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.
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.
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.
This fiberglass sleeve has been cut apart. It is thin and loose fitting on purpose. It is the final heat barrier.
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 is a section of the 8mm black silicone encasement. When cut apart, the waffling is revealed. This aids in keeping the covering in place over the EDPM layer.
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.