 Spark Plug Wire Selection
Spark plug wire selection is usually not too difficult, but we will make it even easier for you with the information below.
There are many types of spark plug leads that manufacturer advertising places in front of your view in parts stores, magazines, and other forms of advertisements. They will exploit having large diameters, lowest resistance, lifetime warranties, and best performance. Trying to wade through all of the choices and associated (BS) to make an educated selection and purchase for your vehicle can be somewhat overwhelming. Everyone claims to be the best, or that you should match your plug wire brand to the rest of your ignition. What is correct?
   Core Choices
There are four primary wire core styles that most manufacturers use: Carbon, copper, stainless, and fiberglass/Kevlar® (otherwise known as spiral core). More details on each are listed below. Some of these are suppression (resistor) wires, and others are not. You do not want too much resistance (per/foot), nor do you want too little on most applications. You need to decide based upon the information we are offering you what is best for your application. There are concerns to be addressed that if your not careful can cause damage to other components in your ignition system. In the least, a bad selection will wear out in a short period of time or cause lost performance from your engine.
 Carbon Core - Carbon core is the most widely used plug wire design manufactured. For years the car manufacturers have supplied carbon core plug wire sets on most all new vehicles. They did this for obvious cost reasons, but also because they are a suppression core so that they do not allow ignition interference in radios, modern electronics, and computers. The main downside is the life expectancy of the carbon core. The more you move the wire around, the greater the chance you have of breaking the core and damaging or completely losing the continuity of the wire. You can expect to replace these wire sets regularly. The resistance value is also fairly high and can cause lost power in high output ignition systems. These are not recommended in applications where a CD amplifier is being used. For a street application just to replace the OEM wires, these are just fine. Also, if you have an old factory coil on your 1950's -1960's vehicle with a breaker point ignition, the coil resistance really does not allow you to run anything else. If you put a lower resistance plug wire on your vehicle you won't have enough resistance in the system for the coil energy to rise, it will almost all leak out, and the engine will never start.
Copper Core - This design is basically tinned solid copper braid as the core and is NOT a suppression style wire. (NOTE: You should NEVER use a solid core wire set with any electronic ignition or ignition amplifier. Solid core plug wires are designed for breaker point and some magneto ignition systems.) The copper core is fairly soft, and regular movement will cause failure of the core. Many manufacturers are phasing out their copper wire sets because they really just are not that good. The breakdown and failure rates are too high.
Stainless Steel Core - The stainless steel core is the primary choice of anyone using a breaker point distributor (with aftermarket coil) or magneto system. It is durable and there is no power loss in the core on its way to the spark plugs. (NOTE: You should NEVER use a solid core wire set with any electronic ignition or ignition amplifier. Solid core plug wires are designed for breaker point and some magneto ignition systems.) There are some magneto applications where you should not use solid core wires, or in the least use a single spiral core plug wire lead for your coil wire. If you have any electronics on your vehicle (data loggers, radio, and more) you should not select solid core wires. If you have a restored classic using a breaker point ignition and no radio, use the solid core wires.
Spiral-Core - This is the premiere plug wire design offered, and you will find variances of it with all ignition manufacturers. The actual core is a fiberglass and Kevlar® stranding, but the resistor is the carbon graphite wrap around the core. It is the most durable and lowest resistance plug wire design (while still being a suppression core). Spiral-core plug wires are the choice of anyone using electronic ignition, a CD (capacitive discharge ignition), or any application where electronics of any sort are in use. A good spiral-core plug wire set will outlast carbon 5-1 or more. You really cannot go wrong spending the money on a good spiral-core plug wire set.
Plug Wire Resistance Values
As stated at the beginning of this page, there are many manufacturers that hype their low resistance values. This is typically on their premiere wires, usually of spiral core design. You don't want too much resistance because that actually restricts spark plug output and the final energy delivered to the plug is substantially reduced. On the other hand though you do not want to little either. OK, it is true a zero resistance plug wire is fine on most breaker point or magneto ignition systems, but not on electronic systems. The wires with too low a resistance can actually cause a voltage leak, explained below.
Voltage leaks occur from the ignition coil when there is not enough resistance in place to hold back the energy that is building in the coil. As the coil voltage rises at such a rapid rate (read our ignition coil information ) if there is nothing holding it back it will lose some of the accuracy and lower the delivered voltage to the spark plugs. Electronic ignition systems are so fast and accurate, that they expect the ignition coils, plug wires, plugs, and everything in-between to be able to meet the same performance. Because of the work they expect of the coil, and how they manage the coil, the possibility of a voltage leak comes up if other parts of the system are not compatible. Sometimes the voltage leaks can even cause misfires.
If you look at various ignition manufacturers, they will have specific values of resistance (Ohms per/foot) with their plug wire sets. This is because the engineers match their plug wires to work best with their specific systems. They will still work well with other brands in most cases, but you need to be aware of what you're getting and why. Ignition brands with more efficient amplifier boxes will typically have slightly higher resistance values in the plug wires they offer than those brands which are not as efficient. The good units do not have to cheat with wimpy coils and low resistance wires, along with big spark plug gaps, to try to force their units to make a powerful spark.
So, in a way it is sort of a good idea to get plug wires that match your ignition unit brand, or at least compare values. The other theory is that if it works, don't fix it. You will find in some applications that a different plug wire set with a different resistance value will improve the performance of your system. In a word, you may need to "experiment".
Shielding Thickness
Here is another advertiser-abused method to confuse and milk more money from the consumer. When you are reading about plug wire size (5mm, 7mm, 8mm, 9mm, 10mm, 10.5mm, etc) you are only being told about the thickness of the cover, or shielding around the core of the wire, not necessarily the quality of the wire.
If you look at the image at right you will see a simple comparison of wire thickness. This is not a dead accurate image, but it gives you an idea. The center cores are all equal, and we've made minor changes to the thickness of red-colored shielding ring. If you actually take a micrometer or digital caliper and see how thick 1mm really is you will see how you've been duped over the years. Most OEM wire sets up until a few years ago were 7mm and worked just fine. The car manufacturers recently jumped on the bandwagon with 8mm and 9mm plug wire sets coming stock on your new vehicle.
Racers have been upgrading from 7mm to 8mm for decades and for good reason. Years ago with the junky 7mm stock carbon wire sets, when you upgraded your ignition system to a better coil and distributor, then built the engine to handle higher RPM, you really needed some better quality in your plug wires. The older shielding materials did not do an optimum job keeping the spark in the wire. Plus, they did not have much help if you think about the cheap, old, stamped steel chrome wire separator kits with the cheesy grommets slid inside. Your spark from the coil is looking for the fastest way to ground, and you hope that is at the spark plug. Having a nice sharp edge of metal before the plug, that is right up next to the edge of the plug wire is very tempting. So tempting that the spark energy may use it instead, creating an under hood lightshow and corresponding misfire.
Now we're sitting here in the 21st century and technologies, materials, and manufacturing processes have changed. The hot setup right now is actually the Accel 5mm plug wire sets. These use their top-of-the-line 300+ spiral core with a 5mm thick, 550° High Gloss Silicone Jacket. These wires are great! We've tested them and they work perfectly on everything we've installed them on, from street and street-strip vehicles, to 1,000 HP drag cars. Accel is one of the Mr. Gasket companies, as is Mallory, so your getting some shared technologies. Plus, with these skinny, coffee straw looking wires, you have improved header clearance and easier installation in tight areas.
The other thing to consider is to just take a look at a Top Fuel dragster. 6,000+ horsepower and monster dual magnetos, but hanging over the engine are simple 8mm plug wires. They do add the Mallory Pro Shield Insulated Shielding, which is mostly for abrasion protection, since they are rebuilding the engine in a mad rush after every pass.
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