Are You Using The Best Spark Plug Wires For Your Application?

There are a myriad of articles on the Internet devoted to ignition systems. The science of creating and conducting an electrical charge and sending it to a spark plug is nothing new. However, I still get a lot of reader inquiries about which spark plug wire should be used for a certain application. Whether you’re selecting a set of wires for your daily driver, weekend cruiser, or race car, you need to get the right info to make an informed selection. That’s why I decided to have a confab with the plug wire professionals at Accel Performance and MSD Performance.

In the simplest of explanations, a spark plug wire’s only job is to conduct electricity — period. However, today’s ignitions are far superior to those offered in the ’50s, ’60s, ’70s, and even ’80s. The latest, technically advanced ignition systems have created a situation where plug wire selection can optimize engine performance or even hinder power output.

A Look Inside

Almost all plug wires are constructed using similar components: a central core, conductive suppression layer insulator, fiberglass braid, and an outer silicone jacket. The center core of OEM-type spark plug wires is made of a carbon fiber that helps minimize radio frequency interference (RFI). However, carbon fibers are not great conductors of electricity and develop a strong resistance to current flow in the plug wire’s ability to carry the electrical current. This reduces the amount of electrical charge that can be delivered to the spark plug. The purpose of this resistance in the plug wires is to help suppress electrical interference with other electronic components in the car. However, the resistance must not be enough to hinder the spark from traveling to the spark plug.

Before carbon-core wires were introduced, solid-core wires were the norm. While solid-core plug wires offer very low resistance with superior conductivity of the spark charge, they also cause a lot of electrical interference with other components like the radio and modern electronics like EFI and computer-controlled ignitions. Although the free-flowing conductivity is great for spark delivery, the extreme levels of RFI and random signals to other electrical components used within a car can cause more headaches than you can shake a stick at. The latest advancements in plug wire construction have nearly rendered solid-core wires obsolete.

spark plug wires

Solid-core wires (left) are great for all-out race cars that do not need to worry about EMI with stereos and such. Carbon-core wires (right) are the standard for most enthusiasts.

“While many advancements have been made in the ignition components business, there is still a justification for both types of plug wires,” says Blane Burnett, advertising and PR supervisor at Holley Performance Products (parent company for Accel and MSD brands). “For those with classic hot rods or vintage muscle cars using carburetors and points- or magneto-style ignitions, solid-core wires can still be used successfully. However, spiral core is a great choice for the vast majority of enthusiasts nowadays. Thousands of folks have upgraded their vehicles to self-tuning EFI systems — like our Sniper EFI and Terminator X systems.

“Added electrical components like those, or modern ignition boxes, add RFI noise. But spiral core wires boast low resistance while maintaining great Electro-Magnetic Interference (EMI) suppression. That helps check the box to use a spiral core for a lot of customers. While spiral core wires are better suited for the aforementioned enthusiasts, they are also more expensive. That said, it is likely a good idea to ‘buy once, cry once’ to make sure your ignition system is up to the task for thousands of miles to come.”

A Look Outside

The outer silicone layer of a plug wire is what you can see. This is usually made of EPDM rubber or silicone. Most wire manufacturers offer different colored outer jackets, but the color is purely cosmetic. However, the outer jacket is more than just good looks. It is constructed in such a way to add protection from heat, abrasion, and deterioration from chemical exposure.

Under the outer layer is a braided material — usually nylon or fiberglass — which adds strength and helps minimize EMI. The material underneath the braided layer is usually a dense, heat-resistant silicone insulation. The heart of the wire is the core. Typically, the conductor core in plug wires has carbon-impregnated fibers. These could be nylon or Kevlar.

A Matter Of Resistance

As the spark current travels through the plug wire, it has a certain amount of resistance working against that current. Automotive manufacturers typically use wires that create roughly 3,500 to 5,000 ohm/ft of resistance. Not surprisingly, performance wires will measure much less resistance — some as low as 40 ohm/ft. This reduced resistance allows the engine to start easily and even produce a smoother idle than an OEM-replacement wire can.

spark plug wires

Testing a spark plug wire’s resistance is fairly straightforward, but you will need a test meter with an ohm setting.

If you are not sure how much resistance your engine’s plug wires are creating, you can perform a simple resistance test with a hand-held volt/ohm meter. With the plug wire removed from the engine and your test meter set to ohm (Ω). Next, measure the length of the wire you want to test and make a note of the length (measured in feet). Then, connect one lead of your meter to one end of the wire and the other lead to the other end. Make a note of the wire’s resistance.

In a performance application, less is more. Accel’s 4000-series Super Stock wires (great for mild high-performance engines) are incredibly durable and offer 3,000 to 7,000 ohm/ft of resistance. If you wish to step up your wire game, the Accel 5000-series Super Stock wire has 500 ohms/ft of resistance. Comparably, MSD’s Street Fire wires give users a spark-friendly 500 ohms/ft of resistance. As an example, if you have 4-feet of Super Stock 5000 or Street Fire wire, you should have an ohm reading of 2,000 ohms.

Types Of Spark Plug Wires

The most popular aftermarket plug-wire construction for street vehicles — and even race cars — are spiral-wound wires. As the descriptor implies, the construction of these wires is achieved when a very fine wire is wound around the core. This winding is typically an alloy made of copper, nickel, and/or tin. This winding is designed to help contain RFI and EMI, and still offer a low resistance within the wire. Spiral wound wires are a mainstay on any high-performance ignition system.

spark plug wire

Accel’s 300+ 8.8mm wires feature a carbon/graphite core that virtually eliminates RFI noise while providing maximum conductivity for ultimate performance in any race or street application. Double silicon insulation eliminates leakage and misfire. The wires feature heavy-duty 8.8mm oversized silicone jackets and 150 ohm/ft of resistance.

Spiral-wound wires do, however, require frequent replacement as the carbon core will break down or crack over time. Plug wires are consumables and considered part of a routine maintenance schedule.

One question I often get asked is, “how often should I change my plug wires?” Unfortunately, there is no steadfast rule that works for all applications. “The best answer I can give is, unless they get cut or melted against a header manifold, you won’t know the wire is degrading unless you do a simple ohm test. Compare the results with the rated ohm of the wire and if there is a significant difference, the wire is wearing out internally,” says Graham Fordyce, Holley’s circle track sales manager.

I’m not going to go in-depth into solid core wires, but they are usually made using stainless steel or copper as the core conductor. Both of these materials conduct electrical current very well. Unfortunately, they have no RFI or EMI suppression capability.

Size Matters

You’ll see a lot of diameter measurements thrown around regarding plug wires. For instance, 7mm, 8mm, 8.8mm, and 10mm are typical wire diameters. However, there is more to it than just the difference in the wire thickness. “The difference between 7mm, 8mm, 8.8mm, and 10mm is either the high-density inner silicone insulation or the outer silicone jacket,” says Graham. The core is the same diameter, regardless of the wire O.D. The advantage to the larger O.D is to protect the core and insulate it from arcing through the jacketing.”

spark plug wire

MSD’s Street Fire spark plug wires feature 500 ohm/foot of resistance for improved spark delivery and power. A helically wound conductor suppresses EMI. They’re also built to last and feature 8mm silicone and synthetic jackets to resist heat and abrasion, silicone boots to protect against high exhaust temperatures, and an Aramid core to provide great strength and durability. A crimping tool is included with universal sets.

Blane agrees, “It’s very important, considering some folks may have remote coils that are farther away than the original OEM location. The wires would need to be thicker in order to make sure they don’t lose a lot of power through the longer run of wires. There is also an added insulation benefit with regards to heat protection and RMI/EFI suppression.”

Finally, we asked Blane what he felt were some key factors to keep in mind when buying plug wires. “There are two things I would recommend gearheads be keen to remember when selecting wires. Be cognizant of the resistance in ohm/foot and of EMI or RFI. The ignitions of today produce more current, which could make its way through the wire. When that is taking place, an electromagnetic field is created. This is important to be aware of because each field can create interference with more sensitive electronic components. Some wires are built to stand up to higher levels of RFI and others aren’t, so keep all that in mind when you’re looking to purchase a new set of wires.”

Givin' It The Boot

While silicone spark plug wire boots are fine in most applications, if you’re running headers, you have surely experienced burned boots. It’s a fact of life for hot rodders — or at least it was. If you’re tired of melted plug-wire boots, Accel has the cure. The Extreme 9000 plug wire kits utilize ceramic boots that will withstand temperatures up to 2,000 degrees. The boots are available in straight, 45-, or 90-degree configurations. Were told the ceramic coating acts as a heat mirror, reflecting infrared energy and heat away from the plug boot. Finally, it’s engineered to resist flaking and peeling.

Selecting the correct plug wire for your application is not a difficult task. If you are running a car on the street as a daily commuter, then carbon core wires will be fine. Add any upgrades in performance, however — especially in the fuel or ignition system — you will probably want to consider a spiral-wound wire. Again, solid core wires will work fine for non-EFI-equipped race cars when using a magneto ignition system.

Finally, Graham wanted to convey how important it is to ensure wire routing is properly done. “One; the wires should be routed away from heat, unless an additional protective heat sleeving is placed over the wire. Two; whenever possible, route the wires away from sensitive electronics.”

Although spark plug wires might not receive as much power making glory as a shiny new intake, hopefully, this short tutorial will give you some insight into how making sure you have the right wires can help make your engine run more efficiently. In the long run, isn’t that just as important?

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About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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