Can someone explain the difference between ingition coils with, and without, an internal resistor. Yeah, I know, one has a resistor inside and one does not! But, when is one needed over the other and why? How do I tell if one has an internal resistor?

I have a Ford flathead 6, 226cid, 95 hp, with a 12V negative ground system and I am trying to understand why I would need one type of coil over the other.

Can someone give me a coil part number for my engine to ask for at a parts store?

Any explaination about the differences would be greatly appreciated.

 

I don't know of any specific examples of an application requiring one type of coil over the other. Most of the vehicles I have worked on include the resistor in the vehicle harness, either as a large white block on the firewall, or a special length of resistance wire that feeds the coil +. You can go either way, though I don't know how hard it is to get one with a resistor built in.

Some of the aftermarket ignition systems may not require a resistor, and the GM HEI does not require a resistor.

So, if you are doing your own wiring, it's your choice. If you don't know what you have, then run the engine and measure the coil + voltage. If it's 12v, then you don't have a resistor and you should put one in. The voltage should be somewhere around 9 volts. This limits the inrush of current, letting the coil run cooler and making the points and or ignition box last longer.