Coils

[Curti]

What is the difference between a 6V & 12V coil? Prestolite used to market a coil for 6 & 12 V. My son has a 1940 Chevy that has loged many thousands of miles with a 12 V coil.

[Guest 47jag]

Curt,

It's all about ratios. The main requirement is that you end up with sufficient voltage to jump the plug gap so there is a winding ratio of primary to secondary. The voltage needed to spark the plugs is a minimum of 10K volts. As soon as you put a load on the engine this will rise to anything up to 20K volts depending on various factors like compression ratios & fuel mixture. The other consideration is that the points are able to handle the current of the primary windings. Too few turns on the primary windings and the current will be higher. Too many and it will take too long to reach saturation and longer to decay which can be a problem in higher speed engines. To get the voltage required the ratio will be in the order of 3000:1. The 12volt coil on your son car is obviously OK but it will not be pushed as hard as it would have been in it's heyday so that it wont make any great demands on the coil. It will be less hard on the points too.

Art

[Curti]

Very interesting, thank you Art. So a 12V coil is ok in a 6V car as long as it is driven reasonably. No Bonniville salt flat runs. I seriously doubt if the 40 Chevy has seen 3,000 rpm in the entire time he has owned the car.

[Guest wombvette]

The answer is nothing really.. There is a slight difference in winding for polarity, but even that is not critical as long as polarities are maintained. Think about it, most 12V coils are operated with a ballast resistor which make it operate in the 6-8V range (at least statically). I know, we can argue the ballast resistor is actually a current limiter, but it does the same thing. A 12V coil will operate just fine in a 6V system.

[Curti]

The answer is nothing really.. There is a slight difference in winding for polarity, but even that is not critical as long as polarities are maintained. Think about it, most 12V coils are operated with a ballast resistor which make it operate in the 6-8V range (at least statically). I know, we can argue the ballast resistor is actually a current limiter, but it does the same thing. A 12V coil will operate just fine in a 6V system.

Maintaining polarities is something else that confuses me. I have found, quite by accident that the car runs OK with the IGN wire connected to either terminal on the coil.

My car is + ground, so my reasoning is to connect the IGN wire to the - side of the coil so it ultimatly grounds + at the points.

[Guest wombvette]

Maintaining polarities is something else that confuses me. I have found, quite by accident that the car runs OK with the IGN wire connected to either terminal on the coil.

My car is + ground, so my reasoning is to connect the IGN wire to the - side of the coil so it ultimatly grounds + at the points.

That is correct.

[Pete O]

And isn't it true that a 12v coil is really more like an 8v coil, because cars with 12v systems usually have resisters built into the ignition circuit to reduce the voltage going through the points to protect them from pitting?

[Guest wombvette]

In effect that is true. 6-8-12 it really don't make much difference. 6V coils usually don't have ballast resistors. 12V usually do. It is a very simple device with a complicated electrical explanation. Since a coil is not a resistor in operation, it cant really be said that the ballast resistor reduces voltage. In operation it is a current limiter.

[Joe Block]

I seen some rebuilt put a modern high-out coil in the old casing then trick it out to maintain the original look of the factory coils. It about 1 $200 modification and you furnish the coil casing, not cheap.

I have tried, but I thinking about trying a high out, if works then putting it a damage burn out coil body to maintain the marginally. Just to many projects. But it great for winter project on these cold days.

[keiser31]

From my last coil experience..never hook up a 12V battery to a 6V coil and run it for very long.....they explode and cover your Model A engine compartment with oil. O.k.....I had an excuse for doing that. I looked under the hood to see an alternator. I, not knowing that 6V alternators were made, thought...mmm...must be 12V. WRONG! Another lesson learned.

[Guest wombvette]

That way you had a V6 coil running on 12V without a ballast resistor. There was no voltage drop or current limitation. Probably got a bit hot.

[Leif Holmberg]

Maintaining polarities is something else that confuses me. I have found, quite by accident that the car runs OK with the IGN wire connected to either terminal on the coil.

My car is + ground, so my reasoning is to connect the IGN wire to the - side of the coil so it ultimatly grounds + at the points.

Thanks Lamar for your answer.I`m glad that 99.99% don`t use this as I meen political argue as Curti does.He think he belive he can change the hole world I think with those input.

Thanks Leif in Sweden.

[Rusty_OToole]

I have started 6V cars on 12V and run them for brief periods. One I let idle for half an hour, eventually the coil overheated and it quit. When the coil cooled down it was good as new.