I have a 1957 312 Y block and I'm installing a PerTronix original Ignitor.  The helpful tech folks at Pertronix informed me via email (and I quote here): "With the original ignitor and a coil with 1.5 ohm of primary resistance (or greater), remove the resistance wire.  If the coil has fewer than 1.5 ohms, do NOT remove the resistance wire."  The tech didn't explain why the resistance wire/ballast resistor should NOT be removed if the coil has fewer than 1.5 ohms, but I think it's safe to assume it's because the coil will become too hot, creating more underhood heat, and eventually failing completely.  So here's my question: how do I determine the ohm rating on the coil now on my motor, and where do I go for a new coil if I need one that has 1.5 ohms of primary resistance or greater?  Many thanks for all thoughts. 

The major aftermarket parts outfits - like Jegs and Summit publish the primary resistance specifications of most of the automotive coils they sell. So just do a little research and you will find one that can do the job.

The Pertronix Ignitor uses a "power transistor" to ground the coil instead of the points. A timing circuit "fires" the transistor, matching revolutions to the distributor rotor - and holding the coil ground for a time interval equal to the (old term) "dwell". The "dwell" and the primary resistance of the coil determine how many amps flow through the transistor and for how long. If you use too small of a primary resistance - the coil alone - or coil plus ballast resistor ....it will fry the power transistor in the Pertronix. Too many amps will flow.

You can test a coil by using a VOM meter across the primary terminals - but the value is typically low and the scale would have to be like "0-10" or smaller - or maybe a digital type.

I would suggest you use an "E-core" coil such as is found in the late Ford TFI electronic ignition system. They are made to run at higher temperatures - and have no oil inside to cook. The steel frame for the coil is shaped like an "E" - they are square and attach with four screws to an upright mounting bracket. There are many of them available - including the stock Ford type - and you would simply need to research the OHMs - or measure it directly.

On my outfit I chose to use an MSD "Streetfire" E-core coil - primary resistance is .44 OHMs and I paired it up with an MSD ballast resistor of .88 OHMs for a total of 1.3 OHMs. But...I am using a different controller than you and at the dwell it provides, the current won't cook the transistor immediately. To use an early Ignitor, the ballast resistance would have to be higher to create a sum greater than 1.5 OHMs.  

 
 Steve Metzger       Tucson, Arizona

Steve:  Thanks for that great information.  I'm heading out the door right now for a 3-day trip, but will email you a couple more questions when I return if that's okay.  Ron

6 volt coils typically have a primary resistance of around 1.5 ohms and 12 volt coils have about 3.0 ohms resistance. Since the standard electrical equation is Volts / Ohms = Amps we see that a 6V coil and a 12V coil both operate at about 4 Amps. Any more amperage than that and the coil melts. That's why you have to run a resistor on a 6V coil used with a 12V electrical system. The easy solution is to buy a true 12V coil such as a NAPA IC14 which is marked "No External Resistor Needed". Back in the 50's when the auto companies switched from 6V to 12V they just put a resistor in line with the coil and still used 6V coils.

awhtx:  Thanks for the tip.  I bought one of the Napa coils today ($27) and I'll install it tomorrow and see how things work out.  Ron

Update: After buying the coil from Napa, I learned that PerTronix sells a 40k volt, 1.5 ohm coil (internal resistor) designed to work with the original Ignitor I am running on the car.  It works without an external resistor and the part number for the black one is P/N 40011.  The coil is available in either black or chrome, and comes either oil-filled, or epoxy-filled for extreme off-road use.  I bought the black, oil-filled coil and it is working very well on my car.  It is especially helpful with cold starts in the morning, and hot starts after it has been run for a while on hot days.  For people running the PerTronix Ignitor II, they offer a 45k coil rated at only 0.6 ohms of resistance.  For the black, oil-filled coil, the PerTronix Part number is 45011.  So the Napa coil I bought is still in the box and will go on a shelf in the garage for a future project.  Thanks for the responses to my original post.  Ron

If you are running an oil filled coil make sure it is installed in a vertical position as it will overheat and burn out.



Most of the older Fords in the Y-block years had the coils installed horizontally.



The epoxy ones can be installed in any position vertical or horizontal position.

marv

Marvh:  Didn't the stock coils come filled with oil and mounted horizontally?  Ron

It has been so long since I cut an original coil apart I do not remember.



These new after market coils today produce substantially higher discharge voltage than the original coils so produce much more internal heat.

Here is a link to the MSD site for their oil filled and epoxy coil as to recommended installation.



http://www.msdignition.com/instructions/Products/8223_instructions.pdf?terms=blaster

marv

Typically a spark plug in ordinary condition in a compressed combustion chamber will fire at voltage in the range of 10,000 to 12,000 volts. Even if the coil has a (dielectric) rating of 50,000v or 40,000v or whatever - you still just get 10-12,000. Makes a good sales pitch though.....the high dielectric strength would be helpful for a Capacitive Discharge ignition (where the coil is being used as a step-up transformer).

The heat in the inductive spark comes from the amps in the coil primary - and importantly, how fast it can get fully energized to "fire". As the engine runs at higher and higher speeds, the time available to "saturate" the coil primary with amps gets smaller and smaller. Modern coils have low primary resistance to enhance the speed of saturation - and they are constucted with high temperature insulation - no more oil to cook.

Older inductive ignition systems had to have both considerable primary resistance in the coil and oil for cooling in order to survive with an acceptable lifetime. This was pretty much matched by the thermal capacity of the points so the system had some similar limits. High performance dual point distributors were invented to increase the available coil charge time (dwell) as the rev's went up - and share the heat load on the points. In some instances the stock coil life dropped in that sort of application - remember when everyone went to using the big Mallory coils for the "hot ignition set-up"? Way more thermal capacity.

When we switch to an "electronic" ignition - using a transistor to ground the coil circuit instead of the points - it has a lot more thermal capacity - can carry more amps with an acceptable lifetime. As a result, its OK to run more voltage into the high temperature / lower primary resistance coils (full battery / charging system volts is fine) and the combined coil primary / ballast resistance can drop to 1.5 OHMs (and less in some circumstances). The spark in the chamber is still the 10-12,000 volts as before - but the heat goes way up due to more amps. Coils for use with electronic ignitions tend to be made for "all position" mounting - but primary resistance could be lower than you want it to be for use with points.

 
 Steve Metzger       Tucson, Arizona