The number in the spark plug is the heat range - a higher number is a "colder" plug. The science of changing plug heat ranges to affect performance and/or cooling is kinda voodoo... (At least my understanding is on about the same level as voodoo...)

What I do know is that the difference between one heat range (changing from 8 to 9 or 9 to 8 or ?) is minimal - you should not worry about running a 9 if it calls for an 8 (or a 10)...

-David
92 500sel
500cc Superbike (not for street use...)