This is not really true.... Heat range is a measurement of the thermal conductivity of the plug, which is basically how quickly it will dissipate heat from the plug. A "colder" plug will dissipate heat faster and therefore might never get up to the correct temperature if its the wrong application. A "hotter" plug dissipates heat slower and therefore will warm up to correct operating temperature faster, but can potentially get too hot and cause pre-ignition if its in the wrong application. The plugs will fire for the same amount of time since the distributor is the determining factor on that. The older method of AC Delco part numbering for spark plugs allowed you to determine heat range and type right from the part number. R= resistive type, 4= 14mm thread, 5= heat range(between 1-5), T= Tapered seat, S= extended tip. The new AC Delco part numbering system does not allow for this. 41= Spark Plug, 1XX-series= Iridium, 8XX-series= Double Platinum, 9XX-series= Double Platinum Fine wire. The last two numbers of the plug are the specific application and don't denote any specific features by themselves.