You are correct, I'm not running a shielded wire, but I am running a carbon core wire and a resistor plug. The Bolly prop, 20X8 is turning 7200 RPM.

As far as the dwell time, I think anything around a 1" diameter will do fine. If the dwell time is too short you want get a spark, if it's too long you will use more current and warm up the coil.

Jim