Coil question

[wirespokes]

I've been doing some research on coils since two are needed for a project. One I'm interested in is the Dyna Miniature DC3-2. Primary resistance is 3.0 ohms which fits for the older bikes like the 85 LeMans. However, Dyna claims these coils will only work on electronic ignitions with dwell control. 

First off, the tech help person said I needed a five ohm coil with a points ignition, showing ignorance of my system right from the start.

When I countered the electronic dwell control statement with "points do control dwell" so what should the dwell be? He responded that I couldn't change dwell because the ignition wouldn't operate properly if I changed the gap. As it turns out, his understanding is that electronic ignitions that control dwell, actually sense the coil and cut the current when it's saturated, and don't really have anything to do with dwell. I guess dwell is an easy concept to communicate.

As far as I can tell, the only problem using these coils with a points ignition would be leaving the key on with engine not running. But that's a problem with any coils - though I think it's worse for epoxy than the oil filled variety. I see no reason these 3 ohm coils shouldn't work on my bike.

Am I missing something here or will they work for me? Anyone used these on a points ignition or non-dwell controlled electronic ignition?

[Kiwi_Roy]

I don't know much about coil but by Ohms Law a 3 Ohm would develop 48 Watts of heat if it stops with the points closed.

Perhaps an electronic module just turns the coil on long enough to saturate the coil?

I think the low resistance coils might also run without a condenser.

Someone who knows will be along shortly

[wirespokes]

Those are my thoughts too, Roy. If it's 3 ohms, how can it be any different than the stock coils? How can the Dyna miniature coils be that different from the regular Green Dyna coils that work fine on these bikes?

The salesman I talked to really didn't know anything other than the sales blurb in the promo.

As for condensers, they're needed in a points ignition. The only way to eliminate the condenser is install an electronic ignition.

[lucian]

I used the standard 5 ohm black dyna coils on my LMII and am very pleased. Plenty of room for mounting compared to the old oil cans. Had to fab a simple bracket with some flat stock . The mini's are more $ if I remember right and are 3 ohm.  I'm pretty sure all the lemans bikes have 5 ohm resistor caps anyhow so either coil will be fine.

[Kiwi_Roy]

I used the standard 5 ohm black dyna coils on my LMII and am very pleased. Plenty of room for mounting compared to the old oil cans. Had to fab a simple bracket with some flat stock . The mini's are more $ if I remember right and are 3 ohm.  I'm pretty sure all the lemans bikes have 5 ohm resistor caps anyhow so either coil will be fine.
 

Except there are several more zeros on the plug caps, 3 Ohm primary. 5,000 Ohm plug caps
I would still worry about the heat generated by a 3 Ohm coil with points, when the points are closed it's just straight Ohms Law. I know they sometimes add resistors in series with the coil but I can't see the sense in that, but as I mentioned I'm no coil expert.

[Kiwi Dave]

I know they sometimes add resistors in series with the coil but I can't see the sense in that, but as I mentioned I'm no coil expert.

Series resistors were introduced in the automotive industry years ago.  The plan was to short them out during cranking to compensate for the voltage drop and maintain the full intended voltage across the coil (roughly).

[wirespokes]

Roy - Here's the game: you want the biggest fattest spark a coil can produce. The stronger the spark, the better the fuel economy and the best power.

Coils with the lowest primary resistance produce the hottest spark.

So why not use a coil with .5 ohms resistance? That would produce the hottest spark of them all. Right?

There's a downside, and that's the ignition system. It needs to be able to power up the coils.

A points ignition will only handle  3 ohm coils - any less resistance and the points will wear quickly from the higher current flow.

If the ignition wasn't designed for a lower resistance coil, the electronics can get stressed and fail.

On the other hand, going to a higher resistance coil, such as a 5 ohm on a 3 ohm system, will give a weaker spark than normal. I'm pretty sure all of the guzzi coils are 3 ohm - I know the LM 4 is spec'd for 3 ohm.

So you really want to match the resistance to what the system was designed for. One way to get a hotter spark (a coil with less resistance) is to add an amplifier (booster) which doesn't boost coil power, but provides the coil with a good current flow that doesn't have to travel through the points. It's like a transistor ignition except the points do the triggering, not a hall sensor or other electronic solution.

As for the ballast resistor, I'm pretty sure that was a solution allowing the use of 6 volt coils in a 12 volt system. But they can also be used to run a lower resistance coil in a system designed for more resistance. I'm not sure if anything is gained by doing that except being able to run a coil you've got lying around. And yes, the ballast resistor was electrically cut from the circuit during engine start up so as to provide a hotter spark by lowering the resistance.

And yes, 5,000 ohm resistor spark plug caps are necessary when running electronic ignition. 1,000 ohm spark plug caps are best for points, though 5K works ok too.

[lucian]

My understanding has been that a lower resistance coil will over tax the charging systems on the older bikes.  My oem oil filled coils on the LMII measured 4.7 ohms on the primary and it originally had points. It now has dyna pick ups and for the reason above stated, Dyna suggested I stay with 5 ohm coils. As long as the dyna's are mounted to a sufficient heat sink, I doubt you will ever cook one from leaving the key on.

[Wayne Orwig]

Coils with the lowest primary resistance produce the hottest spark.

Once you saturate the core, you are at the max energy. A lower resistance will simply saturate the core and make a lot of heat quicker.

Plus , if a spark is igniting the mixture, a stronger spark is not going to ignite the mixture any 'stronger'.

[wirespokes]

The newer low resistance coils tend to generate 40KV or more rather than 25 or 30KV. They also tend to have more secondary windings these days which equates to a hotter spark.  The hotter spark can jump a larger gap which does help with power and efficiency.

It makes sense to me that igniting the mixture with a hotter spark 'gets the ball rolling' quicker. Like the difference between using one stick of dynamite or two. 

Lucian - My Haynes manual for the 74-8 Guzzis only lists one coil with a resistance of 3.3 ohms. My 87 LM is spec'd for 3 ohm coils. I'll bet that's what your LeMans came with originally - about 3 ohm primary resistance. That's also the resistance the older airheads have as well - once you add up the 1.5 ohm resistance each of two coils in series. I get the idea 3 ohms is pretty standard for points ignitions.

[Kiwi_Roy]

2 x 3 Ohm in series would be 6

[Wayne Orwig]

It makes sense to me that igniting the mixture with a hotter spark 'gets the ball rolling' quicker. Like the difference between using one stick of dynamite or two. 

 
Poor analogy. The spark is more like the fuse to the dynamite. Having a hotter spark is like expected 4 fuses to make a bigger bang than a single fuse. Once the single fuse lights that dynamite, the other 3 fuses are a waste.

[wirespokes]

True - there's only so much fuel in the cylinder.  But it doesn't all get 'used' if the spark is weak. I'm familiar with dual plugged bikes and the difference that makes. There you have two sparks instead of one, and a more complete burn because of that. Fuel economy is better, cold start is improved as well.

I can see where you're coming from - the idea is that a spark is a spark, and if there's a spark, the fuel gets ignited and that's all that's needed. But there's another factor - the coil's ability to spark under pressure. It's easier on the coil jumping a gap at one atmosphere but much more difficult at ten. It does work the coil, which tells me the spark intensity could be an issue.

I've heard of engines gaining horsepower with hotter coils. And that was the only change. Maybe I heard BS - but I've experienced better running and economy by cleaning ignition and coil connections. It was sparking before, fuel was igniting, but a stronger spark made a difference.

[Chuck in Indiana]

- the coil's ability to spark under pressure.

Agreed. The whole system.. coil, wire, cap, plug is involved.