The megasquirt site has a great explanation. Forget the 7-pin, as it was rare. The 8-pin is best (85-95 GM V-8). You can control the spark timing w/ a Holley Commander 950 and other engine controllers. If you leave that plug off, it simply doesn't adjust timing from the pickup pulses, i.e. pulses the same as an old Mopar ECU would (but stronger spark).
