My Excalibur has 420HP 454's. Im on a really tight budget. The current exhaust are 3" log cast iron originals. I cant swing 3g for a pair of new manifolds this season, as I have a lot of other things I want to spend my cash on for the boat.

MY question is, would I gain anything by going to a merc cast iron 4" center rise manifold? Those are available very cheap. Right now the boat does about 55 thru the 3" logs. Im sure I would gain a substantial amount of HP considering the 420's have holley 800s, healthy cams, but those 3" manifolds must be choking it. Thanks, MT

if nothing else go with a cast semi tuned manifold

I'm pretty sure that Mercury only used those 3" log manifolds on the early 330's and 370's. Every 420 I have seen has always had 4" exhaust. the early ones had a 4" cast manifold and a cast riser then they all went to the 4" Gil style.
I would think that going to a factory 4" center riser manifold would help your engine breath. The long log manifold didn't flow that well.
I don't know for sure, but I was told that only difference between the old 370 and the 400 was the 4" exh manifolds.
I hope this helps.

My 420's came with Gil's. My 330 has the same log manifolds you are talking about. Everyone says that just switching to center risers will make a difference. Check for the GLM manifolds, they come in cast or aluminum.

Ive got a set of factory center rise mainifolds and risers im about to list in the swap shop if your interested.

I've got 2 sets . They are just taking up space. PM me.

Or get 2 pair of revolutions for around $1,800

What GS42fun said.
You'll prbably gain a little top end.

I have 420 exhaust in very good condition cheap

prob slightly more than you want to spend, but i have 2 complete sets of CMI split tube tops for bravo bbc set up, including switchable external muffers. if interested pm for details