I've been telling everyone for years that I only use W9DC. The book calls for W7DC but I find they're a bit too cold. The R in the plug code is for resistor and you really don't need to use this type of plug because the plug wire has a resistor built into it. This tends to cut the voltage back too much and you want as much as you can get reaching the spark plug.
Dan Caron's
SL Barn
benzbarn@ebtech.net
slbarn.mbz.org
1 877 661 6061