rmbuilder: For what it's worth,the info you provide is among the best I've seen on this site. I was intrigued by your pictures of the Chevy 3 valve head. Rumor has it they will be on next years 427 Corvette option. I too am in the Rochester area ( Fairport). Only been out on Lake Ontario once this season with all the lousey weather. It was nice to hear those blown 572"s wail! G Force

NGK works best in most Mercs but careful with any other brands as many a boating firned tried Rapidfire and other brands and has problems.

What kind of problems?

BH

Now starts the Great Plug Debate!

IMHO you can't go wrong with Champions (for normal CRs) for the SBC marine engine.
RV19YC
RV15YC4 or RV91MC
RV12YC
in order of decreasing heat range. The RV91MC was a joint project between Merc and Champion and has an longer projected nose and stainless steel body. Vortec heads use the RS series (RS12YC)

Lmarth,
Thank you, much appreciated. Should have more updates on the next gen Chevrolet engine soon. Will keep you posted. Alot of people in this part of the country aren't wet yet due to the weather (me being one of them).
BH,
We have used Champion, AC, Autolite, Bosch, NGK, and Denso. I cannot say that there is any significant difference between any brand that specifically relates to the product itself. Often problems unrelated to the brand of spark plugs (ignition, jetting, mixture, wires, timing, etc), are interpreted as "plug performance". It has been my experience that a strong ignition system and a well developed combination is far more important than the plugs it's firing.
Bob