture liberates more energy when sparked increasing the probability of flame initiation.
4) It follows that it is easier to ignite slightly rich mixtures than lean ones - so much for gas mileage, eh?
5) The position of a plug and its electrodes in the chamber relative to the flow inside that chamber.