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How To Make Custom Spark Plug Wires

Author: Bradley Iger | 09/15/2020 < Back to Motor Life Home

Whether you need an OE replacement set or something purpose-built for extreme performance, there’s no shortage of pre-made spark plug wire kits available today. But if your project involves a healthy dose of customization – whether that’s a power plant living in an engine bay it was never designed for, or significant modification to the engine itself – you may find yourself in a situation where a custom-made wire set is best option for your application.


Fortunately, universal sets like those offered by Accel and MSD are designed with projects like these in mind, and you don’t have to be a master electrician to make your own wires. Here we’ll check out what your options are, walk through the process of building the wires, and get a few tips on how to make sure your new wires are up to snuff.

When, Why, And How


With spark plug wires it can be easy to assume that as long as everything’s plugged in properly, everything will remain copasetic ad infinitum. But as Graham Fordyce of Holley Performance reminds us, a spark plug wire is consumable part, much like a brake pad or a filter. “Spark plug wires are designed to flex, and over time, that flexibility will have an effect on the core. When you’re doing a tune-up, you should always check the resistance of the wires and make sure the wire is not degrading.”


And that means that even if a spark plug wire looks fine on the outside, it’s not a guarantee that it’s internally sound. “If it degrades, it’ll normally create more resistance,” Fordyce says. “That might be tough to detect by the seat-of-the-pants unless it’s at the point where the engine is misfiring, but you are getting less energy to the spark plug. And once it starts to degrade, it’s going to continue to do so.”


MSD Wire Stripping and Crimping Tools

Utilizing the right tools for the job, such as this MSD wire stripper (PN35051) and crimper help make the job of making custom wires go smoothly. They also ensure a rigidly secured connector which will allow for years of trouble-free service.



There are also the demands of high performance builds to consider, he notes. “When you build a performance engine, you want as much energy as possible to the plug for a better burn in the cylinder. Any time you start adding things like blowers, turbochargers, nitrous, or you change your camshaft, et cetera, it’s going to demand more of everything else in the motor. And if you can’t burn it properly, it’s kind of a waste to do that.”


That’s where it becomes important to take note of the resistance rating of a given spark plug wire. It’s ostensibly a balancing act between minimizing RF and electromagnetic interference and delivering the maximum amount of spark energy to the plug. In applications where the latter of the highest importance, a plug wire like the MSD Super Conductor might be just the ticket. With only 40 to 50 ohms of resistance per foot, the loss of spark energy in this wire is minimized, ensuring the cylinder is getting all the juice it needs for a proper burn.


But there’s also the construction of high performance plug wires to consider, along with the expected use-case. Sets like the Accel 300+ Thundersport wire offer 150 ohms of resistance per foot to better suppress unwanted RFI and EMI interference, an important feature for both street-driven cars and race cars that use data acquisition systems and other sensitive electronics. They also boast thermal protection up to 600 degrees, something that becomes an increasingly important consideration as you ratchet up the performance in your build.

“Anything you do to make power is going to develop heat, and that’s going to affect the temperatures under the hood,” Fordyce points out. “If you bolt up a turbo in there, for instance, that’s going to radiate a significant amount of heat on its own.”


Exhaust modifications are a significant factor as well – if you change from cast iron manifolds to a set of headers, you’re likely to see increased engine bay temps because the cast iron of the manifold is better suited to retain heat than thin-walled headers are.


Beyond maintenance and performance concern, a new set of properly fitted spark plug wires can provide both aesthetic and pragmatic benefits, too. “A lot of times when you buy a pre-made set, the wires are designed to be routed a very specific way,” Fordyce says. “So if you don’t route them exactly that way, your wires are going to be sort of a mess. And if your wire is longer than it needs to be, that extra length of wire is creating more ohms of resistance, thus taking potential energy away from your plug.”


And custom routing can save time for teams at the track. “A lot of racers like to route them around the valve cover or under the exhaust,” he explains. “So if they want to check valve lash between rounds, it’s just one less thing to have to remove.”

coil near plug engine

Coil-near-plug engines, such as the ever-popular LS, still require quality spark plug wires. Often, coil relocation or non-factory valve covers require custom wires. MSD offers the correct boots and terminals to build wires for these setups as well.



Get It Together


So we’ve established that there’s no shortage of reasons to get a good plug wire that’s designed for your specific application. But in situations where a direct-fit set won’t work, building your own set of wires to the required lengths is the best way to ensure proper fitment, and it allows you to route the wires exactly the way you want to. Fordyce walks us through the process.


“First determine what type of boot is going to allow you to route the wire where you want to route it,” he says. “That could be under the exhaust, or over it, around the valve cover or over it – however you want to do it, you just need to determine which boot will allow you to do that.”


With that squared away, it’s time to figure out the wire lengths you need. “What I find to be the easiest way to do that is to install all the wires on all the spark plugs, route them the way you want to up to the distributor, bring the wire up to the terminal end cap that it’s going to connect to, then give yourself about half an inch of extra room for movement, and cut the wire.”


From there we move on to assembly. “Strip the wire about 5/8ths of an inch, roll the core back over on top of the silicon, and install the terminal. The core goes up against the terminal and you crimp it opposite the side that the core is on, so it sort of presses the core up against the terminal. Then install the boot and you’re done!”



Strategies For Success

Checking spark plug wire resistance with an ohmmeter

Checking the wires with an ohmmeter ensures they are performing optimally and that an error wasn't made in the assembly process. It's also a great way to determine if your old wires are ready to be replaced.


Fordyce also offers a few tips to make the project as hassle-free as possible. “Try to route the wires away from heat sources whenever possible, and as far away from any electronics as you can. And of course you want to make sure they’re not near anything that’s rotating, or any linkage, pulleys – anything like that.”


Using dielectric grease is also an option worth considering, though Fordyce offers a word of caution. “It’s designed to help seal the boots to the spark plug or distributor cap, which helps to prevent corrosion and, to some extent, helps to keep the spark from jumping to ground. You have to be careful with it, though – people seem to have a tendency to use too much, or they don’t apply it correctly and then it gets between the wire terminal and spark plug or distributor cap connections. That’s fine if it’s a tight connection, but if the terminal connection isn’t tight, it could insulate the connection over time.”


And once all the wires are built, it’s important to check your work before they go on the engine. “I typically remove all of them, clean any grease or other debris off of them, and then check the resistance of each of them to make sure there aren’t any issues. If it doesn’t calculate properly, then you know you’ve made an error somewhere – maybe the core slipped when you were crimping and didn’t notice it, or something like that.”


Checking the wires at this point ensures that you’re not going to be chasing gremlins later on when they’re on the car, when the origin of a potential issue might be less clear. But if you follow Fordyce’s advice, you should be back to firing on all cylinders in no time. Enter Text Here

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