How to choose the correct sparkplug leads for your car

The wrong sparkplug leads can prevent your street machine reaching its potential. Dr Tim explains how to choose the correct leads for your vehicle

Photographers: Street Machine Archives

SPARKPLUG leads can’t add horsepower, but they can adversely affect your vehicle’s performance. That’s why it’s important to select a good-quality set of leads to deliver juice to your plugs. Even though they’re simple parts with no moving elements, there’s surprisingly little information about how to choose the right leads for a given application. So let’s try to cover the basics.

This article was first published in the February 2020 issue of Street Machine

The number-one job of any sparkplug lead is to transfer an electric charge from your ignition to your sparkplugs. The challenge is to get the maximum amount of this charge to the plugs without creating electromagnetic interference (EMI) or radio frequency interference (RFI) for nearby electrical components. As the high charges of today’s electronic ignitions move through the leads, a magnetic field is created around them. This field can interfere with sensitive electronic devices, create noise that can be heard through the sound system, and upset radio communications and vehicle computers. On top of that, the leads often have to deal with higher voltages from modern ignition systems and aftermarket coils, and they have to survive in the high temperatures of the engine bay.

Rated in ohms per foot, resistance is the ignition wire’s tendency to resist the flow of electric current. Leads with higher resistance will allow less current to flow to the sparkplugs. This can obviously affect engine performance. Some leads are made to produce minimal resistance, while others are made to suppress EMI. Sparkplug lead manufacturers use different core materials and designs to strike the right balance in achieving their desired result.


IN ADDITION to the all-important core, sparkplug leads consist of several different layers. These can vary depending on manufacturer, but a typical wire will comprise an outer jacket, a braided layer, insulation, a conductive/suppression layer and the aforementioned core.

The outer jacket, usually made from silicone, adds strength to the core and protects the inner layers from heat, abrasion, chemicals and other harm inside the engine compartment.

Usually made from a braided fibreglass or similar material, the braided layer is designed to add strength and provide some EMI suppression.

Below the braided layer, the insulation is usually silicone or similar, protecting the core against heat and minimising energy loss by preventing voltage leaking through the outer layers. It also helps contain interference.

Some manufacturers add a conductive/suppression layer below the insulation as an added safeguard against energy loss and EMI/RFI.

The core largely dictates the performance of the sparkplug lead. Some cores offer low resistance, while others offer better EMI/RFI suppression. Carbon cores are the common OEM style used on modern vehicles. They offer excellent RFI suppression, but also create more resistance than other styles. In addition, a carbon core breaks down more quickly than other materials, so they require more frequent replacement.

Solid-core plug leads typically use stainless steel or copper as the conductor, which conducts the charge very well. For that reason, solid-core leads have lower resistance than others to get maximum energy to the plugs. They’re not suited to electronic ignition systems or vehicles with sensitive communications equipment or computer gear.

Spiral-core plug leads incorporate an alloy (usually of stainless steel, copper or tin) wrapped around the core to suppress EMI/RFI while maintaining low resistance.

More coils around the core will reduce noise but can raise resistance, as the magnetic field is suppressed.

spark plug leadsCHOOSE WISELY

IF YOU’RE running a daily driver, the OEM-style carbon-core leads are fine for you. Carbon-core leads will typically need to be replaced on a programmed basis, as worn leads can cause misfires, reduce acceleration and leak voltage. A healthy set of carbon-core leads will provide adequate performance for stock vehicles, with no annoying noise or interference.

If you have a vintage hot rod or race car with a carby and old-school ignition set-up, you’d benefit from the ultra-low resistance of solid-core leads. The added noise from this style of ignition won’t be a factor unless you have an MSD ignition box.

For high-performance applications using modern ignition or computers that can be sensitive to EMI/RFI, spiral-core leads are the way to go. Their low resistance ensures maximum voltage at the plugs, making them ideal for applications with high cylinder pressures and more fuel, where fouling might otherwise be an issue.

Sparkplug leads are commonly a direct fit for a specific engine application. This makes installation easy, but you can also get universal leads that can be cut to fit something different. There are also plenty of colour options to suit your elite street machine, and boot alternatives to get your leads past the headers and avoid failure due to melted insulation. Once a lead’s insulation is melted, it’s not long before a misfire starts and a dead short results. Remember to keep leads separated using one of the myriad aftermarket accessory items available, in order to avoid magnetic field interference between leads and potential cross-firing.

ignition leadsLEAD BY EXAMPLE

BECAUSE the plug leads just sit there and do their job, they are often the last items to be checked in the fault-finding process, and may be neglected in the maintenance schedule too. Choose the right leads for your application and look after them.