1.5 ohms across the resistor is right IF the coil primary (coil + to coil -) measures about 2.4 ohms and you want the coil fed with 8.5 volts while running with 13 volts in the electrical system.

Math is like this:

Ballast Resistor ohms = Coil primary ohms / (13 / 8.5)

where 13 is the running voltage in the car and 8.5 is the target voltage desired on coil + when the points are closed.

Measure your coil primary ohms (coil + to coil -) with no wires connected to coil + and then plug that number into the formula to see what value you need for the ballast resistor. Unless your coil has a built-in ballast resistor. Then you don't need an external resistor AND the bypass wire does no good at all.

Last edited by Bill Hanlon; 05/31/2026 9:42 PM.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.