From my web site:
The way I see it, it is all about how well the plug dissipates heat to the head.
Look at the size difference in the white
insulator length where the plug threads are.
The more white you have in contact where the threads are, the more heat is transferred to the cooling fins, and the pink is just providing an air gap to keep heat in.
I would also have to argue that an air cooled motor dissipates much more heat much quicker in a freezing wind opposed to a frigg'n hot one.
Engine Speed and Load
* If the engine is to be operated at high RPM
, under a heavy load, or at high temperatures
for long periods a colder heat range
may be needed.
* Conversely, if the engine is to be operated at low speeds
or at low temperatures
for long periods, a hotter heat range
might be needed to prevent fouling.
Ya, I do my homework and test so I know and then share and that particular topic is 2 years old and why I swear by the Iridium plugs, they really do make a difference I can actually feel.
Just pick the number that best suites how you ride and the temp conditions.
Granted it's not as pronounced a power gain as an expansion chamber but for $8 something you can actually feel.