There are two ways to run the coil when converting to 12 volts. You can use a 12 volt coil that doesn't require a resistor or you can use a coil that requires one with a resistor. One of the advantages of the resistor setup is using a resistor bypass when the starter is engaged. This provides a full 12 volts to the coil while the starter is cranking over the engine. It sounds like you have the setup without the resistor and it is working fine. I bought a coil like that last year and it had writing on the side that stated it didn't need a resistor.
