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How to set up two MSD or Mallory boxes for twin plug ignition

MSD or Mallory Twin Plug Setup Install

I recently took on this project and wanted to share my learnings. The install is fairly straightforward, but that’s my opinion AFTER having completed it. Beforehand, I was combing the forums with little direct experience to work from.

The install was on a carbureted (PMOs) SC-based engine that had been running with a pair of Bosch coils and 6-pin CDIs . The distributor is a 964 unit with the internals from an early SC distributor grafted in (Rennsport Systems can help you with this).

I used 2 Mallory 6AL boxes, which are very similar to the MSD 6AL, including the color of the wires but the rev limiter on the Mallory is adjustable (no rpm “pills” necessary). For coils I went with MSD Blaster 2 coils.

Because of the size of the ignition boxes, I decided to place them on a plate in the back seat area with wires running to the engine through a grommeted hole in the engine firewall (you’ll need to do some drilling). Aside from saving space in the engine compartment, the boxes are also protected from the heat, dirt, grease, etc. of the engine. FYI, the Mallory 6AL does not make any sound when it is switched on (unlike the Bosch CDI that has a high-pitched whine when keyed on). The box housings also received their own grounding wires for extra protection.

Rather than crimp the wires from the ignition boxes directly to wires running to their respective locations, I chose to use terminal blocks with ring terminals (McMaster-Carr). The thinking here is that if there is a failure with one of the boxes the changeover would be simple. The other advantage is the ability to easily splice two wires (one from each box) into a single wire.



Wiring harnesses were made with Painless Wiring wire and bundled using a 4 foot length of heat shrink tubing and a 4 foot length of RF shielded heat shrink tubing (for the tach signal wires) from McMaster Carr (McMaster-Carr).

Here’s the rundown of the wiring:

Thick red wires: merged at a single position on the terminal block and run directly to the primary power lug on the starter (I put a small hole in the back seat area directly above the starter and used a grommet).

Black grounding wires: each have their own position on the terminal blockand are grounded together to a bolt placed in the threaded hole that the seat back pivots on (my back seats are gone). The box housings also received their own grounding wires for extra protection.

Thin red wires: merged at a single position on the terminal block and run to a switched power source. The old CDI power source would work if you still have it in the engine compartment, or you could run it to the fuse box. Note: not all fuses provide constant power when starting the car. On my 74 chassis, only 1 fuse has switched power that is constant under cranking. This can create a no-start situation because the ignition boxes will not remain switched on.

Yellow wires: merged at a single position on the terminal block and run to the black and purple tach signal wire on the engine wiring harness.

Orange wires: each has its own position on the terminal block and connects to the (+) side of a coil using a ring terminal. Different colored wire was used for each between the terminal and the coil to keep them separate. I ganged these with the black coil wires from their respective boxes.

Black (coil) wires: each has its own position on the terminal block and connects to the (-) side of a coil using a ring terminal. I ganged these with the positive coil wires from their respective boxes.

Green & Purple wires: these are the distributor signal wires and are likely the most confounding part of the whole project (until now). The first key is finding the correct female connector to attach to the green distributor wire. I learned that these are called 2-pin AMP connectors and are available from EagleDay (2-Pole Female AMP Junior Timer Kit). The second key is getting the wires on the correct side of the connector coming from the distributor. I had some old 2-pin AMP connectors from my CIS so I built a “test plug” that allowed me to switch the sides of the green and purple wires before crimpling them into the shiny, new AMP connector. The way I figured out if I had them right was to start the car and test them in each position. When they’re not oriented correctly, the engine will stumble pretty badly above 3K rpms.



Purple wires: merged at a single position on the terminal block. Ran a single purple wire from the block ganged with a single green wire. The green and purple wires were secured together in RF shielded heat shrink tubing and run to the distributor.

Green wires: merged at a single position on the terminal block. Ran a single green wire from the block ganged with a single purple wire. The green and purple wires were secured together in RF shielded heat shrink tubing and run to the distributor.

The last step once all of the wiring is run and secured is to re-gap the spark plugs. I went with 0.04”, based on the consensus on this board.

With the exception of a wiring mistake with the switched power (see “Thin red wires” section) the car started and ran without issue, including the tach (no need for a tach adapter). Noticeable changes included easier starting (remember, I have carbs), a smoother idle, improved low rpm response, and better transition between idle and main jets.

Hope this provides guidance for somebody in the future looking to undertake the same project.

Old 01-08-2010, 02:06 PM
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Good timing
I just spoke to Steve Weiner a few days ago about ordering a set of PMO's and modifying my 964 dist. with internals from an SC.
He was pretty clear about using 2 CD boxes rather than one.
btw - we are almost neighbors. I'm in Lafayette.

Thanks for a good write up.
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Last edited by SteveWig; 01-08-2010 at 03:08 PM.. Reason: mistake
Old 01-08-2010, 02:55 PM
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Well done, Jon!!!!
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Old 01-08-2010, 03:06 PM
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Quote:
Originally Posted by SteveWig View Post
Good timing,.....
Good pun,.....
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Old 01-08-2010, 03:06 PM
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Quote:
Originally Posted by Steve@Rennsport View Post
Good pun,.....
I couldn't help myself.....
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'75 non-sunroof coupe. 3.5 twin plugged engine in progress
Old 01-08-2010, 03:15 PM
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Jon,
Could you post a pic of where you mounted the boxes?
Thanks
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'75 non-sunroof coupe. 3.5 twin plugged engine in progress
Old 01-08-2010, 03:18 PM
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I'll snap some pics over the weekend and post them. Or, if you want, you can stop by and have a look. I'm dropping the engine (also a 3.5 twin plug) tomorrow.

Thanks for the props, Steve! Your guidance has been invaluable.
Old 01-08-2010, 03:24 PM
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Great write up!
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Old 01-08-2010, 03:59 PM
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If you comb Mallory's site for pdfs, you will find that you can wire the coils up in parrallel to 1 box. It saves a couple-hundred dollars.

Small orange to both coil+, small black to both coil -

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Old 01-08-2010, 04:28 PM
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