I put in some colder champion RC9YC plugs in today gapped at .35. The main reason for doing this is because I'm trying to lower my NOx emissions to pass smog. I had the chamption rc12yc plugs in there and they didn't look too bad, but the gapping was around .42. After replacing the plugs & wires and cap & rotor startup was quicker, smoother, with a quieter and smoother idle. I didn't drive it too much but the butt dyno kinda led me to believe that the power band was a little lower.. I'm Thinking about widening the gap on the new RC9YC plugs, but I'm worried that might negate the effect of getting a colder plug. Anyone have any similar experiences to this?

 

this is for a 93 xj quto locked front and rear with 31" tires and about 500 lbs of armor. I haven't been driving it much because of the smog/registration issue but i'm thinking the lower power feeling has more to do with my 5.7l ram being my daily driver and a lot quicker.

 

Throw a set of NGKs in there of the proper heat range and gap. Be done with it.