I had asked this same question a little over a month ago. Based on the responses I got, I will be changing back to the copper core plugs too at the next plug change.

The platinums I'm running have been in for almost one year, about 7,000 miles, and lately I have noticed a slight miss at idle. Again, based on the responses, I'm betting it's due to the smaller spark of the platinum core.

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Mike Tangas
73 280 SEL 4.5