GT40 is a low-energy coil suitable for use with points distributor or electronic distributor. Since the GT40 has a built-in resistance, you must bypass the external ballast resistor when used with the stock points distributor. The GT40R is slightly different and uses the original ballast resistor.

Of course a high-energy coil is best for use with electronic ignition. The Bosch HEC715 or Nissan Pulsar coil work well.