Dear Dan;
When an engine is cranked for starting the battery voltage drops to around 10 if all is well in the system. Many go far below that.
A 6V coil with a resistor 'in front' will drop the primary voltage so low (4VDC +/-) that they become really hard to start if at all.
Ceramic (wire wound) ignition resistors today are very reliable & made to take the heat in 'normal' operation or if the key is left on with the points closed etc.
Don't get confused here.
