Are you wiring them in series? If wired in parrallel they will in fact blow quickly above 12 volts. Ford originally wired them in series and I'm not sure what year they began to put a choke coil (inductor) in the system to improve the life of the bulbs.



I you have true mag bulbs and they are wired correctly, they will be pretty dim at low RPM and brighten up with speed. They may also blow at a high engine speed with a strong mag but they should not go out at 14 volts.