What size runners on the merlin heads? Are these raised runner heads? What does Merlin have to offer. Did you port match the intake and heads? I would run 800-850cfm double pumpers. What do your plugs look like?

I am running a motor very similar to yours. 8.8:1 or so, merc. rect heads (massaged just a little for flow), and the Merc. dual plane magnum manifold. Last season I ran with a holley strip dominator single plane manifold and I notice this year I have more mid range torque (and I'm running a slightly smaller cam to boot). I'm happy with my set up, have about 24 hours on it now. Oh, and I went with the Merc. intake at KAAMA and Crazyhorse's suggestion. Glad I did.

Mike are you running square bore or spread bore carbs. Most of the mercury ones I run across are spread bores. Look at the black one on the right in the picture I posted. And why the hell are those LS6 intakes so HEAVY I have had good luck with the old C454 intake they use to make.

Holley 4150 model, 750 CFM on mine. Oh, and I'm punched out to 468 also.

Shadow,

Glad to see you're running the Ultradyne 288/296 cams---those are a very good cam combination for the 454 although with only turning 4900rpm's you're not fully realizing the power of those cams----they will make power up to about 5500rpm. Have your Merlin heads been pocket ported? Also, I believe a carb with vacuum secondaries is not a good choice for a marine application engine sepecially when you're looking for performance gains. BGIII has done his engine with very close specs as yours including the exact same cam as you have only he's running with rectangular port heads, Lightening headers and a dual plane intake---can't remember which kind though.