The Science of Spark Plug Wires

Back in the day, when a guy rumbled up to the drive-in and popped his hood for onlookers to gawk at his engine, one of the first ways to tell he was legit was whether it had a full splay of yellow spark plug wires.

You remember those days. A set of yellow Accel wires under your hood showed the world you were serious about engine performance — and they looked pretty cool, too. Now, plug wires in and of themselves do not make power, but if they are not optimized, it will be difficult to get the most from your engine’s performance.

They are often the last part an engine builder orders largely because they are typically the last part they install. But they should not be an afterthought, according to a man who thinks about spark plug wires — a lot.

Mark Vieau is owner and president of Scott Performance Wire, a Mooresville, N.C., company whose entire business is based on transmitting spark from coil to plug. Scott supplies wires for top fuel drag teams and all of the wires for the NASCAR Cup, and Xfinity programs. Vieau pointed out that the early days of performance ignition got underway not when wires became yellow, but when technologies used on internal combustion engines and cars in general began to necessitate the use of new and different materials.

Silicone insulators and jackets meant wires began to increase in diameter, to the point where there are now 10 mm diameter wires on the market now. Things like radio frequency interference and electromagnetic interference and resistance all emerged as ignition systems and automotive electronics became more advanced.

 “I think the biggest transition from that point of, let’s just say from the ’60s to the ’70s, was the advent of suppression core wire,” Vieau said. “Up to that time with points, obviously solid core wire and or magnetos was the big hit. The OEMs certainly ended up with a lot of RFI noise for their new radio systems, their stereos and whatnot. So they decided they needed to fix it, and they fixed it by starting to run resistor spark plugs and suppression core, higher resistance spark plug wires. So that was the first transition.”

Low Ohm or Low Noise?

Voltage causes current to flow. Resistance is anything that impedes the flow of current. Measured in ohms, resistance, when increased, requires a corresponding increase in voltage to allow a given amount of current to flow. Spark plug wires play a critical role in delivering voltage to the plugs while minimizing resistance and EMI and RFI interference.

EMI is what hits our electronics. RFI is a different spectrum, which interferes more with radio frequencies. Maybe your engine data system screen gets wonky or your tire sensors don’t transmit data or you can’t hear your spotter on the radio. Faulty spark plug wires can affect all of those functions. Vieau likes to tell the story from an engine builder who was trying to set timing on an engine, and the timing light would begin to flash when it was 4 to 5 inches away from the wire, and it did the same thing when it got near any of the plug wires, not just No. 1.

“Obviously ohms is an important number, but what we focus on is controlling noise. I look at how quiet the wire is and whether or not it interferes with anything. Electronics, fuel injection, engine control, anything,” Vieau said. “And to me, that’s the most important. And it is surprising to find out that even in today’s world, we’re having more and more problems today than I think we’ve had in a long time.”

Here’s why Vieau prefers to focus on noise rather than ohms. According to Vieau, it takes around 9,000 volts to fire a 400 to 450 horsepower small-block at part throttle. If you have a 45,000-volt coil, you have lot of reserve voltage. Now consider a 650 to 700 hundred horsepower engine, which takes about 22,000 volts to fire at standard temperature and pressure. That means that there is a lot of reserve voltage left in the coil, which is what you want.

Where resistance plays a part in that is if you have a cylinder that is a bit weak, or you have a spark plug that’s a bit worn out compared to the other spark plugs in the engine, it’s going to require more voltage. Because of that, it could use up reserve voltage. If you minimize the ohm rating of the wire or lower the ohm rating of the wire, the coil’s not going to have to work as hard. But it’s essential to minimize ohms of resistance and reduce EMI and RFI noise.

Controlling Noise

There are two ways to control noise, Vieau said. One is to add a lot of resistance to the system. The other is put an extreme amount of inductance in the system, though the end results are essentially the same.

“I call it a choke. It slows down the frequency. That’s all it does. We don’t want to slow the frequency down to a point where it’s killing potentially the energy that’s going to the spark plug and utilizing way too much reserve voltage in the coil,” Vieau said. “You want the system to work as easily as it can and still control the noise. So the potential of sending more energy to the spark plug when it’s necessary is more easily achieved with a low-ohm wire than with a high-ohm wire. That’s the bottom line. But lot of times people think that because they have a low-ohm wire, they’re going to make more power. And that’s a bit of a fallacy.”

Scott Performance Wires sells low-ohm wires to racing teams like those in Top Fuel categories, a series for which Vieau has worked as a crew chief. For performance applications like the cars found at a NASA event, Scott makes a 30-ohm high inductance wire, the highest inductance wire in the industry.

Wire Length

Lots of cars use coil-on-plug ignition and General Motors LS series of engines put the coils near the plugs, but some dirt track cars mount their 2-pound coil toward the right rear of the car for weight-distribution purposes. However, wires lose the ability to carry energy as lengths increase, but that’s where that reserve voltage comes into play.

Vieau said it’s also important to keep the coils cool because when they get hot, they lose efficiency. The farther from heat sources like headers or EGR you can place the coils, the better, even if that means running longer stretches of spark plug wires.

Routing of those spark plug wires is also critical. Keep them away from any voltage source period. Don’t wrap it on a chassis tube or frame rail. Keep it clean and free. Let it cool. And of course, don’t let it dangle. Keep it away from anything electrical to give it the best chance to prevent cross-coupling and potentially getting a signal from a voltage spike, whether it’s an alternator wire or some other voltage source.

“Size does matter, and length does matter, but I agree you want to keep it as short as you can,” Vieau said. “But my objective would be not to be solely concerned about getting enough energy to the plug. It would be about enhancing any noise issues.”

If a voltage spike is strong enough, at the wrong time, it can create a cross coupling to the plug wire and you could end up with a misfire, or you could end up hurting the engine. A single misfire is less an issue with naturally aspirated racing engines running on conventional fuels at sustained high rpm, but lessons learned in Top Fuel drag racing have taught Vieau the importance of transmitting voltage strictly from the coils only at the moment you want it.

If you run the plug wires parallel to one another, use looms or separators to keep them as far apart as you can. Never bundle them with zip ties because that can cause cross-coupling.

Good Grounds

Vieau also pointed out that good grounds are essential in racecars for ensuring spark plug wire efficiency and reducing interference. On V8-powered cars, both cylinder heads need to be grounded to the block and the block needs to be grounded directly to the battery. So many race teams and hot rodders, Vieau said, don’t ground directly to the battery. Without those good grounds, problems can arise.

“That battery plays a really huge part in controlling noise. It’s a shock absorber,” Vieau said. “And what it does is it absorbs voltage spikes, and if there’s a clear path to the battery, the battery has a better chance to control any spikes and prevent any problems.”

Just grounding to the chassis isn’t good enough. Vieau advises going right to the battery because it’s so important for controlling engine noise or EMI and RFI noise.

Materials and Processes

Of course, from a manufacturing perspective, quality raw materials are essential to a high-quality finished product, which means sourcing them from reliable suppliers. For such an ostensibly simple part, spark plug wires are somewhat complex in construction. That’s where a manufacturer’s processes become so important.

Scott wires start with a fiberglass aramid Kevlar blend that is coated with ferrite, a ceramic-like material with magnetic properties useful in many types of electronic devices. Ferrites are hard and brittle, iron-containing and polycrystalline in their makeup. The fiberglass aramid Kevlar blend delivers the yield strength to meet the SAE requirements for spark plug wires. The ferrite also helps minimize and control the noise.

“So there is a difference between spark plug wires. If it’s a ferrite core, it’s a higher end core, it is magnetic, and it will do a better job of controlling noise than a non-ferrite core,” Vieaux said. “Now, they’ll both work in many applications, but one will work better than the other if it is dipped in ferrite, bottom line. And that’s one of the things that we do in our specially designed core.”

Once it’s dipped and it dries, Scott’s fiberglass aramid Kevlar blend is wrapped with .004 diameter copper wire turned at 200 turns per inch. Lots of companies use a copper-nickel alloy, which costs less than copper. That core, that conductor make up the most critical element of the wire. That gets sleeved with a silicone inner core for strength, EMI and RFI control, then sleeved again with a silicone outer core, then fiberglass sleeving and then a final silicone sleeve, which is available in a number of colors to make you legit when you pop the hood at the Steak & Shake.

The number of turns of wire per inch in the diameter of the wire that you use will dictate the conductance of the wire, the capacitance of the wire, the ohm rating of the wire that is repeatable and easy to calculate.

“So what does that do? Essentially the silicone, the outer jacket is high temp. The inner jacket is dielectric,” Vieau said. “And that inside silicone is a mixture of silicone. It’s got EPDM in it, which is just a kind of silicone and it has a much better dielectric. So it helps keep the spark or the voltage contained. So the more silicone you put on the wire, the more it increases the ability of the wire to keep the energy inside the wire.”

OEMs typically use a carbon core, which is basically just the aramid fibers and they just dip it in a carbon based solution. It’s inexpensive, extremely cheap to process, and it functions, but it’s a very high ohm wire.

Scott’s process is to build every single wire like it’s a NASCAR Cup wire. The company has its own wire extruded. It uses .650 strip on all the ends so assembly workers can bend the conductor up to make contact with the terminals that snap onto the distributor or the spark plug. They check every wire on a digital ohm meter to ensure the wire is terminated correctly.

Vieau was quick to point out three things that will kill a spark plug wire: vibration, heat and improper mechanical manipulation, what he calls “gorilla factor.”

“Those are guys that think they know what they’re doing and they grab the plug wire itself by the wire, or they grab it by the boot and the wire. And instead of turning, rotating the boot clockwise, counterclockwise to break the seal, if you will, a little bit, and pulling straight out, they’ll hog the thing up and down a left and right until that thing comes off,” Vieaux said with a chuckle. “And what happens is they end up elongating the terminal and end up potentially breaking the terminal. Even though our terminals are stainless steel and very hardy, they’ve done it. And I would say most of the problems we ever end up with is gorilla factor stuff. So, you’ve got to love on every part, man. You can’t take anything for granted.”

An elongated terminal will create problems, particularly when vibration is present, like at sustained high rpm used for road racing. When a terminal becomes damaged, its ohms of resistance rises. Then there is heat.

Insulation

Like many spark plug wire manufacturers, Scott offers thermal socks for the boots, which can be close to headers and exhaust ports. That’s a high-heat area, but the thermal socks are not always necessary. Vieau uses the “fat finger” method to determine if thermal socks are necessary. Most boots are built to withstand 400 degrees. Putting a sock in an tight area will actually end up transferring heat to the wire.

“If you can take your fat finger and run it around the spark plug in the boot and have clearance between it and the primary tube, in most cases, you don’t need any other heat deflection material,” he said. “The reason is you’ve got enough space to allow airflow. So if you’ve got that clearance, you’re probably on your way to having some success and not having a big issue with any fatigue melting.”

Scott also offers fiberglass fire sleeve by the foot and silicone sleeves, but with either material, what provides the most thermal protection is the fiberglass, which also provides and additional measure of protection against EMI and RFI noise. The sleeving controls noise so well that timing lights might not pick up the inductive signal on a wire with the sleeve on.

“At the end of the day, the secret sauce there is the fiberglass and what it does to control noise. So that sleeve over that wire makes the wire more effective in controlling noise, period,” Vieau said. “It’s not part of the wire. It’s an over-sleeve. Plug wires work without them, but they work better with them.”

Regardless of who makes a spark plug wire, one set will not make more horsepower than another. If your ignition system is sound, you’re not going to see a power difference. You never will unless there’s a problem.

“Nobody thinks about a plug wire until it doesn’t work, to be honest with you. It’s funny, but it’s true. And I get that it’s probably the least sexy part of the whole engine,” Vieau said. “But when you build a race motor to run 500 plus miles on an NASCAR race, it better be right. There’s no second guessing. No, ‘We hope it works.’ It’s got to be right.”