Originally Posted by cliffyk

It depends on whether or not the stock heat range plugs are running too hot--this can easily be determined by "reading" them after making one or more runs at the throttle setting/range you are targeting. To do it properly you make the run and then shut the engine, stop and pull the plugs--we did this all the time on drag bikes way back when there was no other way of electronically monitoring anything.

There are colour charts out there that have been around for years, most of them are incorrect for modern EFI engines where the plugs should be a bit lighter tan than most charts show.

As to a cooler plug's offering an opportunity to run a bit more advance, it might; however unless the plug original plugs were way too hot (nearly white looking) the gain would be quite minimal. If the original plugs were of a proper heats range you would perhaps lose a bit of MPG with no power gain at all.

One other comment, there is no standard for heat range classification of spark plugs, neither a general range or steps between ranges--each maker uses there own scale to rate their plugs, and provide cross references to other maker's plugs. So, just because NGK says that there heat range X is the equivalent to Motorcraft's Y doesn't mean it's an exact match.

This is particularly true when comparing fine-wire and conventional electrode designs where the heat dispersal characteristics of the two designs are quite dissimilar.