What we know and recognize as the modern spark plug wire today began its evolution nearly 100 years ago, and back then, the spark plug wire existed in a completely different form. In the early 1900s, spark plug wires began as simple metal straps, but increased demand for electronics in vehicles led to its evolution. From metal straps, it evolved into a cotton-wrapped, lacquered cable with a metal core that could withstand the voltage demands of the engine it was used in. As motorcycle and engine technologies have continued to develop and improve, so have the demands on spark plugs and spark plug wires, resulting in additional advancements to spark plug wire manufacturing and the rubbery noodle appearance that spark plug wires look like today.
If you’re coming up on a service interval for your Harley-Davidson and are potentially facing the imminent task of spark plug wire replacement, you might be wondering to yourself: what are the best spark plug wires for me to use, and would installing bigger spark plug wires really help my motorcycle to run and perform better? In this article, we’ll dive into what function spark plugs have in your motor, what some of the differences are, and how to know when your spark plug wires may need replacing. There will also be some information on replacement options for you to consider when shopping for a replacement set of spark plug wires for your Harley-Davidson.
A refresher on what spark plug wires do…
Spark plug wires are connected from the coil (which is connected to the motorcycle’s electrical system) to the end of the spark plug that is sticking out of the cylinder and acts as the bridge for the voltage that is fired from the ignition coil or magneto to make it down to the spark plug. One end of the spark plug sticks out of the cylinder, where the spark plug wire is connected; the other end is located inside the cylinder that’s been filled with a mix of air and fuel. Once the voltage makes its way down the spark plug wire and through the spark plug, it delivers the final spark needed in order to create combustion in the cylinder.
How do I know if my spark plug wires are bad?
Spark plug wires tend to go bad due to age and wear, and if a spark plug goes bad or gets fouled, it can create a misfire situation where the engine will not run or idle smoothly due to a lack of combustion in the cylinder. One of the telltale symptoms of a bad spark plug wire can be seen with engine performance; much like you might experience with a fouled spark plug, you will notice your engine suddenly has an unsteady idle due to the lack of combustion, since the voltage isn’t able to make the successful journey down through the plug and cause the fuel in your cylinder to ignite. In addition to this, you may notice your engine occasionally backfiring, or it just won’t accelerate as well as it used to. Your gas mileage could start to decline due to the engine not operating as efficiently as it normally does. You may also discover visible damage to the spark plug wire such as scorch marks or melting (due to improper seating and/or overheating), or physical damage such as nicks and cuts in the insulation that affect the ability for it to successfully carry a voltage current.
Typically, when motorcycle owner is considering replacing their spark plugs, they will also consider whether spark plug wires should be replaced at the same time if the plug wires aren’t obviously damaged or worn. From a maintenance perspective, it makes sense to replace the spark plugs and spark plug wires together at the same time; you’ll be unplugging the spark plug wires when you replace the spark plugs anyway, and replacing both simultaneously ensures that you’re starting with a fresh system to help reduce or eliminate any kind of misfire situation that can happen when re-using old spark plug wires.
Shopping for Replacement Spark Plug Wire Kits
Now, we’ll go over some replacement spark plug wires options that are available for sale, including some highlights and benefits of each as well as which specific Harley-Davidson engines they are tailored to. These spark plug wire sets are all available for purchase on Amazon, and we’ll touch on a few replacement options from a common brand first (Dragon Fire spark plug wires) before jumping over and covering a couple of other alternatives. Each kit reviewed below is sold in custom lengths; as long as you confirm the engine compatibility on the listing site first, no measuring or crimping should be required once the product is received.
1. AIP Electronics Dragon Fire 10.2mm Spark Plug Wire (OEM Fit PWJ188) – Best Aftermarket Option for 103 Motors
These spark plug wires are manufactured to the size and dimension used in many of the Harley-Davidson big twin 103 motor applications, although this particular kit is specifically targeted for use in 1999-2017 Dyna engines. The spark plug wires feature a Kevlar core with a copper-nickel conductor, wrapped in a silicone sleeve that is heat-rated to 600F. The wires are cut to length for use in specified Dyna model engines manufactured between 1999 and 2017, with one wire at 8” and the other at 20”. The core wire resistance is advertised at 150 ohms per foot. These are sold in a kit of two spark plug wires for ~$39.95 USD.
2. AIP Electronics Dragon Fire 10.2mm Spark Plug Wire – for 2009-2016 Harley-Davidson Touring & Trike (OEM Fit PWJ191) – Confirmed Fit for Tri-Glides
With longer wire lengths to accommodate 2009 to 2016 Touring and Trike motors, these spark plug wires are advertised as a direct factory fit. Each spark plug wire has a copper-nickel alloy conductor tucked underneath a Kevlar layer, and additional fiberglass insulation in addition to the silicone sleeve outer layer; just like the previous spark plug wires reviewed, the outer silicone is heat tested to 600 degrees Fahrenheit. The core wire resistance is advertised at 150 ohms per foot in each spark plug wire. If you want to pick up a set of wires that matches the color scheme of your motorcycle, these wires are available in red, yellow, and transparent red. Each kit comes as a set of two spark plug wires and is sold for ~$39.95 USD.
3. Beauticare 10.4mm Spark Plug Wire for Harley-Davidson Big Twin – A Budget Replacement Option for 103 Motors
Like the AIP spark plug wires, this set is also tailored to the big twin 103 motors made from 1999 through 2017, and they also come in a 10mm thickness. There is an option to buy these spark plug wires in either red or blue, depending on your personal preference; you can also get a set of 8mm cables if your particular motor specifies that 8mm spark plug wires must be used. Each set of spark plug wires is custom-cut and pre-crimped with boots, so no need to measure and fabricate the ends of the wires on your own – although unlike the Dragon Fire spark plug wire kits, these plugs don’t appear to have any extra sleeves or construction material to help waterproof or otherwise hold the connector boots in place, nor is there any additional information on the resistance of materials used in the wires. Each kit contains a set of two spark plug wires, and the kit retails for ~$29.59 USD.
4. AIP Electronics Spark Plug Wire Set (OEM Fit PWJ189-RS) – Best Upgrade Option for Twin Cam 88 Motors
Just like the previous two Dragon Fire spark plug wire options, these wires feature multi-layer insulation with Kevlar, fiberglass, and silicone. The upgrade on these spark plug wires is found in the core materials, which feature an alloy conductor that has a resistance of 150 ohms per foot. The spark plug wires are pre-cut and crimped to fit the Twin Cam 88 engine that’s found in Heritage Softail and Fatboy motorcycles, with the wires measured at 8” and 20”. Each replacement kit is sold as a set of two wires for $44.95 USD; the slight price bump over the previous options is likely due to the upgraded core materials, and extra touches in constructions compared to the other two Dragon Fire options.
5. ACCEL Spark Plug Wire Set – 8.8mm – for 1986-2006 Sportster XL Models: A Reputable Replacement Option for Sportster Enthusiasts
If your motorcycle calls for 8mm spark plug wires, and the thought of having a set of obvious aftermarket red spark plug wires just isn’t your favorite thing in the world, you finally have a replacement option available that comes in a more discreet black color. The ACCEL spark plug wires have brass terminals at the coil end, and the wires themselves have a fiberglass braided cover underneath a thick outer silicone jacket. These 8mm spark plug wires are intended for the Sportster series of engines manufactured from 1986 through 2006. Each replacement kit has a 90-day warranty and comes as a set of two spark plug wires pre-measured to the Evolution motor specification. The smaller diameter does equal higher resistance, and these spark plug wires are rated at 500 ohms per foot. The replacement kit is available for ~$35.35 USD.
Frequently Asked Questions
Spark plug wire replacement options for Harley-Davidson motorcycles come in a variety of colors and brands, but more importantly, there are different spark plug wire thicknesses and materials available. While some might argue that bigger is better, it’s wise to know what the added thickness is from, and how that added girth in the spark plug wire translates to your motorcycle’s performance.
The extra diameter you see advertised for different spark plugs happens as a result of the added size of the thicker core (or inner wire) that is the actual conduit of electricity between the ignition coil and the spark plug. Smaller wires have a smaller diameter, and therefore have a higher resistance path for voltage to travel on. The core in thicker wires has a larger diameter, and with the extra space, the larger cores have a lower resistance path for electricity. This reduced resistance results in more power from the ignition coil making it all the way to the end of the spark plug. As a rule of thumb, most spark plug wire manufacturer specifications will indicate resistance details as a measurement of ohms per foot.
You’ll want to verify whether your particular engine has any specific demands that require it to use a particular size spark plug wire, but the argument in favor of larger wires over smaller wires has to do with the lower resistance than larger spark plug wires can yield. Having more voltage available at the termination point of the spark plug wire (and into the spark plug) means that if your engine is tuned or you’re running a power commander, you could potentially adjust the air/fuel ratio so that it does not require as much fuel to run and ride.
So, what’s the drawback to using larger-than-OEM spark plug wires? The larger wires can potentially create more electric interference, referred to as electromagnetic frequency interference (EMI) or radio frequency interference (RFI). When used in an otherwise stock application such as with a stock ignition coil or magneto, the larger spark plug wires may not be able to yield any measurable power or efficiency benefits. In most stock, non-performance applications, using the standard-issue spark plug wire size is recommended over upgrading to a larger diameter wire due to this lack of performance benefit. It is also worth noting that most Harley-Davidson replacement spark plug wire kits come in 10mm, although older engines such as the Evo motor may call for a smaller 8mm spark plug wire to be used.
Lastly, there’s the whole pesky issue of whether larger spark plug wires will fit inside of any clips that are needed to route spark plug wires separate from each other. If you decide to super-size your spark plug wires up a few sizes without checking first, you may end up stuck with making a DIY clip or routing option to keep everything where it should be. It’s not the worst incompatibility to be saddled with, but it’s something to keep in mind if you don’t want any extra handiwork (or headaches) outside of replacing the spark plug wires.
Anytime you are researching a replacement part for your bike, you will always want to verify fitment and compatibility first before you pull the trigger on a purchase. This is especially true in the case of spark plug wire kits; not only do you have to be concerned with spark plug wire diameter and size, but also the length of each wire and the type of connector boot it uses (straight or L-shaped). In nearly all automotive and motorsport applications, you will have spark plug wires in various lengths to accommodate the different distances between the coil and the cylinder. Some spark plug wire replacement kits come pre-measured, pre-cut, and pre-crimped with the connector boots attached, and you don’t have to fuss with measuring and cutting the wires to fit. With these kinds of kits, you just need to make sure you’re replacing the spark plug wires with new ones that are the correct length. However, not every kit on the market comes pre-cut and crimped, and you’ll want to verify before purchasing.
In conclusion…
Finding the right set of spark plug wires for your Harley-Davidson doesn’t have to be overly complicated, as long as you know what effects the differences in size and construction will have on your motorcycle’s performance, and look for those specifications when doing your research. Each spark plug wire on the market is different, with different core materials and different layers and types of insulation, and each one also has different resistances depending on the diameter and type of core material. Knowledge is power in this area, as it can help you discern a potentially well-built budget option from a visually similar product that isn’t built as well.
As always, when it comes to researching replacement parts online, you’ll want to verify fitment and compatibility on any parts you are searching for and/or considering purchasing. In the case of spark plug wires, it may help you to know ahead of time what the OEM spark plug wire length and diameter are before you start searching for replacements; it will help eliminate options that are obviously too long or too short for your engine. Remember, having a bigger spark plug wire diameter is not necessarily better in some applications, and knowing the resistance in ohms can help you compare two sets of spark plug wires to know which one is truly worth your money.
Not much help here for the guy trying to custom place his coil. I wonder if that Sportster wire would work well on a 2003 Twin cam with a relocated coil? How long are they? I’ve bought several sets of taylor 10mm an always get a CEL error with them. OEM wires back on, runs fine.