Understanding coil resistance requirments

[Mike R.]

I have read some articles and a number of threads on this forum on ignition and coil resistance. I guess I am still not clear on whether there is an optimium (ignitino) coil resistance for a TI system or if lower is better(?)

Also the new TI amp board supposedly does not require that a special coil be used. Does this mean that all coils are equal if using the new board?

[Richard M.]

Re: Understanding coil resistance requirments

Great question, I'd like to know too. My first TI experience on a '67 L71 recently has taught me much, and I installed the new solid state module and used the repro 263 coil for "correctness". I bought a spare TI SS module and even a spare distributor pickup coil. I'd like to have a spare ign coil for the road JIC the repro craps out.

Rich

[Joe R.]

Re: Understanding coil resistance requirments

Mike,

Check out the TI Specialties web site, hopefully it has some answers for you:

http://www.tispecialty.com/articles.htm

Joe

[Jack H.]

Re: Understanding coil resistance requirments

The original TI amplifier generated the coil's primary side waveform via germanium based transistor(s). With any solid state amplifier, there are limits on how much output current they can deliver...

Next, the ignition coil is a simple step-up transformer. You have a series of wire wraps around a magnetic core on one side (primary) and another series of wire wraps around the magnetic core on the opposite side (secondary).

Current flowing in the primary windings creates a parasitic flow of magnetic flux in the core. When that flux circulating in the core 'cuts' through the secondary windings, a parasitic voltage is created.

Introductory electrical courses teach the step-up or step-down ratio is based on the turns ratio between the primary and secondary windings. That's not entirely true. The turns ratio concept PRESUMES the wiring on both sides is identical...

The step-up/step-down ratio is actually the product of the actual primary side current flow AND the efficiency (magnetic permeabilty profile) of the transformer's magnetic core. So, what you'd really like to measure is inductance and mutual inductance. But, that's over the heads of most folks...

So, for go/no-go test purposes, they spec a restance range for the primary and secondary side of the coil (transformer) presuming that the typical mechanic has access to an ohm meter. But, from the above discussion, you now understand why this is a necessary but NOT a sufficient test...

What if both the coil's primary and secondary windings are intact, but the core has melted or it's laminations have distorted and separated? If the magnetic flux can't circulate as it was designed/intended to, the transformer (coil) doesn't work like it should...

Taken together, the design of the TI coil (along with its specified winding resistance values) was simply the fall out of designers understanding the safe dynamic current operating limits of the amplifier they were working with.

[Mike R.]

Re: Understanding coil resistance requirments

Thanks Joe, I've read the TI Specialty articles but my questions remain. If the Delco TI coil should be ~0.5 Ohms and the Delco points coil ~1.4 Ohms, would even lower be better for TI? Would the 0.6 Ohm Accel coil be a suitable replacement? It seem that all solid state switched systems would benefit from the same coil characteristics(?).

Mike,

Check out the TI Specialties web site, hopefully it has some answers for you:

http://www.tispecialty.com/articles.htm

Joe