Engine conversions

Designs for inboard or outboard power

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sbreslin
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Engine conversions

Postby sbreslin » Tue May 01, 2007 5:42 pm

I have a 350 chevy in my garage that needs a boat, I have seen an 18 foot ski boat for sale that needs a new motor, 4.3L V6

I was thinking of getting the boat and converting it to my 5.7L. I think it's a Volvo/Penta I/O drive, would I have to upgrade the drive or are all I/O drives the same for V8's and V6's

any other things I should know doing a swap like that?

Thanks for any info on the topic
Sean

Brian Eager
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Postby Brian Eager » Tue May 01, 2007 6:20 pm

First question: is the 350 small block a marine engine? Or just leftover from the automotive world?

Brian
Noah was a first-time boatbuilder

sbreslin
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Postby sbreslin » Tue May 01, 2007 6:53 pm

It's a motor I built up for a T-Bucket that never materialized, though I have some marine parts like water cooled exhausts and a cam driven water pump.

Brian Eager
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Postby Brian Eager » Tue May 01, 2007 7:41 pm

I'd suggest Inboard Motor Installations available from Glen-L as a reference book. You want to keep safety uppermost in the process.

Best wishes with your project!

Brian
Noah was a first-time boatbuilder

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kens
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Postby kens » Tue May 01, 2007 7:44 pm

Probably a little difference in gear ratios on V6 vs V8 outdrive. A good manual/book on the outdrive could determine that. For sure you would get a different prop for the V8.

John K
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Postby John K » Wed May 02, 2007 5:01 am

There is a lot of differences between a marine and automotive block. The ones that I know are the starter, fuel pump (if electric), and alternator have to have spark arrestors. The block is also differernt, I think in regards to water cooling, but not sure.

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narduccimarine
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Postby narduccimarine » Wed May 02, 2007 6:22 am

Actually the marine and automotive blocks are identical. Obviously the electrical and fuel components are different. The fuel pump is a prime example, automotive pumps have a vent hole where it will spew fuel on the ground if the diapraghm ruptures. In a boat this is disaster so they pipe the vent back to the carb. The only internal differences are the cam and pistons. Hypereutective pistons are normally used in a marine engine. The cam is designed to work better with the rpm range used in a boat
All in all though, an auto engine would work fine in a boat as long as the safety issues are taken care of, namely the elect and fuel systems.

A 350 will bolt right up to the outdrive that the 4.3 fits, also uses the same motor mounts. it just adds about 5 inches to the front.
Even the smallest outdive made is used for the v6 and v8, only the ratio is different and you can get around that by changing props.
My 3 cents
I just don't understand, I cut that plank twice and its still too short
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Caber-Feidh
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Postby Caber-Feidh » Tue May 08, 2007 12:18 pm

Biggest dif with a "marine" cam is the spacing, the lobes are kept closer together. The farther apart the lobes, the more high RPM Hp, but reduced torque on the bottom. Marine cams try to keep a flat-long torque curve. That great hotrod cam that barely idles is going to be useless in your boat.

Narduccimarine is 100% correct, The drive is the same. there are no different gears in the lower unit itself.

Drives setup for Volvo built I4 and PRV engines are going to need a few new pieces to connect it to the GM motor.

Dwain the ski king
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Postby Dwain the ski king » Tue May 15, 2007 10:35 pm

As a matter of fact, the marine cams are to medium torque cams(in a chevy) for their medium performance engines. For the old 300hp 327 it was the 390 grind(.390 lift-195 degrees duration @.50) same for the 300hp 350, b-4 '71. My 185 hp 283" Chris-Craft motor used a solid lifter Duntov cam from the 283 Corvette motor. My cam grinder found a "stock" hydraulic grind that had the same specs as my cam. So I wouldn't have to set valve lash(messy job) in a boat with a hot engine. The older motors put a lot of oil to the valve gear, and would actually squirt oil out of the puchrod/rocker arms about 4 feet, at a hot idle(having adjusted the valves on many Duntov cammed Corvettes). Comp Cams 252H or 262H would be a great cam for a SBC in a boat. They make good torque from about 2500 to 5500 rpm. The 252H would be my choice and will run with stock lifters, valves and springs....(206 duration at .050 and .430 lift. ) With flatop pistons and the right 4 bbl. manifold ( 600 CFM Holly or Edelbrock carb)will make about 360 hp. Ran that setup in a 72 4X4 for years and was a good puller....Smooth idle, run up 5500 rpm. When you go much past 210 degrees duration you start to lose low-end torque. Most boats want to start pullin' from about 2000 r's, and the 252 will do that The 262H is either 212 or 214 degrees and .050 so is borderline, but will pull to 6 grand still with stock valves lifters and Z28 springs....OK, all you hotrodders can jump on me now.....SNIcker, snort....Dwain, the Ski King-wheeler dealer-fanny feeler..........

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Postby ET » Wed May 16, 2007 7:55 am

For small block chevy's.......

The blocks/casting are identicle. Heads are identical. Oil pans are a little different in some applications. Cranks shaft flanges on automtive aplications do not always mate up with the proper flywheel used in some marine applications. I used to think that all small block Chevy crank bolt patterns where the same but found that this is not true when watching a local marina run in to this problem when replacing an engine with a crate auto engine with a mercruiser out drive.

In addition to optimal camshaft profiles for ensuring proper power band and RPM efficiency, there are other differences such as stainless steel head gaskets, and brass freeze plugs.

The water pump is also different according to an article that I read-an explaination that the 'pump' for the marine application is truly a water pump, where for the automtove application it does not actually pump, but circulates that water. The difference? I don't know, they look identical, but there's a potential for some lengthy discussion. Therefore if you have a fresh water cooling system do you use the automative style of pump?

For safety - the carberators are not vented as the automotive versions. Distributers, starters and alternators are arcless or spark proof.

The load on a boat engine has been likened to the load on a truck pulling a trailer up hill continually. It's my understanding that marine engines therefore should use the forged pistions and forged steel crankshafts such as the ones used in the heavy duty autometive applications. I wouldnt bet that all marine manufactures or re-manufacturers do this though.

This is complile info from a book I haev on marine engines and experience, but I'm sure open to other ideas.

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narduccimarine
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Postby narduccimarine » Wed May 16, 2007 10:53 am

The difference in the water pumps are a bronze impeller and stainless backplate on the marine unit. The engine water pump on a marine engine is still just a circulating pump. It is considered a non-positive displacement pump, it would not be able to pump water from outside the boat on its own. That is why you need a raw water pump that is positive displacement. On inboard engines and bravo sterndrives, this pump is mounted on the engine and belt driven. On the alpha sterndrives, it is mounted in the lower unit .
Most, if not all mercruiser engines use a cast crank. I thought all marine engines used a forged crank and 4 bolt block but I found out I was wrong!
I think only the very high performance engines used the forged assy.
Probably the 496 but Im not sure
I just don't understand, I cut that plank twice and its still too short

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Caber-Feidh
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Postby Caber-Feidh » Wed May 16, 2007 7:12 pm

The bolt patterns are only different if someone has the incorrect engine series. If the drive is setup with a flywheel for a 1-piece rear seal, it will not match a 2-piece engine. 1-piece rear seals must be used with a counter-weighted flex-plate/flywheel. (Also, oil pans are not interchangeable) Both styles are used in marine outdrives. As a general rule, 1986-present line-production engines are 1 piece, the "target master" and most "Hencho en Mexico" crates are 2 piece regardless of year.

(disclaimer... there are some Mexican motors with 1 piece, check before ordering parts!)

It has been against federal law to vent any carb to the atmosphere since 1974. ANY carb made for automotive use in the USA after that will vent only into the airhorn.

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kens
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Postby kens » Thu May 17, 2007 1:45 am

Caber-Feidh wrote:It has been against federal law to vent any carb to the atmosphere since 1974. ANY carb made for automotive use in the USA after that will vent only into the airhorn.


So does that mean that ANY post 1974 auto carb is a marine carb to satisfy the EPA on the streets?
Maybe that is why the reference to my Rochester Mercruiser carb did cross-reference to some Buicks, Chevys and such. I never could figure that out.

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Caber-Feidh
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Postby Caber-Feidh » Thu May 17, 2007 9:45 am

Not all EPA accepted carbs are 183 compliant, just that the bowls don't vent to the atmosphere outside the airhorn.

The butterfly shafts are different (to prevent puddled fuel from leaking).

Here is the USCG fuel system requirements:
http://a257.g.akamaitech.net/7/257/2422 ... 83.520.pdf

I know allot of the Rodchesters and motorcrafts are USCG 183 compliant.


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