pjv911

My experience with boats has always been family sport cruisers so the performance aspect is new to me.
I am a Porsche tech by trade. So in short my knowledge of what works and what doesn't work is limited when it comes to performance boating.
I just picked up a 21' Donzi with a bad 330hp 7.4 motor (I will not be building this engine).
The base engine im building from (from my other boat) is a Gen 6 385hp magnum MPI 7.4.

Here is the info on what I currently have (engine is apart)
Cast iron rectangular port RHS aftermarket heads. 2.19 int and 1.88 exh valves and 320cc rectangular intake runners.
Cranks is GM forged # 14097044
Rods are GM forged w/ 7/16" bolts
Pistons are forged flat top (Mercruiser issue)
Camshaft is stock issue Mercruiser 385 magnum MPI roller.
Intake is High rise dual plane mercuiser aluminum with brass inserts
Carb is Edelbrock 750cfm marine
Exhaust is Stellings Aluminum manifolds with full dry pipes
Boat weighs 3400 lbs and has 1.5 ratio Bravo one
Sea water cooled (no antifreeze)

Questions
Should I buy a more aggressive camshaft?
Should I stay with the 8.6:1 cr
Any recommendations based on the build so far?

GPM

It all depends on what your performance goal is, how do you use the boat, how fast do you want to go.

phragle

iTrader: (3)

Im building similar.. 454 mag bottom end (fresh) 731 cam, Al's Promax alum heads and a 525sc blower setup. Should come out around 600 hp in a 22 foot boat.

adk61

iTrader: (5)

because crazy is what we do!!!

Baja Rooster

iTrader: (1)

How many hours on the donor motor? I'd say new cam with smaller chamber heads and have fun with it for the next ten years.

pjv911

Ive never used the boat. I bought it two weeks ago with a knocking bottom end caused by a bad oil cooler.
Previous owner said its weak out of the hole and tops out at 57mph on gps. To be 100% honest in my 30 years of boating I've never gone faster than mid 50's in a boat. A jetski yes but not a boat.
My 14,000 lb Searay 330 Sundancer goes 40-42mph topped out (cruise at 30) with twin 400hp Scorpion 383 mercs and that's not so exciting.
I would be happy with the Donzi's stock 330hp performance provided the seller was truthful but then again for how long will I be happy.
So I suppose if I can get slightly better holeshot (more torque) and maybe 60-62mph that would be a nice goal.
I will be enjoying this boat with my family and many times will take 20-75 mile trips so it needs to be reasonably reliable as well as safe.
I also understand this is a 21' boat so the is a line to draw as far as safe speeds are concerned. I just don't know where that line is.
I do appreciate any input you guys can offer.

pjv911

Donor motor is a factory mercruiser reman with a build date of 11/2001. Has 329 hours and its mint. Gunna do bearings and rings regardless.
Just need to know if I should go with a more aggressive cam and maybe go to 9:1cr. I did invest a few $ on the (used but mint) Stellings exhaust so taking advantage of it seems like a good idea.
Plus the RHS heads (believe it or not) came from Mercruiser as OEM on their reman. I spoke to a Mercruiser tech this week and he said the factory rectangular port heads (088 castings) had many issues and it was not uncommon for them to seek aftermarket parts when the GM cores were not in stock. I understand that a smaller combustion chamber will bump compression but the open chamber breaths better. Plus closed chamber heads with flat tops will yield far too much compression. Besides I already have the RHS heads why work backwards?

Boatally Insane

iTrader: (1)

I had 600hp in a 21'boat once... Spent more time in the air than on the water.. ...Hell of a lot of FUN tho'

GPM

Summit sells a nice Wisco piston for your app that comes with rings, reasonable price. The right cam, intake and carb would put you around 500hp.

pjv911

Which cam is the right cam (I'm hydraulic roller)? And what size dome on those Wiseco's?
Sticking with the intake and carb that I have (posted above) is the game plan.
500hp would be really nice!