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Old 03-28-2008, 01:28 PM   #1
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Default SBC Boat Motor

Hey does anyone have experience building marine SBC's. All I want is like a 350 that makes somewhere in the 350 horse range that would be reliable in a boat and be somewhat fuel friendly.

What I want to know is what parts should be stronger, and the biggest question I would have would be the cam cause I want it to make good low end power to pull skiers up.

Thanks

Michael
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Old 03-28-2008, 02:39 PM   #2
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Is this going in a jet boat or a prop boat? What will be your desired rpm? For instance my jet boats impeller is most efficient at 4000-4300 rpm...anything much above that just cavitates.

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Old 03-28-2008, 03:14 PM   #3
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Quote:
Originally Posted by MikeMoss View Post
Hey does anyone have experience building marine SBC's. All I want is like a 350 that makes somewhere in the 350 horse range that would be reliable in a boat and be somewhat fuel friendly.

What I want to know is what parts should be stronger, and the biggest question I would have would be the cam cause I want it to make good low end power to pull skiers up.

Thanks

Michael
Here's a link to the one I just built for my Challenger. It's geared for max performance. http://www.ls1tech.com/forums/showthread.php?t=887661

Give me a rundown of the parts you have..... heads, exhaust, intake, carb, etc.... and I'll give you a few options. Also what type of boat? Drive?

I've ran 5 different SBC's in my boat and I can give you insight on how they all performed.
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Old 03-29-2008, 03:38 PM   #4
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where and how do you plan on running the boat? Do you have an existing 350?
if it's in salt water and raw water cooled you'll probably want to stick with all iron parts. I don't know how well an aluminum intake will last having salt water run through it. But if you can do fresh water cooling then that'll be better in the long run.
The only real things on a boat motor that's different is piston ring gap, you'll want a little larger gap to account for the extra constant load that's put on the motor. That ought to be specified by the ring manufacturer if you're buying a set of rings for a marine 350. And sometimes the exhaust valves are inconel as opposed to the standard 21-4n, depends on how much you want to spend on valves for peace of mind. And theres brass core plugs instead of steel- only matters if it's raw water cooled.
other than that, from readings the best 350 build is typically plain jane vortec heads from gmpp for $400, maybe upgrade the stock valvetrain to roller rockers and just use a mild cam and good springs something in the 220/232 or less range, 600 or 750 cfm holley 4160, and a simple billet electronic distributor from pertronix, mallory, or msd.
And if you're not on a tight budget then a 383 would be the best route and offer the most power, but I'm not aware of the best cylinder heads for a 383. the iron vortec is about the best for a simple 350. The 383 is just a stroked 350 in case you don't know, the 350 has a 3.5" stroke the 383 is a different crank that's a 3.75" stroke.
asking for power and economy in a boat motor is a contradiction in terms I hope you know- about the only possibility is running fuel injection but you'll never recoup the cost of that in fuel savings and probably have more tuning problems on top of it all.

also meant to say, your biggest decision will be on cam selection. you could just go with the stock cam merc uses in the current generation 350, i'll have to look it up.
If it's a small boat or planes easily then it won't matter as much because you'll have enough tq but if it's a larger boat like 22' and a v hull then cam selection will be more important, you'll want less duration and a sooner closing intake so cylinder pressure is higher at lower rpms.
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Old 03-30-2008, 03:53 PM   #5
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Its just a all purpose 19' Larson v hull with an alpha II stern drive. right now It has a 305 with a 2 barrel. I am not wanting a super high performance motor just something stronger then the little 305. I doubt if I would ever want to spin it much higher then 5000.
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Old 03-30-2008, 03:55 PM   #6
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It will never see salt water.
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Old 03-30-2008, 05:28 PM   #7
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Here's a good link with info....... http://www.speedwake.com/upload/show...threadid=52583

http://www.mercruiserparts.com/383_Magnum.htm This is what many replace tired 305 and 350's with. With fuel injection and a Mercruiser warranty its a good deal on a turn key engine.

Getting 350hp from a 350 is very easy. You should easily be able to use your stock manifolds and risers. Get a Edelbrock Performer intake and Holley 650DP carb. A set of 1.5 roller rockers along with a set of Vortec cylinder heads. Gear your cam selection to the heads valvespring capability. If you are wanting to stay with a flat tappet cam then a good upgrade to a Merc 350 is this cam....http://www.jegs.com/i/Comp+Cams/249/...0002/-1/747427
It's a Comp Marine cam that specs out at 212/218 .447/.462 112LSA
This cam should meet your requirements easily and provide just what you are looking for and the stock springs on the Vortec heads should easily handle it.

A stock Merc 350 flat tappet cam specs out a 200/212 .400/.410 110LSA 108ICL.

If you are wanting to go with a roller cam then let me know and I can give you a recommendation for one.. Hope this helps..
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Old 03-30-2008, 07:17 PM   #8
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Just to make sure will all of my accessories, exhaust manifolds, and oil pan off of the 1991 305 bolt directly to a newer block and vortech heads? Also will the stern drive bolt on? I dont really know all of the differences between different years on sbc's. And I can use a car shortblock without having issues correct.

Thanks for those links they provide a lot of good info.

How much more would hydraulic roller valvetrain cost?

Is a place like summit a good place to get all of the marine parts?

Thanks
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Old 03-30-2008, 07:32 PM   #9
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can you put a carb on this intake?

Click the image to open in full size.

It says it is a factory intake manifold off of a crate vortec.

And do vortec blocks still use a flat tappet cam?
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Old 03-30-2008, 10:02 PM   #10
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Quote:
Originally Posted by MikeMoss View Post
Just to make sure will all of my accessories, exhaust manifolds, and oil pan off of the 1991 305 bolt directly to a newer block and vortech heads? Also will the stern drive bolt on? I dont really know all of the differences between different years on sbc's. And I can use a car shortblock without having issues correct.

Thanks for those links they provide a lot of good info.

How much more would hydraulic roller valvetrain cost?

Is a place like summit a good place to get all of the marine parts?

Thanks
Accessories and oil pan interchanging is a question I cannot answer for certain. I would imagine the powersteering and alternator being the same for a 305 and 350. Seeing that you have a 91 motor you probably have the 14" flywheel and starter combo. Pre 87's had a 12.75" flywheel and starter combo and will not interchange. You will need the 14" setup going to a pot 87 1 piece seal 350 block. Your drive should bolt up fine.

Merc gets their blocks right from GM. A reg 350 block will be fine, you just need brass core plugs installed vs automotive steel.

Post 87 350 blocks should allready be setup for hyd roller lifters. Starting from stock that is your best bet. Starting with a pre 87 block you will need retrofit roller lifters and they start around $350.

Get an 87+ 350 block, 2 bolt is fine for your power level.
Stock crank, rods, pistons are fine for your setup. Have a reputable machine shop do the block. Fresh bore, hone, check line hone, new cam bearings, assemble the block. Get a set of Comp roller lifters, roller rockers, pushrods, Vortec heads, Performer intake, 650 mech sec carb, and you should be set.
The stock Vortec heads valvesprings can only support .470" valve lift before they hit the valve stem seal. You need to have them machined and better valvesprings installed. A good cam choice is the Comp Extreme Marine
XM264HR 212/218 .488/.495 112LSA 110ICL

That intake will work with a carb. Yes Vortec blocks and all 87+ blocks should be OEM hyd roller equipped. Notice I say should. There have been people to get early 87 blocks that were not roller equipped.
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Old 03-31-2008, 12:53 PM   #11
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Quote:
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The stock Vortec heads valvesprings can only support .470" valve lift before they hit the valve stem seal. You need to have them machined and better valvesprings installed. A good cam choice is the Comp Extreme Marine
XM264HR 212/218 .488/.495 112LSA 110ICL
When you say "you should have them machined" do you mean the valve stem seal?
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Old 03-31-2008, 05:46 PM   #12
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the one decent thing merc has going for it is mercurypartsexpress.com
if you have your engine serial number and you're transom serial number you can punch that in and get specs for your current motor.

i don't know if the exhaust ports are different on the vortec head which would prevent you from using your current exhaust manifolds, so you might need new exhaust manifolds, ryan might know.

the vortec blocks I don't think allow for a mechanical fuel pump, so you would have to buy an electric fuel pump and wire that up, that's pretty easy.

basicpower.com is a good source for parts, I would get all you can first from summit though.

You can also call basicpower, they are halfway decent over the phone and will give you the info you need and probably prices. If you don't have a line on a 350 yet, check out basic's crate 350 prices, you might make out after you figure all the extra parts that won't transfer over from your 305. Add up 4bbl carb, exhaust manifolds especially if you're current ones are old, ignition, fuel pump, etc. I did a 351w from them that was ford marine factory new for $2800, been running great for about 15 years now.
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Old 03-31-2008, 05:46 PM
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