5 pinners with a functioning 5th pin (OE ECU's) require the 4 terminal "dual" ballast but a 5 pin with a dummy 5th pin or a 4 pin ECU can use a 2 terminal "single" ballast and with that last setup if using a dual ballast you can add a blue loop of wire on the other end of it to make it in effect a parallel "single" ballast to reduce the ohms for a slight degree of performance increase at the possible expense of longevity. You have ALOT of leeway on what you can and cannot get by with in regard to increasing the primary current with a lower ohm ballast. Always carry spares & your meter.