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I'll disagree with you Dave.
IF the engine is misfiring at light load/lean mixture due to too cold an electrode temperature, a plug heat range change may help get more consistant ignition and improve fuel mileage. I have accomplished that in the past on carbed engines leaned out and lightly driven, hotter plugs improved driveability and fuel mileage.
Here it is, you have fresh mixture sitting between the plug electrodes. It is compressed, adding heat to it, all you need to do is trip that fuel over the ignition threshold energy level. Even a weak spark will do it, but wait, if the electrode is cold, mixture lean (low load) and compression low (throttle plate near closed) there just isn't anything to sustain that little kernel of flame and it goes out. Again and again. The losses in fuel and power, and resultant increased throttle opening can be substantial.
Now, IF you have the light load mixture near perfect (toward lean) and the throttle near closed due to light load, you CAN get more consistent ignition if your plug temperature was up near 400f-500f because you installed hotter plugs. Will this improve economy? Darned right it will.
Is there any risk? YES! If you run at full power (WOT) for any length of time you risk spark plug temperatures over 500f, plug damage, preignition, detonation and all the damage that comes with it. So usually plug temperature is set by full power needs.
Modern engineers know all about this. This is the main reason we have such high powered ignition systems these days. Arc weld the darned thing alight! So in most cases, little to be gained by installing hotter plugs IF EVERYTHING ELSE IS PERFECT.
I got this post mixed up with another one that I answered about plug types. So you might find some kernels (that is a flame front pun!) of information.
Steve
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