I recently picked up an Eliminator 24 Daytona with a modified 454 Mag MPI converted to Carb. The build sheet is as follows;

Mark V merc 4-bolt, std bore
Steel Crank
7/16 dimple rods
J/E forged pistons, 9.8-1
ISKY roller cam, intake lift .553, exhaust .578, cam degreed at 108, 114 lob center.
Hyd. roller lifters
Rollers rockers
Square Port GM heads
Hardin Dart copy intake
Merc 500-hp holley marine carb
Mallory magnetic marine dist.
Holley marine elec fuel pump.
Stock 454 mag exhaust manifolds

I'm looking for opinions of the build up and suggestions for future work as $ become available.

What do you think about the GM square port heads. What are thier characteristics (good torque, good HP, what type of power band...)? Would there be a better set up for a reasonable cost?

Are the stock Magnum manifolds adequate. Do they hold the engine back with this build? I was thinking of looking into IMCO, Hardin Marin cyclone's or Gils. Any opinions between those listed and any thoughts on power/mph gain.

It seems like the boat needs a few more MPH to air it out. It runs great into a 20mph headwind. I'm running 68mph on gps at 5200 with a bravo 1 24-pitch(1/2 tank fuel, two people, 150-lb gear and an ice chest)

Your thoughts and ideas are appriciated.

What are the duration numbers on the cam @ .050. The exhaust is restrictive. For best results go with a real tube header like a CMI or a cheaper alternative like Lightning.

I was not provided the duration numbers from the seller. If it matters I would say that the idle is smooth. Not stock but not surging eather.

Any opinions comparing the CMI and Lightning headers. Build quality, fit/finish, longevity, HP gains? How restrictive are the stock Magnum manifolds in your mind. Will I see a 2-3mph diff. Obviously this last question would be a big guess for anyone without experience on a similiar setup.

You may not see much gain---MAYBE 1mph and that's a BIG MAYBE just by adding headers to the stock Merc 454mpi. However, if you are going to make any further mods down the road then a better breathing exhaust system are what you are going to need FIRST! Even an aftermarket manifold exhaust system like a Stainless Marine or a GIL will enhance the looks of your engine(s) and give it a nice tone.

Opps, I mis-read your post---sorry. What I have said still stands true, but now that you have made some nice mods, your engine should respond very nicely to a decent aftermarket exhaust system---even if it is only manifold type of system like a Stainless Marine or a GIL. For the most part, by what you have mentioned with your mods, a good breathing exhaust system will give your engine a pretty good kick and you should see your RPM's increase as well. You COULD see small speed increase. A 2-3mph gain is not unrealistic in my opinion---but it's not a perfect world either.

The cam is the Isky
RR-275/284 Hyd roller
Adv 275*/284*
@ .050 228*/238*
.553"/.578"
108 ICL
114 LSA
Should not be any reversion issues, nor excessive exhaust volume to contend with, as KAAMA said some gains may be had in a decent aftermarket system, but a big ticket system wont give big returns in this case.
Bob

Thanks for the information on the cam. I will file this with the rest of the build info. I am in the process of going over everything on the boat and gathering info before I budget additional mods. Don't need to spend money for no gain. I ran prop tests last week and will be tuning next weekend. Timing, jetting and fine tuning.

Anyone else who wants to ad their .02$ is welcome.

Thanks again

Check out Revolution Marine exhaust. Small diff in HP for a 450-500 HP mtr compared to true headers,and like the Gils except for about $900. I got the powder coated and polished the risers. Full polished Avail. Tell them Allen Pool from KY sent you.

That sounds like a real nice combination. I'd be hard pressed to make it better except for the exhaust. The cylinder heads aren't the greatest but they have plenty of flow on the intake side for a 500 hp engine. The exhaust could use some porting. Some people think oval port heads are better for a 454, but I think you're getting into the range where yuo need the bigger intakes.