PanteraDoug,
So a champion spark plug with heat range 9 is good. I found RV9YC by champion, it's for marine application and so was the NGK BPR6FS I used before. I have no idea what the difference are for marine application.
OK I understand now that I don't need double platinum spark plugs. The more I Google the net the more confused I'm. You are absolutely right about all the cross over charts.
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