Chrysler may have just switched suppliers because of economic reasons, OR, maybe there was some sort of problem and they found these NGK worked better. Sadly, they won't tell you, they'll just issue a new part number.
The center electrode is the critical part of the spark plug, usually when they add a metal name to the plug, its to describe what makes the spark plug different then the others and it's usually indicates something about the center electrode.
Nickel can be alloyed with steel to make stainless steel or a harder, more corrosion resistant steel. Or you can coat steel with Nickel. Thats my guess, just like some plugs have platinum wire, nickel plugs have a nickel alloy center electrode that last longer and performs better then a plain steel center electrode.
Copper plugs, actually have a copper electrode right up to where it extends into the combustions chamber, I guess, the copper would erode away to fast, and the advantage is the lower resistance the copper offers for the 85% of the path through the spark plugs before it gets to the hardened steel tip in the combustion chamber.
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