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crussell85 02-18-2013 09:33 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 10:53 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 04:41 AM

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

vinniekq2 02-19-2013 09:43 AM

wet or dry? power required?

crussell85 02-19-2013 10:18 AM

Its a 2001 Volvo penta with the sx out drive.

crussell85 02-19-2013 10:24 AM

Is this a bad out drive?

TMF Lindbeck 02-19-2013 12:03 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

crussell85 02-19-2013 01:10 PM

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

Richiehd 02-19-2013 05:14 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)

Hogg 02-21-2013 11:44 AM

Here is the Ramjet intake for Vortec heads.

Merc marine intake for Vortec head use. Used on 305/350/383 marine engines, I run this in my 1997 Vortec 350 reg cab shortbox 2wd truck, runs much better than teh stock truck intake, and allows me to run any sized injector I wish(unlike teh stock truck intake)

Same marine intake next to Vortec truck intake manifold

Same intake in Mercruiser red

different angle

Here is the same marine intake with the upperplenum removed. You can see how the runner eopenings are across from the cylinder head intake port it feeeds. the cross ram runners are about 10" long from the opening inside teh plenum to the intake gasket flange.

Inside of Vortec 305/350 truck intake. Lots of injection debris impeding flow.

Scorpion 400 intake, it uses the same tunnel ram lower that is mentioned elsewhere in this thread.


BadBehavior 02-21-2013 03:15 PM

1 Attachment(s)

Originally Posted by F-BIRD'88 (Post 1648087)
If you wnt to go fast and have fun on the water find a good used Hydrostream.
Google it.

Been reading this Post. I agree with F-Bird on all, Except this, to each his own of course, GO fast, Get one of these, put 6 cyl in present boat, sell it put funds into safer boat than the above "chine walker" I know, I owned 2 before I wised up! Sorry had to say it:embarrass

lg1969 02-22-2013 07:40 AM

Hey BadBehavior, That boat is sexy:drool: like a good women. :thumbup:

troutman2 02-22-2013 05:43 PM

We still build that 5.7 engine w/ the same intake as the mercruiser in which is a Bosch injection set-up.

bondo 02-23-2013 09:05 AM


Originally Posted by F-BIRD'88 (Post 1648019)

Originally Posted by crussell85
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.

Ayuh,.... I believe you'll find that the SX drives are cone-clutch drives, which are miles ahead of the Merc Alpha drives, more like the Bravos, but not quite as heavy duty...

It's a dependable drive up to 300 hp, 'n still dependable over that, if the operator has self-control over his righthand...
in other words, skip the WOT jackrabbit starts, 'n yank back on the throttle if by chance ya clear the water, 'n catch air on a wave...

'n,... The thru-prop ain't so much the problem, but the open plenum exhaust manifods are the choke point in the exhaust flows...
The vortex created by the prop, actually helps pull the gases out...


Originally Posted by F-BIRD'88 (Post 1648025)
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.

Only the Ratio changes, each drive is as strong as it's sisters....

'n, of course, a Worn out drive isn't dependable, regardless the amount of power applied...

There's No reason the OP shouldn't move forward with his Plan,...
383s are stuffed into boats, quite regularly...

If crussel would like a more Marine rated conversation 'bout this,...
Feel free to stop by iboats boatin' forums,...
There's many conversations there, 'bout this exact thing...

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