The ignition probe(HT25) voltage is produced by mutual induction between the primary winding and the secondary winding, with the central soft iron core intensifying the magnetic field between them.
The voltage measured at the spark plug is the voltage required to jump the plug gap in varying conditions. This voltage is determined by a spark plug gap, rotor air gap etc.
The plug kilovolt (kV) requirement of older engines tends to be lower than that of the modern engine, as the later designs run higher compression ratios, leaner air/fuel ratios, and larger spark plug gaps.
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