Which Coil for CDI Ignition?

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Stratos23
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Which Coil for CDI Ignition?

Postby Stratos23 » Wed Aug 09, 2017 2:58 pm

Hello,

Being notoriously hard to start, I've installed an Elektrex CDI ignition on my 450 Silver Shotgun (road bike) along with their 12V alternator. Even then, the bike was still difficult to start, despite rebuilding the head and quadrouple-checking everything in the ignition path.

But when I 'borrowed' a 3 Ohm Dyna mini coil off my F1, the bike started a lot more readily and the spark was visibly healthier (the coil that came with the system was a cheap-looking 0.6 Ohm unit). So I emailed Electrex in the UK to ask their advice about how suitable this coil would be with their CDI. Unfortunately, they weren't very forthcoming, playing the "don't want to know card".

So before I buy another 3 Ohm Dyna coil, does anyone have any advice in this dept? Dyna also produce a 0.5 Ohm single output mini coil which may be more suitable. I've been told that lower resistance coils are more suited for CDI ignitions.

Thoughts?

Thanks in advance, Greg

Jordan
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Re: Which Coil for CDI Ignition?

Postby Jordan » Wed Aug 09, 2017 9:25 pm

It's odd that your CDI works better with a coil from an inductive system.
Maybe you could try a coil from a another CDI system.
I'm using a coil/CDI sold for scooters. Low cost, perfect results so far.

dsmess
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Re: Which Coil for CDI Ignition?

Postby dsmess » Thu Aug 10, 2017 12:46 am

The 0.6 ohm coil can only be used with fixed dwell ignition which has a shorter charge period than the standard battery/points ignition (much shorter for low engine speed). This coil would burn out with the old battery/points system.
I am surprised the 3 ohm would charge up enough to produce a good spark.

Stratos23
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Re: Which Coil for CDI Ignition?

Postby Stratos23 » Thu Aug 10, 2017 10:06 pm

dsmess wrote:The 0.6 ohm coil can only be used with fixed dwell ignition which has a shorter charge period than the standard battery/points ignition (much shorter for low engine speed). This coil would burn out with the old battery/points system.
I am surprised the 3 ohm would charge up enough to produce a good spark.


Thanks for your thoughts dsmess. Yes, this is what a few people have said. Am I correct in assuming that the larger resistance coil (3ohm) is slower to charge (more resistance), and therefore less ideal, given the brevity of a capacitor discharge?

I'm not sure either but in real-world terms, the 3ohm Dyna coil produced a better spark (and started the engine more readily) than the 0.6ohm coil that came with the Elecktrex system.

SO, can anyone recommend what coil I SHOULD be using? A 0.6ohm version of the Dyna?

Thanks, Greg

Ventodue
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Re: Which Coil for CDI Ignition?

Postby Ventodue » Fri Aug 11, 2017 8:08 am

Hi Greg,

I claim very little expertise in this area, but I do have this note from Phil Hitchcock at Road and Race:

• Low resistance = more spark energy, but requires more current draw on the battery.

• Higher resistance = minimal difference in spark on a road bike, BUT there is less current draw. So desirable if you run with lights on around town & have a standard alternator.

So there's maybe why the higher resistance coil sparks better. But why not give Phil a call? He sells coils ...

+61 0243 884 211.

ducwiz
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Re: Which Coil for CDI Ignition?

Postby ducwiz » Fri Aug 11, 2017 5:18 pm

Just fyi: A CDI is a Capacitor Discharge Ignition, where a capacitor, which stores the energy for the spark, is re-charged immediately after each spark, by a dedicated high voltage charge coil on the stator. This charge is unloaded through the coil primary winding by closing an SCR. There is no need or even use for any battery, because the ignition circuit is totally independant from the remaining system. It's more or less comparable with the old (LUCAS) magnetos. There is also no dwell, which could define the spark energy, because the CDI's capacitor and the charge voltage define it throud#gh their fixed values.
In contrary, in the Kettering ignition, the coil is charged by a constant current, which in turn is defined by the coil's primary resistance and the battery voltage. The energy which is now stored in the coil's magnetic field, discharges through the secondary winding and the plug after opening the points; so we have a "Coil Discharge Ignition". Now, if the dwell angle of the points is to small, the coil current can't reach it's necessary maximum value, the spark in turn has less energy content.
If you use a low resistance coil, you will have increased charging current, but no increased average current, because low res. coils need shorter dwell.

cheers Hans

dsmess
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Re: Which Coil for CDI Ignition?

Postby dsmess » Fri Aug 11, 2017 10:48 pm

Hans,
Thanks for your detailed explanation.
By quickly charging the capacitor (from charging winding or the battery) this takes care of the dwell problem with inductive ignitions (not enough time to fully charge coil at high rpms). One source states cdi systems have higher energy and shorter duration spark. I'm thinking it might help if Greg increased his plug gap (.045"?). Maybe the 3 ohm coil is better because it increases spark duration. Increasing gap with the cdi coil may have the same effect. Anyway Electrex put this system together and should provide the info. I guess their customer service is limited.

Regarding other cdi coils. Ignitech offers three different cdi coils. One is a dual output. They are described on their website.

Perhaps other electrex users can describe their experiences.

Dave

ducwiz
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Re: Which Coil for CDI Ignition?

Postby ducwiz » Sat Aug 12, 2017 8:32 am

Sorry,

you are right with the dwell problem - but a CDI charges it's storage capacitor to 300 - 400V! No battery can do that directly. In the early days of CDI development (by a Mr. Winterburn, especially for car aftermarket), an electronic HV converter circuit provided the charge voltage. It was powered by the battery, indeed. See https://en.wikipedia.org/wiki/Capacitor_discharge_ignition and https://en.wikipedia.org/wiki/Ignition_system#Battery_and_coil-operated_ignition for more insight.
Btw, I still run my '73 350M3D with the original Ducati Elettronica CDI. As yours, it's spark looks somehow weak, quite different when compared to that from a points/battery system. But the engine starts perfectly, I never had to complain.

I have no direct experience with Electrex, but if their system is equipped with a 0.6 Ohms coil, this was surely done intentionally. A replacement coil should be of same quality.

Energy storage or transformation in an ignition system is subject to a physics law: E = 0.5 * L * I^2
where E is the spark energy, L is the primary coil inductance, and I is the primary current. This current is constant in a point/bat ignition, as long as the points are closed and the dwell is sufficien, but it is allways a function of time (exponentially decaying) in a CDI. The secondary voltage across the plug is dependant on the coil's turns ratio and the switching speed of the primary current. So, not only the primary resistance but also the turns ratio is an important parameter, which has to be defines the coil's characteristics, so must not be neglected.


Hans

dsmess
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Re: Which Coil for CDI Ignition?

Postby dsmess » Mon Aug 14, 2017 1:07 am

Yep, the battery must drive a dc-dc inverter, then charge capacitor from its output.
Dave

Stratos23
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Re: Which Coil for CDI Ignition?

Postby Stratos23 » Mon Aug 14, 2017 10:42 pm

ducwiz wrote:Just fyi: A CDI is a Capacitor Discharge Ignition, where a capacitor, which stores the energy for the spark, is re-charged immediately after each spark, by a dedicated high voltage charge coil on the stator. This charge is unloaded through the coil primary winding by closing an SCR. There is no need or even use for any battery, because the ignition circuit is totally independant from the remaining system. It's more or less comparable with the old (LUCAS) magnetos. There is also no dwell, which could define the spark energy, because the CDI's capacitor and the charge voltage define it throud#gh their fixed values.
In contrary, in the Kettering ignition, the coil is charged by a constant current, which in turn is defined by the coil's primary resistance and the battery voltage. The energy which is now stored in the coil's magnetic field, discharges through the secondary winding and the plug after opening the points; so we have a "Coil Discharge Ignition". Now, if the dwell angle of the points is to small, the coil current can't reach it's necessary maximum value, the spark in turn has less energy content.
If you use a low resistance coil, you will have increased charging current, but no increased average current, because low res. coils need shorter dwell.

cheers Hans


Brilliant Hans - thankyou for such an informed reply (as usual). That's a concise explanation of CDI verses Inductive ignition (Kettering) systems. The Electrex CDI system does include a battery in its circuit, but I'll come back to that in a reply to your next post in a moment...


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