plugs have no impact on timing or true a/f ratios

a "colder" will transfer heat quickly into the surrounding engien material and coolant. thus keeping the tip a bit cooler and being less likely to pre-ignited the mixture. this is why the trailing plugs are stock 9 (colder) than the 7 in the leading as they fire later but in a hotter spot of the combustion event

9s all around is a common thing. as is the big boost single guys running the pain in the ass snowmobile plugs that are 10.5