Aha!
From:
Car spark plugs: Find out which are the best spark plugs for your vehicle.
Quote:
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But don’t use copper spark plugs in high-energy distributor-less ignition systems (DIS) or coil-on-plug (COP) ignition systems. They’ll wear out too quickly.
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My Miata is living proof!
I hope I still have the old iridium plugs that were in it, and that they're in decent condition.