Blaz could be right unless you make a very small change. To do this you need to compare the plugs' ceramic protrusion (material surrounding the electrode shaft). Theory is the further into the combustion flame the tip goes the hotter the plug is due to the longer cooling path back into the head. Be careful of piston contact. There should be some mfgr charts somewhere on line??? Seems like such changes are usually made due to mixture strength, as in older carburetted race engines sputtering at low speeds. IMO