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NGK's are ok Tom but any plug change especially in a BB would show a gain. I tried the NGK's for awhile and did not have the luck I do with running R44S Delco's in my Z.
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When we were dyno'ing one of our L72s back in 2000 I had an ever so slight stumble I couldn't get rid of...kept trying new plugs (strictly AC's), changing heat ranges, gaps, etc...couldn't fix it. Dyno guy asked if he could make a suggestion, and had the parts store run over a set of NGK's...the rest as they say is history.
Unfortunately, the AC plugs you buy today aren't the same as they used to be...matter of fact, I believe they're made by Champion.