Which Coil for CDI Ignition?

[Stratos23]

The ratio of the number of turns between the primary and secondary windings of a coil is what determines the voltage step-up of the coil. The greater the ratio, the higher the step-up. Explanation:

Coils are, in electronic terms, transformers. if you have a transformer with 50 turns of wire on the primary and 100 turns on the secondary, i.e. a ratio of 1:2, and you put 120 volts into the primary you will get 240 volts out of the secondary. Works the other way round too, of course: if you put the same 120 volts through a transformer with a turns ratio of 10:1, then you will get 12 volts out of the secondary.

Remember, several thousand volts are needed to make the charge jump the gap in the spark plug and, in so doing, make the spark! And the further the spark has to jump, the more voltage is required. (And normally, this also produces a spark of shorter duation - but that's another story).

Effectively, the number of turns on the windings in a coil is represented by their resistance. A 3 Ohm coil will have more turns on its primary winding than a 1 Ohm coil.

How this works for you is that with a CDI system, you should be able to run an ignition coil with fewer turns, and therefore with lower resistance (and which will also be physically smaller, to boot) because the voltage that is supplied to it is already higher than with a inductive (points) system. What this means is that the ignition coil has to step up the voltage less - often, much less.

And that's why Electrex have specified a low resistance coil. Now, as you'll understand from what I've said above, what your substitute high(er) resistance coil is doing is stepping up the volage to a level that is high enough for it to jump the gap and give you a spark. The unanswered question is:
WHY is it having to do this?

Ventodue, this explanation is super helpful - clearly articulated in laymen's terms - thankyou. It all makes perfect sense. And yes, I do wonder why I'm not getting a satisfactory spark from the supplied coil. But then it might not actually be the issue - the beefier spark simply improving its willingness to start.

I still wonder whether there's an issue with the inherent problem of a CDI installed on a big single. With a single (where there's no balancing effect of another cylinder), the larger the capacity it is, the greater the variation of crankshaft rotation speed when kicked over by foot. The CDI only knows the speed of the crankshaft determined by when the sensor passes the pickup. It determines the advance based on this. But in the real world, the speed of the crankshaft varies during this rotation due to the compression and 'swinging' effect of the large piston. So it can never really accurately determine the best advance to start the engine. On a smaller single, this effect is less pronounced. Does that make sense? It's only a theory, but I wonder if it's contributing to my issues. Of course with points, this is all not applicable.

As to your question about the rest of the set up - it is to my best knowledge. Everything in the electrical/ignition system is brand new and I've even rebuilt the head.

I'll keep persevering. From the info gathered here to date, it sounds like I should order a 0.6 ohm Dyna or Ignitech coil.

Thanks again and I'll keep you posted!

Cheers, Greg

[Stratos23]

At the end of the day, I just want to improve its willingness to start without damaging the CDI. Any more recommendations out there?

If a part is cheap enough that a failure won't hurt the bank balance too much, you could try? :

Ebay item 171613262872

I have had one on my widecase for several months of occasional use so far, hasn't given trouble yet.

Hey Jordan,

Thanks for the suggestion - unfortunately I haven't had time to check these updates until now, so it looks like the Ebay link you sent has expired and doesn't show up. Was it a condenser you were talking about? If so, then doesn't this only apply to an inductive ignition set up? What ignition do you have on your widecase?

Cheers, Greg

[Jordan]

it looks like the Ebay link you sent has expired and doesn't show up. Was it a condenser you were talking about? If so, then doesn't this only apply to an inductive ignition set up? What ignition do you have on your widecase?

Cheers, Greg

Try:
https://tinyurl.com/ybzsbyvb

It is a combined CDI unit and coil, similar to those fitted to late model singles with Ducati made elec ign.
If you wanted to try it, keep the alternator and pickup unit that you have, but remove the CDI unit and the separate HT coil.
Install the cheap unit. Wiring is simple enough if you follow a Ducati CDI diagram.

The unit is made for an Indian copy of Italian scooters, which had CDI sourced from Ducati's electronic division - hence the similarity.

I made my own CDI up by converting a non-CDI alternator stator, replacing one of six charging coils with the high voltage one needed for self-energised CDI. It was also sourced from India. The work involved was not trivial, but just replacing the CDI/coil unit is no drama, if the rest of your Ducati made CDI system is OK. Spanish made Motoplat CDI parts are not directly interchangeable, sadly.

My Ducati has never run better.

[Stratos23]

It is a combined CDI unit and coil, similar to those fitted to late model singles with Ducati made elec ign.
If you wanted to try it, keep the alternator and pickup unit that you have, but remove the CDI unit and the separate HT coil.
Install the cheap unit. Wiring is simple enough if you follow a Ducati CDI diagram.

The unit is made for an Indian copy of Italian scooters, which had CDI sourced from Ducati's electronic division - hence the similarity.

I made my own CDI up by converting a non-CDI alternator stator, replacing one of six charging coils with the high voltage one needed for self-energised CDI. It was also sourced from India. The work involved was not trivial, but just replacing the CDI/coil unit is no drama, if the rest of your Ducati made CDI system is OK. Spanish made Motoplat CDI parts are not directly interchangeable, sadly.

My Ducati has never run better.

Ha! I know these units well. If it wasn't for cutting my teeth on Vepsas & Lambrettas, I may not have even ended up with Italian motorcycles (I used to repair them to pay my way through uni). I even have a few of the original old Ducati Electronica units lying around somewhere, so will try and experiment with your idea at some point. The major difference I can see with these units is that they have no programmed advance/retard. I remember the timing marks being static on the strobe, no matter how hard you revved the engine. Thanks for the tip though - good idea!

Cheers, Greg

[Ventodue]

I still wonder whether there's an issue with the inherent problem of a CDI installed on a big single.

I'd be surprised, Greg. As mentioned before, my 450 SCR is still happily running its original Ducati Elettrotecnica CDI from 1972 . It may also be worth pointing out that the spark is produces is only a little, feeble-looking, thing - but that's enuf for it to start easy and stay running .

And I bet some of the old Thumpers like Yamaha XTs run battery-less CDIs.

The CDI only knows the speed of the crankshaft determined by when the sensor passes the pickup. It determines the advance based on this.

No, I don't think that's right. I'm pretty sure that on these engines the ignition advance is purely a function of the increasing speed of the pick-up rotor. The CDI box isn't involved. All the CDI needs is for the generating coil on the stator to deliver a big enough slug of electricity to charge the capacitor.

So it can never really accurately determine the best advance to start the engine.

Wrong way round Greg! The ignition timing isn't advanced at engine start. It only advances when the engine speeds up - this to provide more time for complete combustion. The static timing, that which is set up initially, is what the engine needs to start.

HTH

Craig

[ducwiz]

For a deeper understanding of the differences between a CDI and points ignition you might read
http://www.jetav8r.com/Vision/Ignition/CDI.html, http://mgaguru.com/mgtech/ignition/pdf/winterburn_vs_kettering.pdf, http://nebula.wsimg.com/a0048958fdda77c4862ad82c764ec88b?AccessKeyId=967E22DE049163134A29&disposition=0&alloworigin=1 and http://nebula.wsimg.com/760d1b6bb4ff725deb889204e8cc2b44?AccessKeyId=967E22DE049163134A29&disposition=0&alloworigin=1.
But, read carefully, as most of these papers are focussed on automobile ignitions.

Hans

[Harvey]

As I understand it, with points on an inductive system, the primary voltage can go to hundreds of volts when they open.

True, Jordan. When the magnetic field in the coil collapses, a voltage is induced in the primary winding in the region of 250-350 volts - which, as you say, is enough to arc across the points.

It's one of the reasons a condenser - properly called a capacitor - is included in the circuit: the charge in the primary takes the path of least resistance to ground, i.e. via the condenser. Result: no (or much reduced) arcing across the points.

Coil.jpg

That is what gets multiplied at the HT coil.

This is something I am less clear about. Please do say more if you know!

I have certainly read that, once it is fully charged, the condenser re-releases its charge back into the primary winding. This causes the magnetic field in the coil to reform. Which in turn then collapses again, so inducing new charges in the primary and secondary windings. And so on until all the energy has been dispersed by the secondary winding.

However, I find physicists do not always deliver the clearest of explanations .This is about the best I could find, and to get to this took quite some editing. It also implies that when the field first collapses, no current is induced in the secondary winding. Which doesn't seem logical. Me, I dunno. Maybe I've misunderstood.

"When the points open:
- The ground contact is broken, stopping the flow in the primary winding. This causes the magnetic field in the coil to collapse. The collapsing magnetic field induces a current to flow momentarily in the same direction, charging up the capacitor.

This current raises the voltage at the capacitor to about 300 volts. This high voltage stops the current flow. The current then reverses direction, being driven backward into the primary winding by the high voltage at the capacitor. Once the charge in the capacitor is depleted, the current in the coil re-charges the capacitor in opposite polarity. The cycle then reverses. The result is an electrical oscillation which reduces rapidly with time.

The initial 300 volt spike in the primary winding will drive an output in the secondary winding of up to 30,000 volts. The current going to the spark plug will oscillate in harmony with the input current for as long as the output voltage is high enough to jump the spark gap under compression."

As I see it, It says that as the primary current oscillates in the primary the secondary will continue to produce spark at the plug.

If we look at the process the primary needs about 3 to 4 amps flowing in the windings to build the magnetic field, when the points open the current flow stops, and the ,magnetic field collapses across both windings. 30000V is produced in the secondary and 350 V is produced in the primary windings.
Now they are saying that the oscillating current in the capacitor/inductor circuit will continue to produce spark at the plug. This can’t happen as it took 3 to 4 amps to produce the field in the first place, so the current supplied by the 0.3 uf capacitor is no where near enough to produce a spark at the plug, so only one spark.

[Harvey]

How this works for you is that with a CDI system, you should be able to run an ignition coil with fewer turns, and therefore with lower resistance (and which will also be physically smaller, to boot) because the voltage that is supplied to it is already higher than with a inductive (points) system. What this means is that the ignition coil has to step up the voltage less - often, much less.

And that's why Electrex have specified a low resistance coil. Now, as you'll understand from what I've said above, what your substitute high(er) resistance coil is doing is stepping up the volage to a level that is high enough for it to jump the gap and give you a spark. The unanswered question is:
WHY is it having to do this?

To add a bit to your good CDI description.

The main difference between Inductive and CDI ignitions is, in the Inductive ignition the spark is produced when the magnetic field is collapsing, in the CDI, the spark is produced when the magnetic field is rising.

The Charged capacitor can have a discharge current of about 16amps, the only resistance in the discharge circuit is the primary windings, so the primary resistance is important in the performance. The speed that the field is established, produces the fast rise time of the spark, that doesn’t have time to short out across fouled plugs, if the primary resistance is low, a high current will flow to produce the fast secondary rise time, but if it is too low a resistance, the current flowing may exceed the SCR’s capacity and burn it out.
If the primary resistance is too high, there will be less current flowing and a lower secondary rise time, that can short out across a fouled plug to cause hard starting.

The other point in trying to start, is the mixture, if it is too lean or patchy, then the very fast CDI spark may not ignite the fuel, that a slower inductive spark will.

[Jordan]

Ha! I know these units well. If it wasn't for cutting my teeth on Vepsas & Lambrettas, I may not have even ended up with Italian motorcycles (I used to repair them to pay my way through uni). I even have a few of the original old Ducati Electronica units lying around somewhere, so will try and experiment with your idea at some point. The major difference I can see with these units is that they have no programmed advance/retard. I remember the timing marks being static on the strobe, no matter how hard you revved the engine. Thanks for the tip though - good idea!

Cheers, Greg

My Ducati single has normal auto adv-ret ignition action, even with the scooter CDI/coil.

[Ventodue]

For a deeper understanding of the differences between a CDI and points ignition you might read ... <snip>

Thanks Hans. There's some good stuff in there. I haven't digested it all (!), but already it's clarified - for me, at least - how the voltage in the secondary winding in points (Kettering) and CDI systems is created.

I think there's possibly also some relevant stuff for Greg. I note the stuff on the differences between a CDI coil and a points (Kettering) coil. Might explain his problem, if Electrex have supplied him with the wrong type ...?