In terms of amps and arc duration with CD ignition, I think the coils with the more mass in the core laminations and windings dump the more energy in the plug gap, although that is not the only consideration.
If you look close at plug electrodes with a 10X magnifier and pay attention to the type of coil in use, you will see the E-core coils have more pronounced “spark mark” etching of the plug electrodes. The electrode surfaces in the gap should appear eroded by the arc, about like 220 or 280 grit sandpaper. Even if the rest of the plug is carbon fouled or oily or wet with fuel, the gap surfaces should be burned clean by the arc. If the ignition energy isn’t sufficient to etch and clean the electrodes, it is less energy in the gap than the point ignitions of 50 years ago.
Several years ago there were some MSD Blaster oil-filled canister coils that were horrible. They were high voltage, could throw a very long arc in free air, but delivered comparatively little energy to the plug gap. This is consistent with the fact that transformers (coils) wound with higher primary/secondary turns ratio which gives higher secondary voltage also have proportionately lower secondary amperage. With those coils, the plug electrodes would appear shiny and unused and even damp with fuel after extended running on the street or drag race passes or even after a 50 lap main on a half-mile oval.
The lack of spark-mark and other observations inspired me to compare coils because the weak ignition energy was causing people to think they had carb problems and they were criticizing carbs that I knew were identical to a very successful recipe on nearly identical engines. The $16 E-core coil made the carb problems disappear.
I did a simple coil comparison by comparing the sound pressure of the ignition energy of several coils using a 1” dia. plastic tube about a foot long, with a dB meter in one end and a surface gap plug in the other, a MSD 6 triggered with a MSD ignition tester and a Tektronix dual channel 100 MHz oscilloscope to compare coil output wave forms. I had to stuff a wad of paper towel in the tube to suppress the dB to a level that didn’t max the 130 dB limit of the meter. I also used a regular plug with a very large gap to raise the breakdown voltage to a level similar to a running engine and the results were the same in proportion, although the dB was a little more and the arc duration was shorter because the arc gap was wider. (close gap = longer arc duration, wide gap = shorter arc duration)
Of course, this was at atmospheric pressure and room temperature.
Of the several coils, the big MSD HVC coil had the highest dB and the longest arc duration.
A very close second was the cheap FD478 TruTech Ford Thick Film coil; it was only a few tenths dB less on the meter but had less arc duration and faster rise time.
(Ignition system secondary rise time and arc duration have an inverse relationship. In nearly all cases, more of one = less of the other.)
Third, was an old Mallory oil-filled canister coil for point ignition.
The other coils, MSD Blaster II and MSD Blaster oil-filled canister type were short on arc duration and several dB less than the top three. An old oil-filled canister points type bought at K-Mart about 25 years ago was better than the MSD Blasters of either type, Blaster oil-filled can or Blaster II epoxy molded. The little Blaster II was the weakest.
The big HVC, the FD478 and the Mallory points unit were clearly more powerful.
In the late ‘70s and early ‘80s when CD ignitions started to become prolific it was obvious that E-core coils used in the GM HEI systems exhibited more spark energy on plug electrodes than the Blaster canister coil. Some people who changed from the HEI coil-in-cap to the remote coil with a MSD Blaster started having nitrous backfires and changing back to the original HEI coil or an HEI E-core type external coil from a 6 cylinder or a Vega eliminated the backfires. (Mind you, in those days a 250 HP nitrous bump was considered a fairly large amount, LOL.) Stock HEI ignitions did not have the backfire problems that occurred with the new CD ignitions that came with “high voltage” coils.
The opinion I held before I did this simple test was more core mass makes a better CD ignition coil because it transfers more energy by induction from the primary to the secondary and the slightly longer rise time, inherent of the induction process with more core mass, causes longer arc duration.
The Mallory points coil in the test, although it has slightly smaller outside dimensions, is about 50% heavier than the series of MSD Blaster canister coils that were troublesome. I think this reinforces my pre-existing opinion that more mass of core laminations and windings makes a better CD coil because it absorbs and transfers more magnetic energy between the primary and secondary, and also, the greater mass of core takes a longer time to charge and discharge the magnetic flux, resulting in the longer arc duration.
I don’t suppose that answered your question. This was just a ramble while watching the Cup race. I’ve been using the $16 FD478 coil on just about everything for a long time. It’s cheap and it works, it etches the plugs, I’m happy.
Most of the drama is in the marketing and the electrons are not in on the joke.
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