Why change heat range ?
Fix issues x motor to fat. Repair carb or fuel.delivery
Getting oil contaminant in combustion chamber , fix it
Heat range is only the temp that the plug disperse to keepit firing.
Not actual cyl.temp
Not all plugs heat rang alike
NGK
http://www.ngksparkplugs.com/tech_support/spark_plugs/p2.asp?mode=nml