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 Post subject: 750 Sport Coil Choice
PostPosted: Thu May 18, 2017 1:01 pm 
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Joined: Fri May 02, 2014 10:08 pm
Posts: 29
Location: Australia
model: 750 Sport
year: 1988
Hi All,

It appears to be time to update the old coils on my 89 750 Sport.

It has a Marelli Digiplex ignition fitted with original light grey coloured coils and hard wired leads.

Can anyone advise which of the following would be most suitable?

Should I get 5ohm or 3ohm?

Would the following be suitable? Or would the Dynatek brand be better?

http://www.ebay.com.au/itm/182349079482 ... EBIDX%3AIT

I've also seen the "Dynatek DC3-1 (3ohms)" or "DC10-1 (5ohms)" which would be best?

Thanks


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 Post subject: Re: 750 Sport Coil Choice
PostPosted: Thu May 18, 2017 1:19 pm 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5098
Location: southern Germany
model: 750 Paso
year: 1987
probably none of those. The 3-5 ohms coils are for the Kokusan ignition.
According to the 906 workshop manual (the 906 uses a Marelli Digiplex ignition) its coils have 0.34 ohms +/- 10% primary resistance and 3700 ohms +/- 10% secondary resistance.
The best would be you`d measure the resistance of one of yours. Just disconnect one and measure the resistance between its + and - connection.


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 Post subject: Re: 750 Sport Coil Choice
PostPosted: Wed May 24, 2017 7:05 am 
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Joined: Fri May 02, 2014 10:08 pm
Posts: 29
Location: Australia
model: 750 Sport
year: 1988
Testing came back at 4.5ohms on each coil.

This coil claims to be suitable quoted at 4ohms but isn't a brand I've heard of.

http://www.ebay.com.au/itm/271621597212 ... EBIDX%3AIT

Other posts claim the 3 or 5ohm Dynatek will do the job. If the 3 ohm Dynatek will provide a stronger spark over the 5 I'm thinking thats the logical choice. Unless technically neither is the suitable coil?

viewtopic.php?f=1&t=5823&hilit=dynatek

viewtopic.php?f=1&t=7452&p=58973&hilit=dynatek#p58973

FAQ's also state Dynateks are fine for 750's.


Confused……....yes I am.


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 Post subject: Re: 750 Sport Coil Choice
PostPosted: Fri May 26, 2017 11:28 am 
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Joined: Tue Mar 07, 2017 1:39 pm
Posts: 3
Location: Australia
model: 900 SS
year: 1989
Hi deswoodau,

I'm running the 5ohms Dynatek's on mine with no problems, like you, I used this site and any information I could gather from the web and came to the conclusion that they should be fine with the Digiplex ignition. No problems yet!

Your coils, like mine, should be mounted under the battery tray, lots of good airflow but right above a cylinder. From what I deduced the 3ohms do indeed give a bigger spark but run hotter compared to the 5ohms, I figured the spot they mount should get good airflow on the open road but if your doing any city or stop-start, heat soak from the engine below might warm them up quicker and reduce their life.

Why I went 5ohms instead of 3ohms? It's not a race bike for me but a nice Saturday/Sunday 'spirited' speed rider. The spark increase from the 3ohms I'll never use, and personally unless you've reached the point of indexing spark plugs with a full blown race engine and 13:1 pistons I don't think you'll gain anything from running the 3's compared to the 5's. If you're having trouble starting and hoping the increased spark will help, redoing the wires and grounds from the starter to solenoid to battery will be much better. I used the Motolectric kit and that along with new Dynatek coils, new HT leads and some fresh spark plugs has made it easy to start.

I also heard, and I want to stress heard; no proof was presented. That the 3ohms will wear out the Digiplex quicker than the 5ohms. No idea why or how, it's just something I read while in the same position as you, so take it with a grain of salt.

Hope that helps!


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 Post subject: Re: 750 Sport Coil Choice
PostPosted: Fri May 26, 2017 11:49 pm 
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paso grand pooh-bah
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Joined: Wed Dec 05, 2007 12:35 am
Posts: 1587
Location: Newzealand
model: 906 Paso
year: 1990
G is right . 0.3 ohm is standard and about what mine measure out at . If yours are both 4ohm they may not be the original ones . Different ignition systems will have different dwell periods. The dwell angle is the time expressed in crankshaft degree's that the primary winding on the coil is energised. Im picking that the digiplex has a very short dwell period which means that the low resistance will allow a lot of current to flow in that short time to properly charge the primary winding . A higher resistance primary is better suited to something with a longer dwell.
My Guzzi came with stock coils ( 3.5 ohm ) and a fried Dyna cdi . I checked the spec's which said the dyna would work with stock 3.5 ohm coils so I bought a new Dyna and fitted it up , the coils were smoking hot within a few minutes and so was the new dyna ( hence the fried dyna it came with ) so I logic-ed it all out and fitted a 1.5 ohm ballast resistor to each of the coils ( in series ) end of problem..............'cept being a bit OCD I couldn't leave it at that and fitted some dyna 5 ohm coils. The point is that the coils do need to be matched to the cdi, to much resistance will probly leave you with a weak spark , not enough and stuff starts getting hot . Its not just the primary resistance that effects the coil output. The primary resistance controls how much current flows ..... the dwell time controls how long that current flows for.
Having said all that it beats the shit outa me how a iggy system designed to run a 0.3 ohm coil will work with a 5 ohm coil ....... but it does ..... must be something sneaky goin on in that digiplex :)

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 Post subject: Re: 750 Sport Coil Choice
PostPosted: Sat May 27, 2017 5:52 am 
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paso grand pooh-bah
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Joined: Tue Jul 29, 2008 9:50 pm
Posts: 3157
Location: Hilltown,Pennsylvania
model: 906 Paso
year: 1990
Quote:
iggy system designed to run a 0.3 ohm coil will work with a 5 ohm coil



lower resistance kills ignitions

Too much higher will give you misfires

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Ducati,making mechanics out of riders since 1946
There's no problem so bad that a little fixing can't make it worse! : )
If it ain't broke keep fixin it till it is
88 750
90 906
92 907ie


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 Post subject: Re: 750 Sport Coil Choice
PostPosted: Sat May 27, 2017 9:57 am 
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paso grand pooh-bah
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Joined: Sun Jul 18, 2004 12:00 am
Posts: 5098
Location: southern Germany
model: 750 Paso
year: 1987
According to the 900SS wsm the coils resistance is the same as of the 906. I read in another forum about a Paso 750 with Marelli ignition and it also mentioned 0.34 ohms coils. So it's for sure the same on the 750 Sport with Digiplex.
It's interesting that the primary resistance is even lower than what Marelli used for their ECUs for fuel injection.

Just to repeat Hamish and share some thoughts. Every coil has a dwell time, the time it requires to get to its max energy level before it holds and then sparks. The electronic ignition module usually has programmed dwell periods which vary depending on rpm. So it's logic that coils and ignition should match.
As rpm increase the coil has to discharge more often while it has less time to charge. By (my) logic there would be a point at high rpm from which on the dwell time becomes too short, the coil can`t reach its energy peak and spark weakens. Is that the reason for ignition systems with low resistance coils, do they have shorter dwell times and are electronic ignition systems working with higher current reacting quicker? (This is not a rhetorical question :truck: ) If that was so installing coils with higher primary resistance would be the same as shortening the dwell period. It would also explain why some shops sell the 5 ohm Dyna coils for street and the 3 ohms for race use simply because they have a better ignition performance at the top end.

Quote:
Having said all that it beats the shit outa me how a iggy system designed to run a 0.3 ohm coil will work with a 5 ohm coil ....... but it does ..... must be something sneaky goin on in that digiplex :)

I think it may have to do with how the coil is built, how strong its coil and magnetic field is etc. It may work because the Dyna coils have a short dwell time and/or because they have a higher power output as a standard coil. In case they wouldn't work at their full potential they could still be as powerful as an original coil or not far behind. An original 3-5 ohms Ducati coil may not work with the Marelli Digiplex. If I would have to make a choice I guess I would either get some standard coils with 0.4 to 1 ohms or Dynas with 0.5 to 3 ohms.

Quote:
lower resistance kills ignitions

generally true but probably not an issue for the Digiplex or are there coils with even lower resistance? :-P


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 Post subject: Re: 750 Sport Coil Choice
PostPosted: Mon May 29, 2017 1:13 pm 
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Joined: Fri May 02, 2014 10:08 pm
Posts: 29
Location: Australia
model: 750 Sport
year: 1988
Thanks everyone for your responses……much appreciated.

I'll keep you posted once I make some changes.


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