A spark-plug wire is seldom just a wire. Each component used in each type of construction has its place in the automotive world, but some just don't have a place in racing.
First, let's look at what racers often refer to as junk wire. These are the wires that are found as original equipment on today's late-model cars. These are not wires on the inside. They are carbon-dust-coated string inside of insulation. When handled gently, not allowed to flop around, and not removed from the plug or distributor once installed, they may work reasonably well for as many as 40,000 miles. These wires are fragile in that severe movements can break the bonds of the carbon powder. This increases the resistance by the spark having to jump many gaps before it reaches the spark plug. The end connections of the carbon string to the terminal are often delicate, allowing an even larger gap when the wires are removed and reinstalled. With today's stock (though rpm-limited) high-output electronic ignitions, spark plugs often last 35,000-plus miles in a street car. When spark plugs are replaced and carbon-string wires are used, the wires should also be replaced at the same time.
While we are talking about carbon-string wires, you should be aware of manufacturers usually associated with high performance that sell carbon-string wires that look like racing wires. They come in a racy-looking package that may make some performance claims. These wires may have silicone insulation approximately 8 mm thick, be brightly colored, and have all the appearances of racing wires. They are usually priced a little below genuine racing wires. Beware and read the box carefully. The racer needs to be smarter than the average bear.
Why are carbon-string wires even an option? Years ago, the plug wires had to be shielded from the car's AM radio. If you are old enough, you may remember listening to the engine rev through the radio. As more electronic gizmos appeared on production vehicles, better shielding became necessary. Someone discovered that the carbon-string provided the shielding and suppression necessary. Not only that-it was cheaper than using stranded wire. It's called mass production.
An end view of a plug wire....
An end view of a plug wire. This one has a loose fiberglass sleeve over the 8mm silicone insulation. The sleeve adds to the heat and abrasion protection and extends the life of the wire set.
Real racing wires are manufactured with less attention to volume and more attention to racing quality. Most racing spark-plug wires are spiral wound and actually have wire inside. There is a core, which is a string usually made of Kevlar or fiberglass. This gives the wire its pull strength and forms the core on which the wire is wound. The actual wire has a small diameter, maybe only 0.005 inch, and is usually made of a stainless steel alloy that is wound around the core. It should be tightly wound, meaning the windings should be touching each other. I once found a cheap wire on which the windings were spaced so far apart that it looked liked an old Buick grille. I found this out when the "silicone" spark plug boots melted and exposed the wire. Again, beware. In this case, the silicone plug boots were not pure silicone. Since no dealer will let you cut open a plug wire to see how it is made, rely on your intelligence. Read the box closely. Don't buy it because it is the cheapest or has the most appealing color. There have been many gimmicks related to spark plugs and wires in the last 60-70 years.
This fiberglass sleeve has...
This fiberglass sleeve has been cut apart. It is thin and loose fitting on purpose. It is the final heat barrier.
As stated, most spark-plug wires on the high-performance market today are spiral wound, and the construction quality varies considerably. Depend on the manufacturer's description. If possible, get a picture of a wire that's been cut open. All of the leading brand manufacturers make spark-plug wires of a good quality, but there are subtle differences between them when it comes to construction. I'm told that insulator thickness can be as much as 12 mm, though I haven't seen anything larger than 10 mm.
How thick should the insulator be? The simple answer is whatever you think looks impressive on your race car. Beyond that, any real race-quality wire insulator with a diameter of at least 8 mm should be fine with the ignitions used in today's short-track stock cars. That takes care of the spark delivery purposes. The larger diameters will quite possibly give more heat and abrasion protection, adding to the life of your wire set. Both of those factors can be a prime concern.
This is a section of the 8mm...
This is a section of the 8mm black silicone encasement.
One thing to pay attention to is the quality of the terminals and their attachments on each end. The terminal is the point on a wire that often gets the most abuse. Use a spark-plug boot removal tool to pull wires off the plugs. This inexpensive tool removes the stresses from the terminals during removal.
Fiberglass sleeves are available for abrasion and heat protection. If you can, check the silicone wrap on the outer layer of the insulation. The thickness here is important to quality, because the thicker the silicone layer, the better the heat resistance.
The white layer of EDPM is...
The white layer of EDPM is covered with a fiberglass weave to add tension strength to the wire.
Low resistance is also a factor in selecting plug wires. A difference in resistance of a few hundred ohms is of little significance, but a difference of a few thousand ohms can make an impact. Carbon-string wires, especially if handled roughly, will have more than just a few thousand ohms of resistance.
The stranded wires of the past still have their place. They are most often prescribed for use with magneto ignitions and may even be difficult to find. I see no problem with using stranded copper wires on many race cars. Any problems would come from outside your car, unless you use radio communication. Surely, you wouldn't be using electronic traction control. Any electronic devices in your car other than the ignition and tach could be affected when using stranded wires. Interference problems would occur if you ran your race engine in the shop with your wife watching her favorite soap on TV. But then you don't really mind sleeping in the shop. Tell her that is why you spent the money-to get a really comfortable seat for the race car!
The actual wire under the...
The actual wire under the EDPM layer is wound tightly over a fiberglass and/or Kevlar core for strength.
One other concern might arise if another car on the track is using computer-controlled electronics. The lack of shielding with your stranded wires could interrupt someone else's computer-controlled electronics (e.g., fuel injection).
Now you are ready to go to the race car parts store and buy some wires. Looking at several sets, different brands, and more than a few colors, there are a few more things to be aware of. There is the choice of over-the-top or under-the-exhaust routing of the wires. Headers or cast OE manifolds should make the choice for you. Many times, wires are better protected under headers. OE cast manifolds may require over-the-top routing. What is the difference? Length. Over-the-top wires won't reach around and under headers. My recommendation is that you buy a finished set with all the terminals and boots in place. Unfinished kits are available, but avoid these unless you have the exact terminal installation tool needed or this is what you do on your day job. Poorly attached terminals can disguise a host of problems. These should be a consideration only if you have special needs.
This stranded core does two...
This stranded core does two things: It provides a form to wind the wire, and its linear strands make up the greater part of the wire's tension strength.
So, get sparked, get wired, and be smart about your choice.
I have found that these companies (among others) make a good-quality spark-plug wire for racing purposes. Construction may vary somewhat, but the overall result should be satisfactory. Choose your wires carefully and pay attention to their inner construction. Wire set quality extends to more than just the wire and its covering. The quality of the terminals, their connection to the wire, and the plug and distributor posts are important features. Selection should be based on quality.