Spark Plug Buyer's Guide
Spark plugs are responsible for igniting the air-fuel mixture in your gasoline engine. Each cylinder has its own spark plug (with the exception of certain engines, like the Hemi, that use two per cylinder). Replacing your spark plugs based on their service interval keeps your engine running smoothly and can restore lost fuel economy. Choosing the right plug comes down to fitment, electrode material, and heat range. This guide covers how spark plugs work, the key specifications, and how to choose the right set for your engine.
How Spark Plugs Work
Inside the spark plug is the center electrode that stretches from the terminal to the tip of the spark plug. High-voltage electricity (20,000+ volts) from the ignition coil travels down the plug's center electrode and jumps across a small air gap to the ground electrode, creating a spark. That spark ignites the air-fuel mixture, driving the piston down and powering the engine. A ceramic insulator surrounds the center electrode so the voltage does not leak out elsewhere.
Modern vehicles use a coil-on-plug system where an ignition coil is placed directly on top of each spark plug and then controlled by the electronic control unit (ECU). Older vehicles may use a wasted-spark system or a mechanical distributor-based system. Read more about how ignition systems work here.
When to Replace Your Spark Plugs
Spark plugs wear out over time and are replaced based on their service interval. Spark plugs should be replaced every 20,000 to 100,000 miles depending on the plug material. Worn plugs can cause rough idle, misfires, and reduced fuel economy. Meanwhile, a physically damaged plug (cracked ceramic, for example) should be replaced immediately. Check your owner's manual for your engine's recommended interval.
Electrode Material
The electrode material primarily determines the service life of your spark plug. This is usually the first choice you'll make.
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Copper: The cheapest, with the shortest life (~20,000–30,000 miles). Most common on older, distributor-based systems.
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Platinum: Platinum is a harder, more durable metal compared to copper. Platinum spark plugs typically last 60,000 to 100,000 miles. Double-platinum plugs use platinum on both electrodes for even longer lifespans.
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Iridium: Even harder than platinum. Iridium spark plugs can last upwards of 100,000 miles and are the most common choice for modern and performance engines. Interestingly, their fine center electrode allows them to fire with less voltage.
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Ruthenium: A newer electrode material advertised to last 100,000 to 120,000 miles.
Heat Range
A spark plug's heat range describes how quickly the plug dissipates heat from the tip into the cylinder head. The tip must stay hot enough to burn off carbon deposits but cool enough to avoid pre-ignition. This typically means that they should operate between 500 °C and 950 °C. Spark plugs that dissipate heat quickly are called cold plugs; spark plugs that retain more heat are called hot plugs.
You should generally use the heat range specified by your vehicle. If you significantly increase power with a turbocharger, supercharger, or higher compression, step to a colder plug to manage heat and prevent pre-ignition. Don't go too cold, or you risk fouling and poor performance. Typically, you should only go 1-3 steps colder depending on how aggressive your setup is.
Note: Different spark plug manufacturers use different heat range scales, so refer to the reference image below when comparing between brands or adjusting your heat range.
Reach and Thread Size
A compatible plug must have the correct thread size (the diameter of the threads, e.g. 14mm) and reach (the length of the threaded section, e.g. 0.750"). Both of these specifications must match your engine exactly.
Gapping Your Spark Plugs
The spark plug gap is the distance between the center electrode and the ground electrode. The gap must fall within your engine's specified range to achieve reliable ignition. Most plugs are pre-gapped from the factory, but this does not mean they are correctly gapped for your specific engine. Always check the gap against your owner's manual or by searching online.
If you have boosted your engine, you will likely need to reduce the gap. Higher cylinder pressure makes it harder for the spark to jump, so reducing the gap ensures a reliable spark under boost. Look up specific recommendations for your target boost level.
To gap a plug, measure the existing gap with a feeler gauge or gapping tool, then gently adjust the ground electrode. Take care when adjusting, particularly with iridium and platinum plugs.
Conclusion
Spark plugs are a straightforward maintenance item with a meaningful impact on engine performance and reliability. Choosing the right spark plug comes down to picking the electrode material for the service life you want, matching the heat range to your application (stepping colder for forced induction), and ensuring fitment and a proper gap. You can find the manufacturer-recommended part number in your owner's manual, and all the parts you need to plan your build on RideBuilder.
Additional Resources
