The voltage on a stock coil is reduced through the ballast resistor to about half of the avaiable voltage so 6.8 sounds about right. Read the papers that came with your new coil. Normally you have 12 volts to the MSD box and it supplys the voltage to coil. If you are not using an MSD box putting to much voltage to the coil will cause it to overheat and shorten its life.