[blndweasel]

When selecting a heat range for your spark plugs, it is entirely dependent upon how deposits are burning off (nor not burning off) the terminal of the plug. The only reliable way to check the plug color is to run the engine at or near WOT then immediately shut the engine off and pull a plug out.

A hot plug will show white burned deposits all along the anode (or the part that attaches to the threaded outer portion of the plug). A cold plug that isn't burning deposits properly will be totally black. You want your plugs to run in the middle somewhere, so you should see a little bit of white/grey at the tip, then turning tan/brown, and still some black showing at the base of the anode.

spark plug temperature ranges won't affect power in any way, but they will ensure that deposits aren't accumulating on the spark plug.

This is a much more common practice in carbureted engines, as constant changes in A/F mixture and timing call for on-the-fly adjustments in plug temperature range.

good resource: hotrodders.com

good luck

tbw