There is only one vacuum advance/electronic timing control distributor that I'm aware of. It fits mid-80's 305 V-8 truck engines. There will be an oval connector coming from the distributor housing with 4 wires, plus the 3-wire connector that mates the module to the coil connectors. The module on that distributor has 5 pins, and requires a remote timing retard module and a detonation sensor that screws into the engine block coolant drain hole ahead of the starter motor. That system was worthless when it was new, and it hasn't improved with age!
Later model computer-controlled distributors have a 7-pin module, no vacuum advance, and the 4-wire computer connector. They require a full computer-control system for timing advance.
The pre-computer HEI system has a 4-pin module, a vacuum advance, a 3-wire connector from the distributor housing to the inner connector on the distributor cap, and two connections for external wires. One cap terminal is marked "BAT", and the other one says "TACH". Connect a #10 wire from the ignition switch to the BAT terminal, being sure to eliminate any ballast resistors or resistance wire in the process. The distributor needs full battery voltage and lots of current. If you want to trigger a dash-mounted tachometer, connect the trigger wire to the TACH terminal.
HEI systems have an open-circuit secondary voltage potential of 50,000 volts, so special coils and other high-performance gimmicks are totally unnecessary. With a little sneaky tweaking, I've run round track race engines on completely stock-appearing HEI distributors at 7,000+ RPM and 500+ HP.
Jerry
