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post #16 of 30 (permalink) Old 03-31-2012, 06:36 AM
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Richard,
Is the ACCEL 35361 you used a direct "bolt-in" replacement for the stock MarelliPlex/GM module? I assume it would enhance the MP performance even if the coil were not upgraded?
Thanks
Wil
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post #17 of 30 (permalink) Old 03-31-2012, 09:27 AM
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MarelliPlex

[QUOTE][Richard,
Is the ACCEL 35361 you used a direct "bolt-in" replacement for the stock MarelliPlex/GM module? I assume it would enhance the MP performance even if the coil were not upgraded?
Thanks
Wil
/QUOTE]

Yes the Accel unit is a direct drop in fit. Cost about $34.00 at O`Riley`s Autoparts. The stock overthe counter replacements are prone to sudden complete failures and high RPM breakdown as are the coils.

The Marelli Plex coils are really not much voltage wise. Both Crane & MSD sell high(er) voltage coils that are an improvement over any thing "stock or Autoparts stock replacements". Just be sure that you get one with correct resistance for the trigger module (common HEI type) non CD ignition.
Oil filled performanmce one`s are better, but still not as powerful as the Crane PS91 type.

The Accel module and PS91 coil are really unstressed in any Alfa application as they were designed for 8 cylinder motors at high RPM.
At 8000 RPM our 4 cyl coils are triggering at a rate equal to only 4000 RPM on a V8.....

Richard Jemison
RJR Racing

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Last edited by Alfar7; 03-31-2012 at 09:40 AM.
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post #18 of 30 (permalink) Old 03-31-2012, 10:36 AM Thread Starter
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It would appear that the Chinese made amplifiers (i.e., most over the counter replacements) are prone to quicker heat degredation and failure. I can attest that the amplifiers get quite hot. After a few seconds testing on the Auto Zone rig, the three good amplifiers I tried were almost too hot to hold in your hand. As noted earlier in the thread, a good quality heatsink compound is designed to conduct heat away from the amplifier module to the heat-sink. Some of the over-the-counter chepies can be had for as little as $10 bucks. I have one of those and decided not to use it for obvious reasons. Of the two other good ones I'd collected, one was an original GM brand and the other was more curiously branded both GM and Marelli. Both have been in my parts boxes for at least 20 years.

Reading other threads on the 'net turns up discussions of people looking for original modules from period GM, 80's cars apparently because these cars typically travelled many thousands of miles without amplifier failure. You can usually pick these up at PickNPulls for next to nothing. Just make sure you have them tested before using them. That said, I think an even better choice would be the Accell Richard mentions. It really isn't that much more expensive than most of the over-the-counter modules and, if it'll survive racing, it'll do well in a little 'ole street Alfa.

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series

Last edited by 180OUT; 03-31-2012 at 10:39 AM.
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post #19 of 30 (permalink) Old 09-12-2012, 11:25 AM
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If one is replacing their MP coil, this coil (PN 8207) - at least visually - seems made for it. I would assume the heat-sink would no longer be needed. I actually have my control module on a separate heat-sink from the coil.

Now, can anyone say if it would be a drop in or not?

Also, Richard, what is the condenser for on your set-ups? My MirelliPlex came with a small condenser which I believe was for the electronic tachometer. I never used it and the tach works fine.
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post #20 of 30 (permalink) Old 11-25-2012, 09:13 AM
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Quote:
Originally Posted by Biba69 View Post
If one is replacing their MP coil, this coil (PN 8207) - at least visually - seems made for it. I would assume the heat-sink would no longer be needed. I actually have my control module on a separate heat-sink from the coil.

Now, can anyone say if it would be a drop in or not?

Also, Richard, what is the condenser for on your set-ups? My MirelliPlex came with a small condenser which I believe was for the electronic tachometer. I never used it and the tach works fine.
I had one of the SS Blaster coils installed on my alfa 33.
After 200 miles I got stuck in peak hour traffic with no spark!!
Touched the coil and it was hot with a burning smell from the engine bay.

Luckily i had my original Bosch and hooked it up in 10 seconds and was on my way again.

This MSD says 'MADE IN CHINA' on it. (my bosch has been working for 25 years now)
Maybe I was just unlucky.
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post #21 of 30 (permalink) Old 12-30-2012, 03:11 PM
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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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post #22 of 30 (permalink) Old 12-30-2012, 05:52 PM
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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.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500
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post #23 of 30 (permalink) Old 01-04-2018, 04:45 PM Thread Starter
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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).



Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series
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post #24 of 30 (permalink) Old 06-14-2019, 09:05 AM
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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...i-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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post #25 of 30 (permalink) Old 06-14-2019, 10:02 AM
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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.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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post #26 of 30 (permalink) Old 06-14-2019, 11:30 AM Thread Starter
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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.

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series

Last edited by 180OUT; 06-14-2019 at 11:54 AM.
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post #27 of 30 (permalink) Old 06-14-2019, 11:46 AM
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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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post #28 of 30 (permalink) Old 06-14-2019, 12:00 PM Thread Starter
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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.

Jim . . . '72 Super 1300, '70, 1750GTV, 2nd series,
'62, Lancia Flaminia Zagato3c, 2nd series
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post #29 of 30 (permalink) Old 06-14-2019, 12:03 PM
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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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post #30 of 30 (permalink) Old 06-14-2019, 12:58 PM
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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.

Ed Prytherch
79 Spider
85 GTV6 3L
76 Suzuki GT500
2011 Jaguar XKR

A little government and a little luck are necessary in life, but only a fool trusts either of them. - P.J. O'Rourke
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