The OP is correct,12 volts WON'T do that as the insulation of air doesn't start to break down until we get to between 2000 and 3000 volts[li].
Are you sure it is actually arcing across as you've got individual coiplacks,so there's no HT floating around the engine bay like there is in mine? Even if it was ignition coil HT I doubt very much if it could even to ground across more than about a one inch gap.
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