It is the spark gap that determines the firing voltage. They fire at the same voltage whether driven by H.O. coils or stock coils.
Lower input voltage also lowers secondary voltage. If you use a ballast resistor and it drops the coils + terminal to 10volts it will make lower secondary voltage. if a coil is rated 75,000 volts at 12 volts....it won't make that at 10 volts.
I know coils are affected by saturation and other factors too...but don't tell me output voltage is strictly based on gap...
You do seem to be confusing coil output voltage >potential< with the actual firing voltage.
Here are the basics of the how the Kettering system employed on the SOHC4, actually functions.
When the points close, current flows through the primary and a large electromagnetic field develops in and around the metal core of the coil. At some point the core becomes saturated and the magnetic filed can develop no larger with the given voltage. This is the minimum required dwell time and is stored energy. When the points open, the magnetic field collapses and as the collapsing lines of flux cross the many turns of the secondary (which also shares the metal core with the primary windings), a voltage is induced. This voltage builds until it is large enough to ionize the atmosphere between the spark gaps of the plugs and turn it into conductive plasma. The formation of the plasma bridge across the electrodes completes the coil output circuit allowing current flow. This rapidly dumps the energy stored in the coils preventing any further voltage buildup.
While "H.O." coils may have the potential to induce higher voltages, unless the gap atmosphere or the gap distance between the electrodes changes to require a higher output voltage, no increase in actual firing voltage will occur with only a mere coil change.
Similarly stated, if both HO coils and stock coils are both fed 12V, neither will develop more spark voltage at the spark plug tips using the Kettering scheme, even though HO coils are capable making higher voltages, the spark event limits the voltage development to what is needed to jump the gap.
Although there are similarities between transformers and Kettering ignition coils, they are not used under the same types of stimulus and load configuration, therefore actual voltage multiplication behavior is not identical between the two devices.
The coils don't deteriorate with age.
insulation does with heat and will eventually break down causing breaks in the windings.
XS650's were notorious for stator windings failing due to poor varnish insulation used in the 70's. Not abuse...heat and age combined with pooer technology.
I think both your assumptions are faulty and ill-advised. In addition, they are based on an irrelevant comparison between two different devices.
You assume (without support) that the insulation used in the two devices as being the same without foundation.
Copper windings will maintain properties both physical and electrical a very long time, so long as exposure to atmospheric contaminants is avoided. First layer of protection is the job of the individual wire strand insulation. There are many materials that can be used during manufacture to provide that insulation. Acrylic, vinyl, varnish, Polyurethanes, epoxies, and myriad plastic compounds have been tried and employed. The selection is tied to the manufacturing process, material cost, application technology, as well and breakdown voltage requirements.
I don't actually know what material was used inside the Honda coils. (A likely candidate is Formvar.) But, I do know that the selection is interactive with the mil diameter of the wire core.
Stator windings have comparatively large diameter wire in them, say 14 gauge or 64.1 mils. Adding a wire insulation layer can increase the diameter to 66.6 mils or even 68.2 mils if it is heavy build, making the insulation account the 4 mils of wire diameter.
By contrast, size 36 magnet wire is only 5 mils core and may have only .7 mils insulation.
To achieve the desired thickness, it is common to use a different insulation material to achieve uniformity along with the design handling capabilities used with a more delicate wire core.
Wild assumptions that the insulation material is the same on both products seems highly contrived and speculative.
To me, this is like saying all drinking vessels must have the same container material, and thickness of a beer keg, because that is the only similar vessel you were able to observe.
Still, there is concern over the individual wire insulation breakdown characteristics. Certainly, enough heat to melt the copper will damage the copper and the insulation. End of coil. However, I classify such heat as abuse.
Next, is enough heat the damage insulation without melting the copper. Some may be familiar with Formvar, which was a common material used for coating magnet wire. This was a "modified polyvinyl acetal resin". Today the same thing is manufactured und the name of Vinylec. This product lists its heat distortion temperature as 84-93C, which is about 195F.
Given the coils are mounted under the gas tank As opposed to inside a hot engine case of an XS650, I don’t see the coils reaching this or higher temps as routine.
I have no doubt that you can cook an air cooled engine with no airflow and have its heat radiate to the coils to over temp them. But, is this not abuse?
The stock coils are encapsulated with yet another plastic coating, possibly epoxy. The point of encapsulation is prevent damaging atmospheric contaminants, as well a destructive UV light from reaching the inner coil components, including the wire insulation coating. Damage to the outer encapsulation I would also consider abuse.
The encapsulation coating can be damaged by UV in sunlight. Since the stock location is under the tank out of sunlight, any owner that exposes them to sunlight would be abusing the coils.
I classify the HT leads as separate from the coil proper, even though they are cast into the coil’s encapsulation. It is well known that the HT lead insulation (Vinyl based, I think) becomes hardened and brittle with age as this material has no protection from out-gassing/de-polymerization effects.
Old leads need be treated with care to avoid sharp bends which can lead to cracking. The HT lead’s cut back stubs can be spliced onto new wires and the old coils will continue to function as they nearly always do. But, if the technician is incapable or unwilling to do such work then the next viable alternative is to replace the entire coil/lead assembly. And that is a matter of personal choice, rather than an unavoidable imperative.
Edited: For clarification and to address coil aging and various typos.
