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3 ohm or 5 ohm coil?

Classic Norton Commando Motorcycles.

3 ohm or 5 ohm coil?

Postby pantah_good » Mon Mar 20, 2017 8:04 am

No luck on searches for this. I'm about to order the Pazon Altair ignition for my '72 Roadster Combat rebuild and am not sure whether to use the Dyna 5 ohm DC7-1 or 3 ohm DC6-1 coil. Pazon replied to that question by saying the 5 ohm coil should be fine, especially if a street bike. It is destined for the street (with maybe a track day or two), but I was wondering if the Combat spec compression might prefer the 3 ohm coil? What harm could be done using a 3 ohm coil if a 5 ohm coil was all that was really needed? I'm not an electrician.
Bill
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1972 Norton Roadster Combat
1974 BMW R90S
1997 Ducati 748/853
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Re: 3 ohm or 5 ohm coil?

Postby kommando » Mon Mar 20, 2017 8:31 am

I used a Dyna 3 ohm on a Boyer, it actually tested as 2.9 ohm. It would work ok on cold days but on a hot day (for Scotland thats 60F) it would go 20 mins before the bike ground to a halt. Turning off the ignition and waiting 15 mins cured it for another 20 mins. I put it down to the Boyer box overheating as when I changed to 2 6V PVL's with a combined 3.4 ohm the symptoms went away. Pazon may be ok but I would be looking for just more than 3 ohms.
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Re: 3 ohm or 5 ohm coil?

Postby o0norton0o » Mon Mar 20, 2017 11:35 am

I've used the 3 ohm dyna coil for over 10 years with a boyer microdigital (red box) ignition. I've never had an ignition problem since installing that combination. That's my single data point...
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Re: 3 ohm or 5 ohm coil?

Postby pommie john » Mon Mar 20, 2017 11:04 pm

Boyer reccommended the 3 ohm coil for use on my race bike because it was running without an alternator. They said that with an alternator I should use the 5 ohm.
The thinking is that a race bike with just a battery would have about 12.5 volts, but with an alternator it would be nearer 14.5v which would heat the coil more.

This info was based on an old Mk3 Boyer, the newer Pazon may work with both.
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Re: 3 ohm or 5 ohm coil?

Postby pantah_good » Tue Mar 21, 2017 10:49 am

Thank you gentlemen. I just don't picture this bike living in the higher rpm range, and since I quit smoking I can ride for more than 20 minutes before needing to stop. Not very sound engineering reasoning, but I ordered a 5 ohm.
1972 Norton Roadster Combat
1974 BMW R90S
1997 Ducati 748/853
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