 Re: heat range
Buttanic is on the money. While air fuel chaqrge temp can affect plug temp it is only slightly. When they say a plug removes heat from the combustion chamber they are over simplifying a bit. The part we are trying to control the temp of is the ceramic core nose. Too cold and it will collect carbon (foul), too hot and it will begin to glow causing preignition. The heat path is Combustion --> Core nose --> Spark Plug Shell --> Engine Head --> Cooling jacket. So technically the core nose is removing a little heat from the cylinder but that is not it's purpose. It's using this heat to keep itself clean. Heat range is "adjusted" by contolling the length of the core nose inslide the spark plug before it contacts the shell. A longer core nose creates a hotter spark plug and vice versa since the tip of the core nose is further from the ultimate cooling source. The coldest plug is a surface gap which essentially runs at the temp of the shell.
The real question here - and it is often misunderstood and misapplied is - Why do you want to change your heat range? There are very limited reasons to do so - AND unless your plugs are constantly fouling or you have melted a piston, there is no advantage to changing heat range. Changing heat range has virtually no affect on engine performance unless you go cold enough to foul or hot enough to melt.
What does have a significant impact on performance is spark position within the cylinder and that's where extended core nose plugs have an advantage over surface gap (if the engine is made for them).
I was a techincal service engineer for Champion for many years and we used thermocouple plugs to test engines. These are $200 spark plugs that have a thermocouple embedded in the core nose. A full set is installed and connected to instrumentation and then the engine is run through all it's ranges and conditions and also with some "fudge" thrown in to simulate "worst case" i.e. lean mixture, overadvanced timing, etc. Hundreds of hours of testing goes into recommending a particular plug for an engine. Unless you have a compelling reason to do so, changing your own heat range puts you at great peril with reguard to doing engine damage (that will not be covered by warranty).
If you told me - I troll all day , never see WOT, and always foul plugs - that might be a reason to go up a heat range. Otherwise, leave it alone.
Steve | 
That really clarified some things for me. After reading that on NGK's site it got me to thinking hence the question was put up. 
