Measure your voltage at coil positive, if it is 12 volts or higher with a full battery charge you will either want a 1.5 ohm or higher resistance coil or a 1 ohm ballast resistor. If you measure 8 or 9 volts you do not need a ballast resistor. If you are doing this with a PerTronix Ignitor installed, do not leave the ignition on for more than 30 seconds while reading the voltage or you can fry the module.