coil for 250 racer

[flynbulldog]

Hi guys, been away for awhile but work on the 250 racer is progressing, albeit slowly.

I'm going with a total loss system with stock points, the flywheel and all of the associated electronics are gone.
I'll be running a 12v battery and I need to know the primary resistance value needed to find an ignition coil.

anybody have any ideas on a coil for this system?
Thanks

[Jordan]

Why do you need to know the resistance?

Jordan

[DewCatTea-Bob]

____ It's quite likely that such things have advanced since the last two decades when I was last aware of what's available, so I have no advice for a modern-ign.coil,, however I'd still expect that running with total-loss would require an ign.coil that didn't have too low of a resistance-rating.
But if all you find are with very low primary-resistance (well under 3-ohms), then you may wish to install a suitable ballast-resistor in series with the coil's primary-circuit, (of which,, as it heats-up [from a cold start], it then increases the total-resistance of the ign.coil primary-circuit), which will cut-down waste of stored battery-juice.
Also, I believe most ign.coils are merely 'RATED' as either for 6 or 12 volts, but that rating is not actually real important because the coils are not wound to create any particular secondary-voltage figure. _ Ya just need enough high-tension voltage to create a reliable spark (for the spark-gap ya wish to use), so that's the concern if you use a rated-12v.coil with a 6v.battery,, and the only concern of using a rated-6v.coil with a 12v.battery, is possible overheating of the coil's primary-winding.
So, you may consider using a '6-volt' ign.coil (with your 12v.battery), especially if you also employ a ballast-resistor.
__ I-myself have always been against 'total-loss' for racing, cuz with that setup, ya start-out with decent spark but as the battery continually gives-up it's dwindling supply of stored juice, it then gets weaker & weaker and along with that the ign.spark must also get weaker & weaker.
To cut-down that situation, a by-pass switch could be used to by-pass the ballast-resistor, (say just before the end of a race when the battery has become significantly weakened), so as to then restrengthen the ign.spark just when it's most needed.
__ It's my understanding that aircraft-engines use especially high-quality ign.coils, so you may wish to check-into that for a possible choice too, before you make your final decision on what ign.coil to go-with.


Dukaddy-DUKEs,
-Bob

[Stu65]

I use total loss and points on my race bike with a 3 ohm coil and haven't had any problems. The point about dwindling power on a total loss system is a fair one to make, but we did a 5 hour endurance on the 250 Monza, we changed the battery after 2 1/2 hours as a matter of course but that battery still showed good output, so unless you are planning very long races I shouldn't worry if all else is good.
Stu

[JimF]

The higher the resistance of the coil the less current will flow through it making the battery last longer.

The duty cycle is rather small being as current is only flowing when the points are closed.

I kind of have to think that the coil's output voltage is not related to the input resistance, that is to say that a 4-ohm coil might generate 10KV while a 12-ohm coil might also generate 10KV simply owing to the ratio of the windings between the primary and the secondary.

So while you might want to find a high resistance coil to extend battery life, the coil resistance may not tell you anything about the voltage at the high tension cap.

And as Stu points out, 5 hours is a long time on a 3-ohm coil. Perhaps the thing to do is to forget about the coil resistance for now. Find a coil that works any maybe write its resistance on the coil with a wax pencil in case you have a problem later.

You might want to consider at this time the size (not the voltage) of the battery as it's physical size will have an effect on how much power it can store. Also be sure that when you leave the bike sit on race day that you switch the ignition off, lest your engine rotation come to rest with the points closed as the battery power will just pour out through the points and the coil with you none the wiser until you go to start and use the bike.

If you do experience short total-loss ignition life with a given coil and battery you can then either opt to try and fit a larger battery or look for a higher resistance coil that works in your application at that time.

Jim

[flynbulldog]

Thanks guys, all very good information.

From what I can find most racers are using dyna 3 ohm green coils. I don't know what the primary resistance is but it seems to work for them.
I can sure find coils for a lot less money (the dyna are sold in pairs and run around $150) most of the coils I've found have a primary resistance somewhere between 0.700 to 1.400 ohms the 3 ohm rating is secondary resistance.
Honestly I don't know what all of this means but the coil manufacturer that I spoke to wouldn't sell me a coil unless I could tell him the primary resistance I need for the application.

I think I'll just buy the dyna and call it good...

[Jordan]

<the 3 ohm rating is secondary resistance>

All those turns of very thin wire and only 3 ohm? That doesn't sound right.

Jordan

[Harvey]

Thanks guys, all very good information.

From what I can find most racers are using dyna 3 ohm green coils. I don't know what the primary resistance is but it seems to work for them.
I can sure find coils for a lot less money (the dyna are sold in pairs and run around $150) most of the coils I've found have a primary resistance somewhere between 0.700 to 1.400 ohms the 3 ohm rating is secondary resistance.
Honestly I don't know what all of this means but the coil manufacturer that I spoke to wouldn't sell me a coil unless I could tell him the primary resistance I need for the application.

I think I'll just buy the dyna and call it good...

I would go for a 4 ohm primary. On 12 V, that will run 3 amps, for 5 hours, thats 15 amp hr. The battery should handle that ok.
The coils with the lower primary resistances of 0.7, 1.4 Ohms are for an electronic ignition where the primary current is controlled by the electrics.
The 3 ohm rating that you quote would be for the primary winding, the secondary would be about 8/10kohms.

Harvey.

[flynbulldog]

Looks like I'm certainly confused by all of this. Can anyone recommend a coil?

[DewCatTea-Bob]

" Can anyone recommend a coil? "

____ Why not simply use your stock Ducati ign.coil ?


" I can sure find coils for a lot less money (the dyna are sold in pairs and run around $150) "

____ I sure don't think any ign.coil could be so superior (to a common/ordinary ign.coil), that it would be 'WORTH' 75-bucks !
I'd say save your money (by getting an ordinary/cheap coil), and use your savings to instead invest in modern/more-advanced batteries !


Dukaddy-DUKEs,
-Bob