Spark plug wire length?

CTripps

CTripps

Active Member
Hi folks.. I've searched a couple of times, can't seem to pin down an answer to this.

I've been wondering... Does the length of the spark plug wire matter? Or maybe the better question is how much does the length of the plug wire matter? The stock ones I've handled all seem to be within and inch or so of each other for length, and I've used a similar length when replacing them, but does it matter if you use a 10" long wire over say a 6" long wire?
 
The added resistance of a few inches will not matter. In fact you probably couldn't even measure it with a standard ohm meter. 10" is not excessive in the least. Use a longer wire with comfort.

Tom
 
if you replace it with a better wire longer dosent matter at all it will still have lower resistance than stock
 
Thanks guys, that's what I suspected but it's good have other opinions on it. It's not like I'm going to use a 3' long Ignition wire (or a 3" one for that matter) but now I know I have the freedom to play a little with it on the next build.
 
JonnyR said:
if you replace it with a better wire longer dosent matter at all it will still have lower resistance than stock
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Not necessarily. Ignition wire from most auto parts stores today is resistance wire. The one that comes with the kits has a metallic core. It has lower resistance than the wire from the parts store.

Some expensive high performance resistance wire is okay but the majority will show a higher resistance reading than the kit supplied, metal core wire.
In any case the slight increase in length is not a factor to be concerned with.

Tom
 
So wait, to get optimal ignition performance, should I replace the stock spark plug wire and boot with say spark plug wire and boot i get out of a car? Or should I keep the stock one? Im confused
 
The kit supplied plug boot should go in the trash, not on your bike. They are by far the weakest link to good performance. Any auto parts store should be able to provide you with an automotive grade plug boot that will be superior to the kit item. The spark plug will need to have the top cap to use a good quality boot. The kit boot requires that the cap be removed and the contact inside of it is inferior to the auto grade boot which snaps on and completely surrounds the cap providing a good electrical connection.

The kit supplied wire is a metallic core wire which has lower resistance than any so called 'high performance' ignition wire. Almost without exception the wire you'll get from an auto parts store today will be resistance or surpression wire. The problem is the kit makers often only give you a scant few inches of wire which requires you replace it if you want the CDI located somewhere further from the engine than on the down tube.

Like Venice said, a copper core (conductor) wire is the best way to go.

Tom
 
But wait wait,why do modern cars use wires with higher resistance? This confuses me, shudnt cars use un suppressed metal core wire too? Could I find copper core wire on a more recent say 90s VW car? The reason being, I love finding excuses to go the auto wreckers, and I would just go there and give em a few bucks to rip a wire out of a car there.

Where storewise, could I buy a copper core wire from then like Venice suggested?

Also, I should mention i have a basic digital tachometer and one of those bike speedometer things that you can buy at Canadian tire. Not sure if that counts for anything because i dont really understand interference and such from electric current.
 
Frogster said:
But wait wait,why do modern cars use wires with higher resistance? This confuses me, shudnt cars use un suppressed metal core wire too? Could I find copper core wire on a more recent say 90s VW car? The reason being, I love finding excuses to go the auto wreckers, and I would just go there and give em a few bucks to rip a wire out of a car there.

Where storewise, could I buy a copper core wire from then like Venice suggested?

Also, I should mention i have a basic digital tachometer and one of those bike speedometer things that you can buy at Canadian tire. Not sure if that counts for anything because i dont really understand interference and such from electric current.
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New cars with all of their solid stae electronics, computers and elaborate sound systems need resistance wire to keep the RF signals generated by the ignition system from screwing up their delicate electronic balance. They sacrifice a few ohms of resistance for this. The small, relatively weak ignition sysyems on our bike engines need all the help they can get and metallic conductor wire is one way to do that.

Yes, you can run resistance wire and spark plugs and probably never notice the difference but the fact remains that the lower the resistance between the ignition source(CDI) and the spark plug's electrodes the better the plug will fire.

Tom
 
if my science memory is correct, the speed the spark travels through a plug wire is close to the speed of light.

i don't think a few inches matter... :)

all my cdi's get hidden under the motor near the bottom bracket, which makes my plug wires about a foot or so. never had a problem, and i don't have an ugly black box with electrical taped wires in plain sight.

i use 7mm copper core wires , too.

the reason cars have suppression wires, is because they have electronics, and the RF caused by the spark messes with that.

RF leakage is the reason your car stereo will buzz or your idle fluctuates in time with your turn signal.
 
Okay awesome, where could I get a copper core wire though? I cant go to the auto wreckers cus none of the cars there will have it.

Also, will the non suppressed wire mess with my bike speedometer??
 
Frogster said:
Okay awesome, where could I get a copper core wire though? I cant go to the auto wreckers cus none of the cars there will have it.

Also, will the non suppressed wire mess with my bike speedometer??
Click to expand...
Not sure about Canadian auto parts stores but many here in the States carry copper core wire. They sell it by the foot from a bulk roll. You usually haver to ask for it.

If your speedometer is the wireless type the RF from the ignition will probably have some effect on it. The wired types seem to be less effected but even so some builders have found that using resistance wire helps eliminate the interference.

The loss of performance with the suppression wire will be negligible in most cases. The bug-a-boo is getting a good electrical connection at the CDI and the plug boot. Resistance wire has a thin carbon impregnated thread as its center core and it must be in contact with the metal parts in the CDI and boot. A metallic core wire makes that a no brainer but the suppression wire takes some care when installing it. You need to have a clean, square cut at the end of the wire, locate the core and carefully thread it onto the CDI contact threads. The plug boot metal contact often has a sharpened tank that when crimped will pierce the outer insulation and into the core or the core must be exposed and folded back into the metal contact.
I even go so far as to solder my copper wire into the plug boot contact for trouble free and a permanent installation.
Location is also a factor. Keeping the speedo wires as far from any engine related wiring or parts helps as will locating the CDI someplace other than the downtube. Like Bairdco said, mounting the CDI under or to the rear of the engine will keep some distance between the speedometer and the source of the electrical noise.

Good luck.

Tom
 
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