I picked up a 68 coupe today, been sitting for a while. I'd like to try to fire the engine, but it has the transistor ignition and the coil is missing. Is there a common parts store coil that can be used with this system just to see if it will run?

 

Summit has one that I bought for a spare for my 1966 L72 with factory K66 ignition. The part number is SUM-850496

 

68, have you tried that coil? Curious if it would work. I see the primary ohms are in spec with a GM t.i. coil. Might be a good alternative to an expensive used GM coil.

 

I'd be curious as to if anyone has actually ran one of those, it's a lot cheaper than an original. And would be great for this car, as it will never be an ncrs type car

 

Someone just posted a service replacement “207” TI coil for $250 in the for sale section.. that’s a good price!

 

No, I have not. I bought it on recommendation from a thread in the C1/C2 forum where this problem has been discussed at length. Regarding the USED 207 coil there is no way of knowing how long it may last. It is a 50+ year old wear item after all.

 

$250 is probably a great deal for someone with an original numbers car, but so much has been changed out on this one already that I would go to standard ignition before spending that much on a coil. Along with not knowing if the control box or distributor works or not

 

I tried that coil that 68 Hemi suggested, the old car fired right up. Hopefully it will continue to work

 

Glad to hear that.

 

The turns ratio is noticeably lower than the stock TI coil, so that tends to be a bit rougher on the module switching transistor and protection diode due to higher primary flyback voltages. May or may not be a long term concern with the older/original modules depending on the plug gap size and cylinder pressure/compression.

Not trying to scare anyone. The original TI coil had a high turns ratio specifically to reduce the primary flyback voltage to prevent long term damage to the "fragile" germanium switching transistor.

 

I am far from an EE, thus my question. Would a diode (the correct one, LOL) prevent that 'flyback' problem?

 

How would this coil effect one of the modern replacement drop in modules, if at all?

 

Excellent question.

The primary winding flyback voltage is both a problem and a matter of physics. It's there, it can cause damage, but ya just have to live with it (if you clamp it to a low voltage the plugs won't get a high enough secondary voltage to fire). The TI circuit board has a zener diode across the output transistor to clamp/limit any excessively high flyback voltages when the coil dwell is shut off. As long as the zener diode is in good shape, and its solder joints are good (ie: not marginal or resistive), there's not much you can do to improve the protection. Adding an additional zener in parallel would allow a little current/wattage sharing during clamping, or be available as a redundant safety measure if the original zener failed due to age or thermal cycling mechanical damage, but you're still stuck with trying to allow "high" flyback voltages to insure that the plugs can fire reliably, but also keeping the flyback voltage "low" enough to prevent blowing up the power transistor.

That's a basic description of the issue. The world is full of restrictions and compromises.

 

I don't have a part number or data sheet on the power transistor (and protection zener if equipped) used in the newer replacement boards, but I have heard talk/discussion/rumor that the transistor has better/higher voltage tolerance than the original PNP part (If anyone can look and tell me the part number of the power transistor on the newer boards I could look up the specs and give a better answer).

(Erased the earlier paragraph here because it was getting too long. I'm going to try to keep this short if possible.) The original TI coil has a turns ratio of 150:1 IIRC. Assuming at some RPM and WOT that the plugs needed 20 kV to fire, that would mean that the coil primary winding flyback voltage would be 20 kV/150 = 133 volts. That is what the power transistor sees as a voltage spike every time the coil/plugs fire. The Summit coil is advertised as a 116:1 turns ratio part. This causes a voltage spike on the power transistor of 20 kV/116 = 172 volts. (A typical 100:1 turns ratio points coil would obviously cause a flyback voltage of 200 volts in this circumstance.) The TI coil was designed with this high turns ratio to keep the flyback voltage low, making life easier on the internal transistor and protection zener. Now, just for amusement, let's assume that the protection zener on the circuit board clamps the primary winding flyback voltage at 160 volts. The original coil would deliver the needed 20 kV no problem. The alternate coil, however, would only be able on the secondary windings to output 160v x 116 turns = 18.5 kV, a possible misfire at that RPM and load.

So, as long as the newer boards have sufficiently higher voltage capable power transistors in them (which I suspect they do), then there's most likely no big/obvious concern about using a coil with a lower turns ratio.

Summit lists that coil as having 4.50 mH primary inductance. I haven't heard any definitive information on what the primary inductance is on the stock TI coils. If someone knows, I would appreciate your passing that information on to me.

 

OMG my fat head just got a little fatter! I'll never find a hat that fits me now!

 

Well, that's just one of the risks you take when diving into the technical discussions.

 

It got worse... I opined on a legal question(s) with Town Counsel and he was most impressed with my approach. His complemenatry reply was copied to a number of people. Damn, my scalp near popped off!

 

So, in a nut shell what you’re saying is IF a repo amp box has an extra zener the Summit coil should be safe but with a factory original amp box maybe not so much?

 

No. An extra zener is only a last chance bandaid on an original circuit board. A repo/updated circuit board will most likely have a protection zener across the output transistor (just like the original circuit board), but also a higher breakdown-voltage power transistor. The high turns ratio of the TI coil was necessary to lower the primary winding flyback voltage to reduce the chances of damaging the "delicate" power transistor used in the original boxes. The newer circuit boards most likely have more robust, higher-breakdown voltage power transistors in them, which allows them to survive the higher flyback voltages inherent in coils that have lower turns ratios than the original TI coil.

 

How about this coil
https://www.tispecialty.com/coils.htm