The variable dwell system is simple and effective. The energy from the coil is finite per time. The engine wants a fat spark of a given energy per stroke, irrespective of revs so it follows that the finite energy from the coil is thinly distributed at high revs so the energy per stroke is significantly less than at low revs. Variable dwell takes care of this by restricting the energy at low revs by varying the dwell. If follows that a coil that is capable of producing the energy required at high revs is needed, the energy from which, the ignition system restricts at low revs. It is therefore important to use the correct coil as advised by the ignition manufacturer. Our variable dwell product, Magnaspark requires a 0.6Ω coil.