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PostPosted: Sun Jan 15, 2006 8:08 pm 
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Looking for the discussion thread for this mod?

That's here.

Why do this?

Why should you upgrade from points or a Mopar-type electronic ignition control box to a GM HEI module? In short, the HEI gives a hotter, longer-duration spark for better ignition and more complete combustion. It eliminates the ballast resistor, which is a common failure point with Mopar points and electronic ignition systems. Cavisco, a user on this board, explains it this way:

"The Mopar system uses the positive temperature coefficient of the ballast resistor to regulate the primary coil current with changes in duty cycle (engine rpm). A simple, relatively reliable system for most daily drivers. The disadvantage with the ballast resistor is the slow response time of the resistor resulting in the field of the coil not being fully charged during sudden acceleration (a spark that may not be as hot as is possible with more modern systems). The GM HEI module is typically based on a Motorola (now Freescale) MC3334 IC or equivalent. The MC3334 regulates coil current using an internal variable voltage reference. This internal variable voltage reference is able to respond much faster to changes in duty cycle resulting in a much hotter spark durring times of sudden acceleration, and less destructive heating to the coil during low-rpm operation. The MC3334 also imposes a 1 msec off time to ensure complete discharge of the coil field during a spark event. At very high rpms coil current is limited due to the slope of the coil's primary charging ramp time which is determined by [the vehicle's line voltage as seen by the module] and the primary impedance of the coil itself."

Here is a comparison of the ignition performance, visible on an oscilloscope, of Mopar electronic ignition versus GM HEI in a slant-6 engine. You can see the HEI spark is of longer duration and of sufficiently high power to support a larger spark plug gap (0.045" is the standard gap on GM engines originally equipped with HEI) for higher secondary voltage and better combustion in the cylinder:


What you'll need

Use an ordinary Chrysler electronic ignition distributor with a single pickup and vacuum advance. If your car hasn't already got one, you can get a brand new genuine Chrysler electronic distributor (not a poor-quality Chinese knockoff or thrown-together "remanufactured" unit) from Old Car Parts Northwest.

Distributor cap and rotor
You'll want to spend more time than you thought you needed thinking about the distributor cap and rotor. This is a long but very worthwhile thread on the subject. Nutshell version: today's caps and rotors are not very well made, but some extra effort and cleverness can improve them a lot.

Ignition module
Get a good quality HEI module such as an ACDelco № D-1906, or Standard-Bluestreak № LX-301. If you're doing a high-RPM, max-performance build, there are "high-performance" modules available, but they offer no advantage on a Slant-6 retrofit, and they cost a lot more.

Your stock coil might work OK for awhile, but really isn't a good choice. At the very least, replace it with a coil meant for use with electronic ignition. Such a coil would be a Standard Ignition № UC16. It's much better, though, to use a coil intended for use with a high energy ignition system. If you'd like to mount the module off by itself somewhere, then a good coil option is the stock external-mount E-core coil used with Ford and Mazda high-power electronic ignition systsems. That's a Standard Ignition № FD478.


You might be tempted to get the cheap Chinese "Tru-Tech" version, FD478T. Don't; you'll regret it when the car stops and leaves you stranded somewhere inconvenient. If you're going yard-hopping, you'll find this coil in '82-'97 Ford cars and trucks, '84-'94 Lincolns, '82-'95 Mercury and '85-'89 Merkur models, as well as '94-'97 Mazdas. You can grab the connectors and a few inches of primary wire, as well, or if you'd like to have a new connector with wire, use Standard № S539:


Another fine coil is the GM HEI coil used on '75-'84 GM cars mostly with 4-cylinder engines, Standard Ignition № DR35.

Mounting the parts

There's a bracket available to mount an HEI module to the underside of a Chrysler distributor. This is a nice, clean option for V8 engines with the distributor is up top, well away from any possibility of road splash and with plenty of clear space for some extra equipment under the distributor body without interfering with the ability to rotate the distributor to adjust the timing. That's not the case with the slant-6 distributor's low, rather cramped location, though.

Instead, mount the module to the inner fender or wheelhouse, just above the distributor and high enough up to be kept away from engine heat and road splash. Fetch a piece of aluminum ¼” thick by about 2” by about 3½”. Drill two holes in it to match the HEI module's mounting holes. Heat sink compound comes with every new HEI module. Squirt some onto your mount plate, put HEI module on the plate, and secure the plate and module to inner fender. Strictly speaking, this aluminum mounting plate is optional. It helps assure temperature and mount stability of the module. Some have done the mod by just bolting the module to the inner fender and had acceptable results.

Combination coil/module/heatsink/bracket

A tidy option is to use the combination HEI coil/module/mounting bracket originally found in just about all gasoline-powered GM pickups and RWD SUVs from '96 to at least '00. It includes a nice sturdy mounting bracket, heat sink for the module, and good E-core ignition coil, all in one:


Wherever you are, you can probably pull these out of wrecking yards all day long for very little money, and you can just grab the watertight connectors for the coil while you're there. This way you take advantage of GM's own work. The newer style module included with this setup (and shown above) won't work with the Mopar distributor; it's looking for a 0-5 volt square wave, and that's not the kind of signal the Mopar distributor puts out. You can use the early type (4-pin) module—part numbers specified above in the discussion of module selection—on the newer heat sink bracket with minor mods to the heat sink. Then just bolt the combination module/coil/heat sink unit to the inner fender:



If you use this combination coil/module assembly, the original coil has three terminal pins:

A is the coil's primary (+) terminal; gets ignition-on 12v and is connected to module pin "A".

B is a tachometer signal output. Not used if you don't have a tach.

C is the coil's primary (-) terminal, goes to module pin "D"

The only other "gotchya" is that many of these E-core coils have a male (spark plug type) output terminal rather than the female type on our original coils, so you'd need to get an appropriate coil-to-distributor-cap cable such as Standard Ignition № 814CA, which is a Ford item, or its equivalent from Magnecor or Aurora (those are my favoured spark plug wire suppliers; or you may have favourites of your own).

Module ground

A proper module ground is important. Although the module will probably ground OK via its mount, it's best to run a wire from one of the module mounting bolts to a good ground (alternator housing, for example) to be sure.

Hooking it all up

Remove the ballast resistor from the vehicle. Discard it. Take the two connectors that used to connect to the resistor, and connect them to each other instead.

Hook up the module: Holding the HEI module with its convex side down or toward you, the upper left is terminal “B,” battery; lower left is “C,” trigger; and the two on the right are for the distributor pickup coil. Which pickup coil wire goes to which module terminal is determined by trial and error: If engine is difficult to start or runs poorly after installation, you swap these two wires.

So, the two wires from an ordinary Mopar electronic distributor go to the module's two right-side terminals, the module's upper left “B” terminal gets connected to coil (+) primary so that it gets +12V from ignition switch with no ballast resistor in between, and the module's terminal “C” goes to coil (-) primary.

Power relay

You'll want to add a power relay to guarantee full line voltage to the ignition module at all times. If the ignition module is starved for power, it'll work unreliably and it'll die prematurely. The module really needs to see full line voltage, and the wiring in most of our cars isn't up to that task after all these years. So, when setting up your module wiring, it's best to install a relay that'll provide full line voltage to the module via the coil + terminal.
Use a good brand of relay (Bosch/Tyco, Omron, and Hella are three good picks). You need an ordinary 4-prong normally-open ("SPST") relay. The prongs will be labelled #30, #85, #86, and #87.

#30 is your power input. Connect this via a 14ga wire to a good, solid source of line voltage. Good options for where to pick up this power feed include the battery positive terminal, alternator B+ terminal, large terminal on the starter relay, large terminal on the starter. Put a fuseholder in this wire as close as possible to your power takeoff point. You'll want a 15A fuse, and you'll want to carry spares.

#85 is your trigger ground. Run a 16ga wire from this one to any decent ground.

#86 is your trigger feed. This one needs a 16ga wire that's live when (and only when) the ignition is switched on.

#87 is your power output. Connect this via a 14ga wire to the coil + and to the module's power terminal.

Here's an annotated diagram of an HEI installation on a 1965 Dart. On this particular car, the installation was done in conjunction with some other electrical modifications not part of the HEI job, such as the addition of a master fuse panel under the hood:



Here is a thread with pictures and a video link, describing a low-dollar HEI installation.

Here is another site with instructions on how to do this swap. The author of the linked page says you have to have the GM module connector, but that's not true. All you have to do is make sure you use the correct-size female spade terminals (three of the four terminals on the module are ¼”, while the fourth is 3/16”.)

• The improved spark quality with HEI permits you to increase your spark plug gaps from 0.035” to 0.045” for improved starting, idling, driveability, and mileage. If you're running the 1960 to 1974 cylinder head with spark plug tubes, now's a good time to put in a set of the NGK № ZFR5N extended-nose spark plugs for further driveability and mileage improvements. Remember, if yours is a 1963 to 1974 head, remove the spark plug gaskets (metal ring washers) before installing the plugs. Just grip the ring washer with slip-joint pliers and unscrew the plug from the washer. If yours is a 1960 to 1962 head, you do need to use the spark plug ring washers. Details on the plug washer issue can be found here.

• A hotter spark needs a "sturdier cage" (better spark plug wires). Links to Magnecor and Aurora are above. The other thing about good wires is you won't have to think about your spark plug cables again for a very long time.

• Drive and enjoy!

Looking for the discussion thread for this mod?

That's here.

Photos and contributions to this post courtesy McNoople, DonPal, and Olafla

Too many people who were born on third base actually believe they've hit a triple.


Last edited by SlantSixDan on Sat May 03, 2008 9:03 pm, edited 7 times in total.

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