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Typically, with a stock engine, the stock plug will give you the best performance. Especially if it's an NGK. Aftermarket multi-points or platinum/iridium do nothing that a brand new set of stock plugs can't.
Most dyno tests show no improvement, and on those that do show a difference, the difference is so incredibly slight that it is below the run-to-run variability of the test itself.
Those of us who use propane in our cars play around with different heat ranges to prevent plugs melting, but that's a very different proposition from a basically stock gasoline car.
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