Well, I've got this Excel spreadsheet that might be of help.

And then, this area of NGK's site might be useful as well:
http://www.ngksparkplugs.com/apps/watercraft/


1998 Silver Frost SVT Contour born on...8/28/01[/i]
American Iron Shootout Radial Tire 2 Class Champion, Cecil County Dragway April 20, 2002