The Mopar Electronic Ignition System, introduced in 1971 is essentially a more reliable evolution of the Kettering points & condenser ignition system that's been around since the 1910 Cadillac.  It still relies on a ballast resistor to control the primary current to the ignition coil.  A ballast resistor has a variable resistance that varies directly with temperature and is therefore dependent upon current current flow through the coil.  Since the Mopar EIS has a fixed dwell, the length of time that the coil's primary current is flowing during the dwell period decreases as RPM increases.

To prevent the coil from burning out at low RPMs, the higher current causes the ballast resistor's resistance to increase.  This effectively compensates for RPM by increasing the current to the coil with increasing RPM.  The early 5-pin Mopar EIS used a dual ballast resistor that had a compensating side (0.5 ohm, connected to Pin 2) and an auxiliary side (5 ohm, connected to Pin 3).  The purpose of the auxiliary side was to protect the ignition module's electronic circuitry from excessive current.  The later 4-pin Mopar EIS control modules eliminated Pin 3 and only used Pin 1 to power the EIS module.  Even though the ballast resistor compensates for RPM, the Mopar EIS only allows full coil saturation (maximum magnetic field strength) at lower RPMs.  The ballast resistor is also relatively slow to respond to quick changes in RPM so that stomping on the gas at low RPMs results in poorer spark as RPMs climb (see GM four pin Module).

GM's HEI system eliminates the need for a ballast resistor because the ignition module's circuitry has a variable dwell that limits the maximum current to the coil to 5.5 amps (compared 1.9 amps @ idle with the original 1965 points ignition system).  The HEI's module's circuitry also adjusts the dwell time based on the previous spark event to maintain the maximum primary coil current.  The coil's low primary resistance of 0.5 ohms and higher primary current keeps the coil fully saturated up to around 3000 RPM on a V8 or to around 4000 RPM on a six-cylinder engine, which is much higher than fixed dwell systems (~1000 RPM with conventional V8 ignition systems).  This results in a longer and hotter spark.

There are many web pages devoted to the 4-pin HEI ignition module upgrade.

All accounts of the HEI upgrade are positive with reports of quicker starting and improved idle. I followed the HEI Retrofit Guide on the slant six forum for the most part by sourcing an ignition coil from a 96 GM V8 pickup truck and the 4-pin ignition module from a large cap GM V8 distributor.  I ran into two problems along the way:

  1. The OEM coil location on a slant is tight and the bracket I made for the HEI coil interfered with the fuel pump. I would need to fabricate a better fitting bracket. Fabricating an HEI e-core coil bracket isn't very difficult and I have one for sale now (see HEI E-Core Coil Bracket)..
  2. Spark plug wires to connect an HEI coil to a Mopar distributor cap are custom items.  NAPA doesn't sell individual parts to allow you to modify your existing high tension lead to the coil.

The simplest solution to both issues is finding a canister coil with similar performance as the e-core HEI coil.  The HEI system runs fine with the old-style canister coils used on points & condenser systems.  In fact, using the 1.5 ohm NAPA ECH IC12 coil is commonly done with GM small cap HEI conversions (see DAVE's small-body HEI's). However, these coils have much more primary resistance than an HEI coil and will therefore not be able to provide the HEI's full performance.

The coil wire is more of a challenge since it the necessary terminal and boot components are not readily available so suit GM coils.  High performance spark plug wire manufacturers can custom-make one for your needs but this adds more cost to the upgrade.