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I believe the condenser is needed to help extend the service life of the spark module while improving the efficiency of the coil, much like it does in a conventional points ignition system.
 

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The function of the condenser in a Marelliplex system is different from it's function in a traditional points ignition.
In a points ignition the condenser (or capacitor) increases the energy that goes into the spark at the plugs by eliminating energy loss in the arc at the points if it is not present.
In a Marellliplex it may be connected in series with the wire to the tachometer if the tachometer is unstable. I did not need to use one in either of my cars that had 'plex ignitions.
 

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Discussion Starter #23
I found these old IAP tutorials on rebuilding a Marelli Plex distributors. In contrast to distributors with shaft bushings, the 'Plex distributors have bearings which can be replaced. (Although this references a Fiat distributor w vacuum advance, the 'Plex mechanicals are very close to Alfa which means the Alfa 'Plex distributor should be hopefully similarly rebuildable (if anyone has rebuilt an Alfa Marelli Plex distributor please comment).

In particular, the instruction on setting the air gap between the distributor sensor and reluctor is particularly useful (use a torn off box top).


 

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We have a Marelli Plex on the race car we are building. But it did not come with the heat sink, coil or controller, and we're looking to buy what we need. According to folks on this thread, we can run a strong coil, without adding a resistor. It looks like the Bosch Red is a good option. But, we'd like to stay with Marelli if possible. Also, it is for historic racing, so we have to be mindful of what we do.

Does anyone know what Marelli coil would be best? I found online the Marelli BK2A - but some ads say it is NOT for Plex, then at least one says it is FOR Plex. Very confusing. Then I found a Mako Marelli 207 A-Mk1 that says it is specifically for the electric ignition for 79-on with Plex. https://www.midwest-bayless.com/Fiat-850-0_p-10006-9938064-oe-ignition-coil-for-electronic-ignition-marelli-fiat-124-spider-brava-lancia-1979-on-wmarelli-plex-oe.aspx

Anyone have any thoughts on this? Is the Mako an inferior product? Is anything built now likely inferior to NOS?

And to confirm, it looks like no matter how strong we get with the coil, the Plex system is not going to burn out like a Pertronix, right?

Finally, the Plex coil came with a heat sink all around it. I know the control module needs to be on a heat sink. But does the coil as well? I was told that the coil will actually run cooler on a race car because it is at low rpm where the coil gets hot. Assuming that's true, we can just have a little heat sink for the module.
 

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The GM HEI module is attached to the same heat sink as the coil. You should use GOOD heat conducting paste between the module and the sink or it will fry.

Almost any coil should work but one with a resistance of about 0.7 ohms and an inductance of about 6 mH will give the strongest sparks, particularly at high rpm.
 

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Discussion Starter #26 (Edited)
Last time I checked, the Fiat sites were selling the heat-sinks separately. The 'Plex was popular on period Fiats and Lancias as well as Alfas. Also, I'll bet if you put a "wanted" post here on the bb someone will most likely have a heat-sink that they'll sell you. I'm pretty sure APE will have one too.

As an alternative, you could dispense with the heat-sink and HEI Trigger and just use an MSD6 ignition. MSD sells an adapter (shown in the photos earlier in this thread) that makes the MSD plug-compatible with the 'Plex distributor sensor. It completely bypasses the heat-sink/HEI trigger and allows the MSD to drive the distributor sensor. If you are interested in original appearances, you can mount the MSD in the passenger compartment and rout the wiring through grommets into the engine compartment.

Probably, the best coil to use with a 'Plex is a low resistance, non-ballasted MSD Blaster. A well knows Alfa and Fiat race mechanic I know runs 'Plex ignitions exclusively in his race cars and substitutes the Blaster for the "Plex coil. If you're interested in original apearance you can always paint the MSD coil black.

It seems that, despite their age, HEI 4 pole triggers continue to be commonly available. There are a number of aftermarket brands of improved versions which offer hotter spark, etc. I'm using an ACCEL 4 pole trigger on my my hot-rod Super's engine that was recommended by RJ. Typically, it turns over only two or three times and then starts right up. That's pretty good for an inductive discharge ignition.
 

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Thanks all. Someone did just reach out about a heat sink he has.

Another question. Are there different versions of the Plex? I see that some references call it Plex 201 some call it Plex. Is that the same? If there are different versions, how do I tell from the one on our car which one we have?
 

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Discussion Starter #28
The 201 is the aftermarket version and uses a single distributor sensor. Some Alfas reportedly used dual distributor sensors, although I've never seen one of those. The replacement distributor sensors now being sold are single version. Centerline recommends mounting single sensors when fitting replacements.
 

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Thanks. I thought all the Plex were aftermarket. But then I did see a reference that possibly 79-80 Fiats might have had them from the factory. Or maybe the distinction is the 2 sensor / 1 sensor that you note.
 

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It is true that you can fire a CD ignition from a plex sensor but based upon my experience with several MSD and Crane units in several different cars, they are not worth the extra dollars and complexity and they are implicated in the failure of other parts of the ignition system. Keep it simple.
 
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