This is what i've learned about spark plugs:
Copper is most conductive
Platinum lasts longer, but is less conductive
The thing that gives you more power is having a higher output ignition system with larger gaps. Having the spark travel farther is the only way to get faster ignition, the spark chooses the shortest ground and as a result only 1 of the prongs in those 4 prong spark plugs even gets used. The reason you want a better ignition system with the larger gaps is so that there's enough applied voltage to overcome the resistance posed by the gap and also because increasing the gap (thus increasing resistance) has a tendancy to stress most stock ignition systems. Don't waste your money, regular scheduled plug changes with the recommended plugs are the best and cheapest way to go and it keeps your car running it's best. A full multiple spark after market ignition is the best way to get power from you ignition system as well as all the matching components/wires/plug gaps/plugs is the best way to go for more power through the ignition system. Good luck, hope this helped.
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'84 Mustang 5.0 LX
My car