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crussell85 02-18-2013 10:33 PM

Boat Engine 383
Working on getting a boat engine together. I have the rotating assembly for a 383. Scat 9000 crank, 5.7 rods, kb dished pistons with a d shaped cup 18cc. I have a set of L31 Vortecs sitting on the shelf but have been looking at the Gmpp small port bowtie heads. Chevrolet Performance Vortec Small Port Bowtie Cylinder Heads 25534421 - I am also looking at Chevrolet Performance LT4 Hot Cam Hydraulic Roller Camshafts 24502586 - for a cam that is suggested from a Marine Performance book that I have been reading. Not really sure what would have to be done to the stock vortecs to run that cam but if someone could throw me a link to a book or some good info. Another thought on a cam is Chevrolet Performance Hydraulic Roller Camshafts 14097395 - which I should be able to run with the stock vortecs. I'm don't have a target hp/tq number in mind I just want the boat to be fun! Great acceleration and good top speed. The boat is about 20 ft and weighs app 3,500 without passengers (typically four adults).

I'm not sure where the point of saying "it's pointless to go that much on a cam or heads because you still have the wet log manifolds which are staying.

I am also looking at an efi system from Marine: Affordable Fuel Injection these guys live about fifty miles from me so it will be nice for tuning and such. Just not sure if I want to go tbi or mpfi. I've heard tbi makes more torque? But mpfi burns more efficiently and will allow the engine to run smoother and last longer?

I know this isn't a boating forum but it's my favorite forum


vinniekq2 02-18-2013 11:53 PM

what type of exhaust manifolds are you using.some cams do not work well with wet manifolds. What rpm are you planning to run the engine at most frequently. How much horse power do you need to get the speed you require. or what RPM is your prop most efficient at? are you concerned about fuel burn or power ? I will likely try to talk you out of the vortec heads.
building a good/reliable boat engine costs more than a car engine

crussell85 02-19-2013 05:41 AM

Running the stock manifolds. The vortecs came stock on the 350 engine. Max rpm is 5000

vinniekq2 02-19-2013 10:43 AM

wet or dry? power required?

F-BIRD'88 02-19-2013 10:51 AM

If you have the good hi perf Mercruiser Magnum center riser marine exhaust you can build a real fun
Custom version of the Mercury Black Scorpion 6.2L Ski boat motor that makes even more power
and costs a lot less.

Use the complete Mercury Scorpion EFI system (looks alot like a tunnel ram) or the GM Ram Jet EFI system
9.5:1cr 92 octane unleaded
Use a Crane HR-284-2S-12 IG hyd roller cam or the GMPP LT4 hot cam (the hot cam uses 1.6 rockers for .525" net lift)
Use the GMPP bowtie vortec heads

You need thru the hull open exhaust or the "Silent Choice" exhaust as the thru the prop exhaust routing
will really limit this motors power output.
Will not run on 87 octane gas.

The GMPP ramjet EFI system is not coast guard approved
as far as I know.

These are both custom build motors so you need to set up the EFI on the dyno.
thru the Mercruiser exhaust.
But it will GLH.

Port match the Mercruiser exhaust manifolds to the Bow tie vortec heads. The ex port exit shape is different.
just make it match. Not that hard.
You can clean up the Bow tie vortec heads too to good effect from out of the box form for more power.
They don't need a lot of rework.

Fine on a Alpha Drive that is in good condition but you will need to custom prop it.
A radar gun and a few borrowed different pitch props to try will show you what the boat needs.

Not designed for 87 octane gas. 92+ octane is best.

This custom build Black Scorpion 383 revs higher than 5000rpm. When you need to go fast just open it up.

crussell85 02-19-2013 11:18 AM

Its a 2001 Volvo penta with the sx out drive.

F-BIRD'88 02-19-2013 11:22 AM


Originally Posted by crussell85 (Post 1648012)
Its a 2001 Volvo penta with the sx out drive.

sorry about your luck.
You need a 1.5:1 ratio drive that can take some power.
Not real sure how a SX drive will stand up. The thru the drive exhaust will kill the power any way.
This is a high perf motor. Needs a hi perf drive and exhaust system (thru the hull or silent choice or Marine water jacketed headers)

crussell85 02-19-2013 11:24 AM

Is this a bad out drive?

F-BIRD'88 02-19-2013 11:37 AM


Originally Posted by crussell85 (Post 1648015)
Is this a bad out drive?

I'm not real up on this Penta drives. I am of the opinion they are light duty.
This hi perf 383 motor build recipe I laid out is no wimp.

Hot rodding a boat is not the same as hot rodding a car.
It is never cheap.

A Mercruiser Alpha drive (V8 {1.5:1 ratio} in good condition is the minimum.
Mercruiser will tell you you need a Bravo Drive for this engine.

Any all V6 engine drives will not work.
The stock default thru the drive exhaust routing will not work.

F-BIRD'88 02-19-2013 11:57 AM

If the SX drive on your boat is not in excellent condition do not hot rod the engine.
A worn drive will not last behind a powerfull engine. A 4 cyl V6 drive or drive for a 305 is light duty and usually the wrong internal ratio.

If this is a light duty drive just enjoy your boat for what it is.
Or it will soon become a lawn/driveway ornament.

TMF Lindbeck 02-19-2013 01:03 PM

There are many calculators on the Internet to figure your hull speed and the hp required to go a given speed in knots I would google for a hull speed calculator. I've found on my 16 ton 42" dive boat the first 7 knots ill need 60 odd hp but if I would want to go a additional 4 knots at 10 kt ill need a additional 400 hp. My leingth at the waterline and tonnage Re holding me back. For your little 20 ft'er you can really get going with little hp but you'll find a point whare it takes more than 100 extra hp to just gain a knot or 2. Basically what I'm saying is you can build a 350 hp engine and get a max speed of 8.5 knots or a 900 engine and go 12 knots in my case. What would be the better bang for the buck? Easy choice for me was a cat 3208 as opposed to a MAN with triple he hp and only going marginally faster

F-BIRD'88 02-19-2013 02:03 PM

Do you know how to make a boater into a Millionaire?

Find a Billionaire and give him a power boat, and wait... :)

crussell85 02-19-2013 02:10 PM

So... ... maybe just look into the 7395 cam and the stock vortec heads?

F-BIRD'88 02-19-2013 02:30 PM

If you wnt to go fast and have fun on the water find a good used Hydrostream.
Google it.

Richiehd 02-19-2013 06:14 PM


Originally Posted by crussell85 (Post 1648077)
So... ... maybe just look into the 7395 cam and the stock vortec heads?

In my opinion that 7395 has too much duration for wet exhaust. You may end up with water reversion, and a no start in the need to pick a marine performance cam or speak to someone that knows marine camshaft design. (Mike Jones Cam design, Denver NC)

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