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ignition question...

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ignition question...

Postby SRX 440A » August 15th, 2006, 3:04 pm

anybody ever try running the new automotive 8 or 8.8 mm ignition wires on the vintage sleds? would there be too much resistance? i know the core is different,old style is regular stranded wire. new auto type has a soft flimsy core. i know a auto coil probably has more fire,but just 1dering if anyone has ever tried it.
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ignition question...

Postby ZR800XC » August 15th, 2006, 3:10 pm

tried it on a 76 Merc with the Thunderbolt ignition...couldn't get it to run right. For some reason the throttle response suffered. Also tried it on my old John Deere pulling tractor...that didn't work out either. Don't know why, I figured the new automotive wire would have been better.?!


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ignition question...

Postby SRX 440A » August 15th, 2006, 3:18 pm

whew! thanks. thought i was losing my marbles. i wanted to see what kind of response i would get. i already tried it on 1 sled and did the same thing-off idle response was terrible,almost like a bad bog. i thought perhaps it was this hot weather+fat jetting. did yours do kind of a funky bog? like maybe 1 cylinder wasn't quite getting lit?
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ignition question...

Postby shewelt » August 15th, 2006, 3:29 pm

Automotive plug wires are usually a carbon-core type grade materials. The resistance per foot is high (5K-ohms or more) for surpression of RF noise & to round off edge of the pulse going to the plugs. The fast-edge is what creates much of the ignition noise and radio distubance. The automotive ignition systems are also matched to this "stage" impedance, so it needs the resistance to transfer 100% power to the plugs...per the original design.

On a Vintage snowmobile CDI, especially if you are racing, you want the least resistance possible in the plug wires...less than 1-ohm per foot is normal. Stranded copper core is what you want. The CDI will transfer the most energy to the plug & the lower resistance will create the sharp pulse edge you need to get the spark to jump with the high-cylinder pressures used back in the day (more pressure requires more voltage to jump said gap). Using ohm's law, dividind the output voltage by 5000-ohms instead of less than 1-ohm really cuts back on the available current to the plug gap, once the spark begins...That's why it won't go to throttle-up Houston .

So, don't waste your time with automotive wires unless they are SOLID CORE RACING wires....which are not real common with the electronics used in vehicles these days. Stick with the available stranded copper wires from most catalog suppliers...it's cheap & the best for the need. Hope this helps.
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ignition question...

Postby ZR800XC » August 16th, 2006, 12:50 pm

Ya, SRX, that's what happened...acted like it had one bad coil and wouldn't come up on the pipe. The way Scott explains it makes perfect sense. As the revs got higher, so'd the cylinder pressure. Thus not allowing the spark to jump.

I do have a question though. How is the automotive ignition different? The the wires have 5K ohms of resistance, how does the automotive ignition shove the electrons through the wire with ease??? Higher voltage? Amps? Or does it function a different way? Just curious


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ignition question...

Postby shewelt » August 16th, 2006, 3:47 pm

Automotive ignitions do more "pulse-shaping" to meet noise standards these days, so likely are different than the CDI's of old. If you round off the pulse, but add duration, the 5K-ohm resistance would be OK. Some of the automotive ignitions dump 50,000 volts from the coil to the plugs. Even with 5K-ohm's resistance, with the right pulse-width & that much voltage...sparking 8.5:1 compression isn't that difficult.

The Vintage CDI's have a very short duration pulse-width, so the increased resistance really stomps out the fire before it even generates. The CDI needs to hit hard & snappy....less resistance does that. That said, never run a resistor plug in a Vintage CDI sled...same reasons as the added resistance in the wires. A side note...a small diameter spark-plug electrode will also help concentrate the spark (magnetic field)....as will a platinum center electrode on the spark plug (more electrons to jump off w/Platinum)....but I just run B9ES myself......
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ignition question...

Postby AHood » August 19th, 2006, 4:10 pm

if your looking for "fancy looking" copper core wire try taylor/vertex the magneto manufacturer. they make some nice solid copper core wire with high quality plug boots and what not.
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