⚡Spark Plug

Spark plugs are sorted by electrode material and by heat range. Copper conducts best and wears fastest, so it belongs in older engines and small equipment that expect frequent changes. Platinum lasts far longer, and iridium longer still, which is why factory long-life intervals are quoted on iridium plugs. Ruthenium is the newest tier and is marketed on wear resistance again. Material sets how often you replace the plug. It does not set whether the plug is right for your engine.

So which spark plug should you buy? Heat range is the decision that actually matters, and it is not a property of your engine family. The same Subaru EJ appears twice on this shelf with two different plugs that are not interchangeable: the factory-spec plug is heat range 6 and carries a Subaru genuine part number, while the range 7 plug one step colder is correct for a boosted Stage 2 build and, by its own manufacturer copy, raises fouling risk at stock power. Buying by engine code alone and ignoring boost state gets this wrong. After heat range, the buying gate is the cross-reference: several plugs here are the confirmed original-equipment part with a carmaker number to prove it, while others are aftermarket equivalents where the cross-reference is the only thing standing between you and the wrong thread reach. That is not academic. One plug on this page shipped with the wrong thread size for a listed application even though the retailer fitment checker said it fit. Confirm the part number against your own engine, not against a search box.