Inliners International Forums General Discussion Engines internal or external resistor Coils?

I have alway bought coils with internal resistors. I just borrowed a coil from a friend that is not internal resisted. He said he has never ran a ballast resistor even with that coil. Another buddy said he burned a set of points in his VW with in a couple miles. What is correct? Why do they even need to be resisted? It seems that the more juice the better.

Dear Dan;

The external resistor used in the 50s, reduced the voltage from +12V to about 6.8V when it warmed up. The coils used then were basicaly the same as the older (6V) ones which ran at 6.8 to 7.2V.

This allowed the coil to have full voltage in the 'start' mode, lowering it in the 'run' mode, which added to point life.

Later (60s) the resistor(s) were "internalized" or done away with & the coil primary winding made heaver, where some MFG used a 'resistor wire' in the primary circuit.

Happy trails.

Does that mean I can run my 6v coil on a 12v car? I never ran a ballast resister with the 6v, so I think it is internalized.

I don't think a 6v coil will last very long hooked to 12v . I've tried it before for short periods of time when testing a used engine & found that a 6v coil will get very very hot.Early Chevrolets were wired with a "by-pass" wire to feed full 12v to the ignition when starting & then route the "juice" thru the resistor in the normal run position. The wire on later models is left bare on the starter solenoid or left off later solenoids alltogether.

You can't use a 6V coil on a 12V system. Older vehicles that were 6V just ran full 6V to their coils. No resistor was needed. If you ran a resistor on a 6V car you'd end up with less than 4 volts going to the coil.

Dear Dan;

No you can't as Tom & Mike stated.

I would work in the run mode (6.8) but the first time it got +12V for any lenght of time, it would be 'toast'.

cabbie,
you can use the old 6 volt coil in the 12v system if you install a resistor in front of it. problem is the resistors are unreliable. alot of folks years ago would keep a spare in the glovebox. $$$ is better spent just buying an internally resisted coil. just my 2 cents.
robert

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.

I've tested many GM coils with a meter,some were stamped 6 volt on the casr,some were 12 volt.All of them measured 1.2-1.5 ohms between the primary terminals.That means all the coils I tested had the same primary resistance reguardless of listed voltage which also means the coil cares less about the system voltage so long the voltage it sees is around 7.5 -9 volts for proper points life.Putting a voltmeter on a running 12 volt system after the ballast resistor but before the coil, shows about 9 volts on GM vehicles.
12 volt points ignition systems uses a total of 3.0 ohms or so,half of that figure comes from the ballast resistor.Bypassing the resistor on starting is benificial on many engines,but not all.

Dear Tony;

The later (12V) coils have a heaver primary winding which will not show on a ohmeter or 'continuity test'.

An early (6V) unit will work in a "pinch" but not worthwhile in the 'long run' because their designed for the lower voltage.

You can "spin the numbers" if you like but the two are different.

Happy trails.

Yes,they are different of course.Wire gauge size relates to the current in the secondary side,not voltage.The heavier windings allow for a stronger magnetic field in the primary,which gives a higher voltage spark on the secondary side.When 12 volt systems came out,the engines were more powerful,turned higher rpm's,so the coils needed to make a more powerful spark.I also believe the secondary windings were changed as well.
A 6 volt coil run on the slightly higher ballast resistor controlled voltage of a 12 volt system won't have a shorter service life,but it will have less voltage and current available to jump the plug gap.

It also allows for the higher voltage in the primary as well. The higher (output) voltage is just an added benefit for a 12V system, hence the change to it.

A 6V coil (early) with the lesser voltage design primary, will not "put up" with the 12V 'spike' it gets every time the Ing. switch is turned on with the points closed, before the resistor heats/the starter engages.

If your resistor were a higher/constant value, then the voltage during the (starter draw) phase will be to low for a proper start (if any).

This is why there are 2 types (6 & 12V) of coils.

It maters NOT where (external/internal) the resistor is, which was the question.

Actually the ballast resistors used on early 12 volt systems was bypassed whenever the starter was engaged so you got whatever voltage was available (less than 12 volts due to starter current draw) and did protect the points somewhat if you left the switch in the run position whether the engine was running or not. You knew when you needed a new ballast resistor whenever the engine would start but not run when you released the key.

I too have measured several "6", "12", and "24" volt coils and all had the same primary resistance. Both the 12V and 24V systems used 1.5ohm ballast resistors. I didn't quantify the outputs but there was no noticable difference in the maximum length of the spark/voltage between stock and hi po coils when triggered by points. In other words, a stock coil is fine for points ignitions. The important parameter is the current. Too little won't saturate the coil and give you full output; too much overheats the coil, burns out the points, and provides no additional energy. The main benefit to going to higher voltages is that the coil saturates quicker. The was needed to go to the higher rpm and 8 cylinder engines which had less dwell time.

Thanks for all the info! This week end I gave the coil back to my buddy who used it to start up his fresh built 383. He hooked the coil up straight to the battery, and it got very very hot. It ran, but I am still going to gets a new one with an internal resister. I know I can just buy a resister, but I dont what any thing on the motor, or firewall that didnt come originally.