So my coil went bad over the winter storage. Cost of TI coils cost at least 3 times more ($140.00) vs $50 for standard unit. Can someone explain to me why they are different please?
Thanks,
Jim

 

The TI coils are different (than your average points type coil) primarily in that they are lower inductance/resistance to help get more energy into them at higher RPMs. There's a couple points type coils that are built close enough that they would deliver higher energy amounts at higher RPMs too, but there's one more very important requirement for a coil to operate properly in a TI system. The TI coil needs to have a higher turns ratio than a points coil. The old wive's tale is that the higher turns ratio provides a higher plug voltage. That's incorrect, as it's the plug that determines the arcover voltage, not the coil. What the higher turns ratio actually does is reduce the primary winding flyback voltage that the sensitive switching transistor has to deal with every ignition event. A regular points coil (with about a 100:1 turns ratio) could possibly blow up the TI switching transistor, but a TI coil IIRC has about a 140:1 turns ratio. If you take a somewhat extreme plug arcover voltage of 30,000 volts, the points coil primary voltage would be 30000/100= 300 volts. The same 30000 volts with a TI coil would impose 30000/140= 214 volts on the switching transistor (and also the zener protection diode). A bit safer for fragile antique electronics.
For amusement a few weeks ago I was looking through the Summit catalog and did see one or two reasonably priced can coils that looked not perfect, but promising as a replacement in a TI application. (Disclaimer: I don't lose sleep if my car isn't NCRS compliant.)

 

69427,
Thanks for the information. Can you tell me which ones from Summit are the ones you think would work? I'll be calling Summit tomorrow.Also ordering a high torq starter.
Jim

 

Some numbers to ponder.

 

These look like good candidates.
I didn't check any of the other brands.
https://www.holley.com/products/igni...ils/parts/8145

https://www.holley.com/products/igni...s/parts/8140HV

 

buns and Berry,
Thanks for the help.
The Accel 8145 looks like the one. Just checked Summit because I have other things to order and they don't sell the 8140HV on their website.

 

I'm currently tied up at a family gathering and not near my Summit catalog, but if you're doing some research on the coils be sure to look for the actual turns ratio, as DC resistance measurements of the secondary windings is a bit more of a crapshoot than I'm comfortable with. I'd also ask the actual brand/manufacturer techline guy what the maximum continuous primary current rating is, as IIRC unless the engine is actually rotating, the dwell is turned on 100% in TI systems, similar to having the ignition on when the points are closed.

 

After speaking to summit I have a couple more questions.
-The chart above indicates that for a GM TI ignition I need (Primary .38-.51 / secondary 8.2K-12.4K ohms)
The Accel #8145 coil is(Primary 0.7 ohms)is the primary at .7 is too high.The tech supper guy thought it was too high. Secondary is fine.
Is the 0.7 too high for the primary?
Thanks
Jim

 

Jim

Are you running the an original TI ignition board or one of the newer solid state modules? Most of the solid state modules let you use a standard coil with the system.

John

 

John,
I am running the original TI system at this time. After reading your response Im going to check into upgrading to the new internals for my system. Thanks for posting attachment.
Jim

 

Some good circuit board info from JC. The cost of the updated circuit board is pretty reasonable (compared to the TI coil prices), and the more modern output transistor allows more flexibility in coil choices. Just for additional understanding, I'd ask the replacement circuit board manufacturer if the circuit board has safety circuitry in it to turn the coil off (slowly!) if the key is left on too long when the engine isn't actually running. This feature just keeps the coil from turning into a small toaster oven from all that continuous current (about 30 watts). (This is a nice safety function, but I'd be surprised if any aftermarket manufacturer had this capability in their product.)

I wouldn't sweat the .25 ohm difference. All the tech specs for these coils isn't spelled out, but chances are the energy output will still be sufficient for proper engine performance.

There's very little series resistance in a TI harness (about 1.1 ohm IIRC). Add in the .7 ohms in the (above) primary windings, and the coil will have to deal with about seven amps in the primary circuit. I'd consult with the coil manufacturer to see if they can confirm that the coil is robust enough for that kind of amperage (points coils usually only deal with about four amps, and at a much smaller duty cycle too).

My suggestions regarding talking to the manufacturers is just caution, to try to minimize blowing up circuitry or burning up the internal coil wiring due to the higher currents.

 

Update:
I bought an Accel 8145 to replace the 30 year old back up that I put on the car after the 1st one died. You won't believe this but the 30 year old coil that I temporarily put on it, when I spun it around it had the sticker that says it's the correct coil for TI ignitions. I never remember ever seeing that sticker on there before.
Guess the new Accel is now a back up..
Thanks all,
Jim