Electronic Ignition Info
Older Points-Type Ignition
Shown above is the layout of a generic points style ignition system. Chrysler's implementation is identical to the above, using a "ballast resistor" as the primary resistor which drops the voltage down to lengthen the coil's life. Although a stable workable system, drawbacks include routine points maintenance, such as gap settings, condensor (capacitor) lifespan, and poor performance in low and high RPMs. In the mid seventies Chrysler switched over to an electronic style ignition system. Click on the following to see the original chrysler manual's ignition spec :
Electronic Ignition Theory
Basically the improvement on the old system is to replace the points and condensor with a basic rotor and electric pickup coil. This makes the primary circuit maintenenace free. Essentiually the switching is converted from a mechanical switch, to an electrical transistor switch, as pictured below.
Chrysler's 4-Pin Electronic Ignition
Chrysler's implementation of electronic ignition put the transistor in a module mounted on the firewall. It receives full power from the ignition, input from the pickup, and shunts the coil to ground at the right times.
Chrysler's 5-Pin Electronic Ignition
Almost identical to the 4-pin, the 5-pin module has an external ballast input to step down the voltage. The 4-pin did it internally, and I guess was a weak point in the design.
Mallory Aftermarket Electronic Ignition
Mallory improves on the Chrysler design with their Unilite series by putting the transistor inside the distributor itself, thus eliminating the need for an external transitor module. The ballast is still used, and the upgrade is equally easy from points or electronic. Mallory part#
Make sure your ignition wires are spiral type suppression (resistive), and your plugs are also resistive. This unit can use any aftermarket or OEM coil, but they suggest using their coil and an additional voltage spike suppressor module.
- 4757801 for mechanical + vacuum advance
- 3757801 for only mechanical advance
MSD Aftermarket Electronic Ignition
Similar to the Mallory design, MSD has a Ready-To-Run drop in distributor. They use HEI style distributor terminals, so you need their wires too. MSD part#
- 8387 mechanical + vacuum advance
- 32739 RED 8.5 Wires
- 32733 BLACK 8.5 Wires
Im using Accel's high performance coil, Part# 8140 :
Testing an ignition coil. (within 10% of spec. is acceptable) :
- Primary Circuit resistance : Measure from coil (-) to coil (+)
- Secondary Circuit resistance : Measure from coil (-) to distributor coil output
Wires & Plugs
There are 2 types of wires/plugs original performance, and radio surpressive. The difference is the amount of resistance built in. Ie original plugs and wires were solid core, and had zero resistance. Eventually it was found that they caused large electromagnetic interference everywhere, and interfered with car lectronics. Nowadays, everything is resistive/suppressive type.
Mallory SPIRAL CORE PRO SIDEWINDER
MAXX Performance 8.5mm Ferrite Spiral Core Spark Plug Wires
Part# MAXX 505R
Moroso - Ultra 40 Race Wire
Moroso - Blue Max Spiral Core Spark Plug Wires
For plugs in order of popular preference, AutoLite, NGK, AC Delco, then Champion are preferred.
Application - 1968 Dodge 440/7.2L
||#75 - Non-Resistor (discontinued)|
#85 - Resistor
D7 (first digit = heat range)
||#XR4 (Stock# 5858) - V-Power|
#XR4IX (Stock# 7189) - IX-Series Iridium
||#R43S - Standard|
#7 - Rapidfire
||#RJ12YC - COPPER PLUS|
#RJ14YC - COPPER PLUS
The heat range of the spark plug determines how much heat the plug is capable of removing from the combustion chamber. The heat range of the plug does not affect horsepower. It only changes the temperature of the tip of the plug. If the heat range is too cold, the deposits will not burn off the tip and the spark plug will foul. If the spark plug heat range is too hot, erosion and pre-ignition could occur. A change in the heat range of the spark plug used will change the temperature of the spark plug tip by 70-120 degrees! Always start with a colder plug, and work up, as tip-fouling will be the only issue. The heat range of a spark plug has only a minute effect on combustion chamber and overall engine temperature. A cold plug will not materially cool down an engine's running temperature.