Ohm me, ohm my: Spark plug wires demystified

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Fuel, air, compression, and spark. Those are the four things a gasoline-fuel engine needs to run properly. While it sounds pretty simple, the reality is that even vintage engines are quite picky about how each of these four items is attained. Today, let’s dive into spark plug wires and the developments in technology that these simple conductors have experienced over the years.

That’s what a spark plug wire is—a conductor. At its basic level, the spark plug wire exists solely to transmit electricity from the coil or distributor to the spark plug. Over the years the demands on these humble wires have increased, and thus the technology wrapped into them has also advanced. Now the market is flooded with options when it comes to plug wires. We’re here to help you make sense of the features and technology out there.

Size matters—but not in the way you think

The spark needs of a stock 216-cubic-inch inline-six Chevrolet are drastically different than those of a supercharged big-block Ford. Plug wires are often advertised by the diameter of the wire, making it easy to look at the options for plug wires and immediately think if 7mm is good, 8.8mm must be great. There is more to consider though.

Those larger wires typically have a lower resistance per foot—and this, not size, is the key thing to pay attention to. Plug wires of larger diameter could simply have larger conductors inside the wires, but their size could also be due to other features. Early spark plug wires were very simple—just metal that conducted. There wasn’t even insulation. The automobile had been around for at least 50 years when my 1930 Ford Model A was built, and guess what, it’s plug wires are just straps of metal from the distributor to the plugs.

Model A spark plug straps
The simplicity of the Model A Ford never fails to amaze me. Kyle Smith

Those simple conductors evolved to require insulation to shield the ever-increasing voltage from jumping to an unintended ground rather than through the spark plug. Not only did insulation join the features list for plug wires, increasing use of electronics as the decades wore on required the development of EMI and RFI suppression. Electromagnetic and radio-frequency interference can be a real pain in the butt. The most common instance of radio frequency is the phenomenon that occurs when a home mechanic replaces the plug wires on a vintage car and afterwards the radio has a pinging noise whenever the driver turns on sweet tunes while the engine is running. That pinging noise is the radio-frequency interference from an uninsulated plug wire.

Wire cutaway of an Accel wire

The EMI and RFI suppression keeps the electrical noise of the ignition charge moving within the wire to a minimum. That electrical noise can wreak havoc on everything from AM radios to engine and transmission control computers. So if a little insulation is great, more must be better, right?

Resist the temptation

Yet again, not quite. Like everything in life that is good, that insulation has side effects. The side effects are resistance and cost. The resistance of many wires is measured in ohms per foot. A wire with high resistance can be in the 5000 ohms per foot range for especially high-output ignition systems, whereas a low resistance performance wire could easily get double digit ohms per foot. Lower resistance means more of the ignition coils energy is going towards bridging the gap or the spark plug. That hotter spark can create a more even burn of the fuel/air mixture in the cylinder, and thus, more power output from the engine.

Diagram of a spark plug wire

Also, all the shielding inside a wire is not free. The best spend of your money is to buy a wire that protects against just enough interference to keep the rest of your car functioning properly—but no more. That is a tough balance to strike on modern computer-heavy cars, or even racers with sensitive data logging equipment. For stock applications, an OEM-spec wire might be best despite it having greater resistance than an aftermarket wire.

Ignition systems which use a magneto are often best served by the lowest resistance wire possible. The higher voltage of a coil on plug or distributor system can create enough voltage that a slightly higher resistance wire will not greatly affect spark strength, but the relatively lower output of a magneto means using a lower resistance wire is more important to ensure a hot spark from the spark plug.

Get the connection right

Once you have your wire chosen, you’ll need to connection the distributor cap (or coil, for coil on plug applications) and the spark plugs. There is often the option to purchase pre-terminated wires, where the wires have been cut to the proper length for your application, but if you have something custom or want a cleaner appearance than factory, building your own spark plug wires might be the way to go. High-temperature-rated boots might be required if you have headers or other routing that puts the sensitive wires near a strong heat source—mainly the exhaust. Headers are notorious for causing problems, so invest in some extra insulation to prevent your new wires from literally melting down.

traditional female distributor cap
Here is an example of a female distributor cap. This style is popular on OEM applications from the 1960s and ’70s before the switch to the HEI style. Kyle Smith
HEI-style distributor cap wire
Here is an HEI-style distributor cap and wire set. This is often an upgrade, but the function is the same as the female-style cap above. Kyle Smith

It’s not a difficult process; it just takes a bit of patience and attention to detail—like any other automotive project. Installing new plug wires is a part of most basic tune-up to-do lists, and if your favorite cars wires haven’t been changed in a decade or so it might be time for you to start shopping. If you have any additional tips, be sure to leave them in the comments below.

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