I need some help in making upgrade decisions. My Donzi 28 zx is equipped with twin mag mpi's, and, id like to start adding some HP. The boat has the stock Merc manifolds with Corsa thru hull exhaust. I thought about buying CMI E Type headers and Thru hull open pipes, but am a little perplexed with the Merc heads. If i go this route, and add a set of heads later (say next year), are all SBC exhaust configirations going to bolt up to these "stock head" headers, or, should i wait and do the top end of the motor first (am i limiting myself by going this route)?

Is it possible to build one engine at a time (say over two seasons). Ultimate is ~500 HP 383 with aftermarket mpi (BBC is out of the question).

This is a freshwater boat.

I believe most or nearly all aftermarket SBC heads have the same exhaust bolt up configuration. I'm sure the resident experts will chime in if this is incorrect. I went from stock Merc/GM to Dart heads and there was no issue. The only item I noticed was there were two bolt hole locations for the intake manifold that weren't as deep as the stock ones which necessitated 'slightly' shorter bolts.

Although getting 500 hp out of a 383 is possible the amount of money spent for those specific small block parts may justify a big block build. If you want to try 500 hp in a 383 just for kicks that too would be a fun exercise. Depending on your existing EFI system you may be able to tune it your self or have one of the locals here tune it assuming it isn't the newer 555 system. You might exceed the rpm/flow capabilities of the stock inductions system/throttle body to get the RPMs required for a 500 hp stroker. An aftermarket system might in this case better suit your plan.

As far as stretching out the build over two seasons, I'm assuming you mean build one motor, install and run it as a mixed set one season, then do the remaining motor. While it could be done I think the mixed combo would be awkward at best. Prop mixing, load sharing, handling characteristics and other unknowns may make boating in this manner less than desirable.

Good luck and let us know how things go.

http://www.mercruiserparts.com/400_scorpion.htm


Run for another season or two, save your money and get two of these. Then sell you 350s, it will be cheaper and more reliable in the long run.

Here's the thing... The "tinker factor", or "built it up myself" thing is in play here. I like the drop in option. I did consider two LS7's (used), but again, there's no polished goodies in there, etc. Very easy way to get over 1000 HP with two of those.

I think i'll do the header route for now. Thats still $4600, and without piping through the transom!

Doing the lower ends over next winter is doable (probably another $6000), then i can follow up with the top end including trick mpi system from someone like FAST.

It wont be cheap, but it will be over the coarse of 3 or 4 years, which does fit my budget (there's the props too!)!

If you go with CMI's make sure you tell them which heads you are using as some heads have the spark plug rotated up closer to the center of the piston . If this is the case they cut out that area of the flange so you can get your plugs in . The Trick Flow Twisted Wedge heads i used on my 383 were like that . You'll love the power of twin strokers and 500hp can be achieved . Make sure you get the dry tailpipes with your headers or you will limit cam choice and there goes your 500 hp .

There may be better choices than the FAST system for EFI. I know the Holley systems have been well supported and built for marine applications.

Hmm... Interesting on the EFI... Also, i've been reading horror stories about "straight pipes" and decibel levels. I've been around drag racing enough, but my boat will have to be somewhat quiet!

Surely CMI's baffles work like header baffles on bracket cars! -but don't restrict HP? If not, i would think theres a huge market share here for someone to take advantage!

My short term plans got a little delayed... Looks like I tore up a drive coupler over the weekend! I'm going to try sliding the engine forward to replace it. Of course the drive will have to come off for proper engine alignment. I dont have the means to pull the engine properly... I would think a set of blocks built from 4x4's and 2x10's that then sit atop the stringers will woek just fine. Anyone tried this before?

Where do you boat? salt or fresh? What kind of speeds are you seeing realistically (honest) now from your Donzi and what speeds realistically are you looking for from it?
This will determine where you go from here, how much money you need to spend and what you need to do.

BE REALISTIC-WORK OUT THE NEEDS- SET UP THE BUDGET AND FUNDS-SET A PROJECT TIMELINE-ENGINEER THE EXACT CHANGES NEEDED- DO THE UPGRADES WITHIN THE PLAN !

This is how you should approach this type of project, DO NOT USE THE SHOTGUN APPROACH!

In other words don't work on something like this as "I might do a little of this, then I might do a little of that" You'll frustrate yourself, probably make weak decisions, achieve weak results and ultimatly spend to much money and time and never really get where you want to be with your boat.
"Plan your work-work your Plan!"



Best Regards,
Ray @ Raylar

Raylar - thanks for your response (and thanks to everyone else for your suggestions). I guess that's why I'm poking around here, asking for advice or what others have tried with SBC's. Seems like the boating world is exponentially more expensive than cars, and I follow what you suggest.

we made basically a giant sawhorse to drop a 454 short block into a sportfish. with a perf boat, you should be able to drop a 4 by 4 or 2 across the top of the gunwhales. cut a couple of 2 by 4's and wedge them betweem bottom side of gunnell and the deck to brace it up. then you can lift it with a 30 buck come-along from harbor freight or lowes.