Replace the stock internal coil of your HEI with an MSD. The coil features lower inductance and high temperature windings which allow the coil to charge faster delivering increased spark energy at higher rpm. The MSD Coil mounts in the stock HEI cap and installs in minutes. To receive the most from the coil, it is recommended to replace the module with MSD's powerful HEI Module, P/N 121-83645.