What is the higher voltage difference of spark plug firing in open air , as compared with plug in engine combustion chamber under compression. 6000 volts air spark to ? in engine. Diodes have to be used to protect against this higher volts.
Typically somewhere between 6 - 20KV for the arc to strike, then it drops to 1 - 4KV for the duration of the arc.
It depends on the plug gap and compression etc.
This page has examples of the voltages and waveforms:
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