A bit of history is in order:
Back in the early 1900's when "Boss" Kettering and associates at the Detroit Electric Company (later Delco) developed the "modern" high tension ignition system, they determined that breaker points could only tolerate roughly 3-4 amps of current flowing through them without burning excessively. These were six-volt systems, so they designed the coils to function properly at that level and life was good in the relatively low-performance engines of the day.
Somewhere in the 1950's most American vehicles got higher compression ratios and switched to 12 volt systems, but they used the same coils (why not, they worked fine). The higher voltage pushed higher amps through the coil (and points) which tended to burn the points. Enter the lowly ballast resistor, which dropped the voltage to maybe 7-8 volts (roughly what a six volt system charges at). This cured the points burning, and soon most vehicles also had a circuit to provide full 12 volts to the coil while cranking the engine for better starting, but automatically went back to reduced voltage during normal operation.
Later on, with the advent of smog controls, it was determined that leaner air/fuel mixtures would help reduce emissions; trouble is, leaner mixtures are more difficult to ignite (that's why we have chokes, to richen the mixture for cold starting). What was needed is bigger sparks, larger plug gaps, and higher voltage to jump those gaps. Now we need a coil having higher inductance. Trouble is, to do that we need more current flowing through the primary windings of the coil, WAY more than our puny breaker points can handle. Transistors were considered to be pretty 'high-tech' back then, so they were employed to carry the increased current loads demanded by the hotter coils. Some of the early systems used breaker points to trigger the transistors; the points only carried a fraction of an amp so they lasted almost forever.
It didn't take long for someone to figure out a way to eliminate the breaker points entirely, and the so-called "electronic ignition" was born (a misnomer because they are ALL electronic). At any rate, the current limiting feature of the breaker points was now history, and manufacturers were now able to build truly high energy ignitions.
But they didn't. For some bizarre reason, Ford continued using the very same coil on the DuraSpark systems, complete with the ballast resistor wire. My guess is they did this for cost-cutting reasons, possibly to use up old stock and keep from having to build new. Whatever the reason, General Motors ran far, far away from Ford with their HEI systems, and even though Ford later deleted the ballast resistor wire on the later DS2 ignitions it wasn't until they came out with the TFI ignitions (really just a copy of the GM HEI stuff) that Ford had a truly high powered ignition.
Please note that the GM HEI and the Ford TFI ignitions do NOT use a ballast resistor. This is on stock systems, of course, but one has to spend some serious coin to really improve significantly on a TFI/HEI coil. In fact, the much vaunted DUI ignition uses a GM HEI coil, and this is actually one of their selling points due to the fact that replacement parts are readily available. I'm sure that the DUI is a good unit, but the sparks just ain't gonna be one whit different than a GM HEI or a Ford TFI.
Ya pays yer money and ya takes yer choices.
Have fun,
Joe
