Read my plugs, was carbon fouling on 93 pump gas but not on 96 vp?

QuickDart360 said

Maybe a good time to try so?e non projected ngk R5671a-7 or ngk R5671a-8 which is colder than the one ending in 7.

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Whats the benefit of a non projected plug? Also what is the determining factor or what you look for to determine heat range?