A friend of mine has older 454's built to 420 specs. He currently has the stock merc center riser iron manifolds. Would going to a set of new style 496 aluminum manifolds be worth some HP over the old style iron ones?

They would flow better and weigh less. Might be worth 1-2mph.

Motors were 370's, but now have hot cams, and roller rockers, etc. I bet a free flowing exhaust would really wake them up.

YEP !

Regards,
Ray @ Raylar

Would the 496 exhaust bolt up to the old style 454/502 rectangular port heads? I thought there were issue with the bolt pattern. Had to redrill stock CMIs to get them to fit a new 496? Something like that?

I havent run yet but have fitted my 91 502's with 496 manifolds all lines up nicely, they do exit 1 inch lower

The old Merc 370 is a very good all forged parts engine that came with Crane Gold roller rocker arms.

Also, I believe you are correct about replacing the stock cast iron Merc exhaust with a better free flowing exhaust....especially if you've added a hotter cam. Your engines should respond with some kick...especially in the midrange and maybe 1-2mph on top as Griff said.

merc is replacing the old aluminum 496 exhaust with iron exhaust because of leaking problems...be careful which you put on..