Electronic ignition upgrade.
If the Mopar box doesn't vary dwell, then tell me, how does it retard timing at high RPM? Because it DOES.
The Mopar box's dwell is not constant either. It's a very simple design from an era where transistors were expensive and using an integrated circuit in a module would have been unthinkable. (At one point, Chrysler patented a voltage regulator design, which they never used, that had only one transistor because a second one would drive up the cost.) The way it behaves is that it normally keeps the transistor in an on state. When it receives a pulse from the distributor sensor, this shuts off the transistor (like opening the points) for a duration controlled by a timer circuit. The circuit that detects the pulse has a very short response time - but it's long enough that it retards the timing a tiny bit. Since it's a somewhat fixed amount of time that delays the spark, but there's less overall time in between sparks, you get a measurable amount of timing retard at high RPM.
Only real ways around this are to make a more sensitive trigger circuit, which would have meant more complexity and cost, and possibly a risk of sparking in response to electrical noise. Modern computer controlled ignitions either have a variable that predicts the amount of delay, or have it factored into a computerized curve that's a bit more complicated than anything you can do with a mechanical advance.
The race modules have a shorter timer circuit - if the off time is too long, you don't get enough dwell to get decent spark energy at high RPM. Conversely, a shorter off time and lower resistance ballast resistor makes for more heat at low RPM. I haven't personally investigated Chrysler's report that their ignition combinations tailored for high RPM overheat at prolonged operation below 2000 RPM, but it's consistent with the way they operate. The module is tuned for an optimized dwell at one particular RPM, and the ballast resistor limits the current below that point.
