What constitutes running one step colder plugs? I haven't really found a definitive answer. It seems like people are saying for highly modified cars, not sure if that's the case for me. Or the jcw NGks are the best because the tip doesn't erode as fast or something.

Anyway I need to replace my plugs, and there is a possibility I may be having engine clicking when on throttle no cels though.

 

Is the N14 and N18 engine using the same plug? NGK Platinum or Iridium which is better, what is the part number? Thks