This topic is jarring old memories when it was told the only way to check out an ignition system was with a scope even with some outrageous statements, via the spark line, could tell if cylinder #4 had low compression or a leaky intake valve. Was started I believe in 1948 by Dumont with the introduction of an ignition scope and followed with the likes of Sun, Snap-on, Allen, and Marquette that basically purchased a 20" off the shelve black and white TV set for the display mounted in a huge console. These were purchased by dealers mostly to impress their customer base with a huge complex console mechanics didn't like to use has there were alligator clips everywhere to make connections.
The high voltage pickup was a capacitive clamp that clipped unto the coils high voltage lead, but developed huge problems for them back in 72 when the HEI first came out. Really no access to the secondary circuit in the V-8 distributor. The 4 and 6 cylinder versions still used an outboard coil. But then a clamp on plate was developed, forgot by who and was patented in an attempt to read these signals that clamped unto the top of the distributor. But these weren't very useful with the HEI as these point replacement electronics, just did that, barely replaced the points.
A key change as a opposed to a conventional point ignition system was the elimination of the so-called condenser, in parallel with the points. When the points opened, this condenser was in series with the primary of the autotransformer that would resonate the coil and that condenser could actually be adjusted to obtain the longest spark line as if it was too large or small, like tuning a radio, wouldn't get the correct resonant frequency.
Further complicated by the fact, no one had an idea of what the capacitance of that condenser was, or how to adjust it, these condensers were never mark with basic parameters like capacitance and break down voltage. But could be done with mylar capacitors, but those didn't have screw tabs to tie them down.
With electronic ignition system, a bipolar transistor was use that only conducts in one direction, placing a capacitor across that would reverse bias that transistor and burn it up in an instant. So a 60 V breakdown zener had to be used, and goodbye condenser and that longer spark line. With electronic, would only see a single spike, in this light, ignition points are far superior in that both positive and negative alternations were generated and the spark would last even up to a millisecond. Amplitude only told you that the plug was shorted due to carbon built up or had electrode gap wear with a seemingly good much higher voltage.
Recall joking about these consoles, paying up to $20,000 back then to check a 50 cent spark plug. But the public caught on, and this phase of ignition testing died. Plugs were routinely changed anyway.
The key to getting energy in an ignition systems deals strictly with the size of the magnetic core used in the coil and to have an ample supply of current to magnetically saturate that core over the speed range. Conventional coils to limit this current is to use a high resistance primary that was nothing more than a waste of energy and damage to the coil as it would overheat. HEI does use a very low impedance primary that is pulsed by a monostable circuit feeding a controlled current output stage that is sized just to saturate the core of the coil. If you leave the ignition switch on, draws very little current as opposed to other electronic ignition systems or conventional points if closed that can burn up the coil if the ignition switch is left on.
If you want to bench test an HEI, would have to mount 8 spark plugs to a plate with each distributor lead connected to each one, have a 12 volt supply, and a means to turn the shaft. My variable speed lathe does a good job at that. Even turning the shaft by hand will show a firing sequence. One of the main killers of electronic ignition is an open spark plug wire, all that energy has to be absorbed by that zener diode, can never be made large enough. It will short out killing that pulse.
