it,s to do with the heat range of the plug, the lower the number the hotter the plug type and the better it dispels heat and therefore stays at it's optimum working temperature and would be better in a bike that's ridden hard/race bike or in hot conditions

check out ngk tech support
http://ngksparkplugs.com/tech_support/s ... p?mode=nml

edit: there's also a good wright up here
http://www.strappe.com/plugs.html

Yes the 9 is a colder plug but you'll be ok with them

okay thanks guys.