Marinizing a GM 350

Discussion in 'Gas Engines' started by JohnnyDude, Nov 27, 2014.

  1. JohnnyDude
    Joined: Nov 2014
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    Location: Orland, FL.

    JohnnyDude Junior Member

    I am currenty in the process of what I think is marinizing an automotive engine. I've been reading up on this subject for a couple of years. I have a 1998 Chaparral 205. It came with a 4.3 and before I ever took it out I expected it to perform better than it did. Cruising is O.K., but I like to wakeboard or even slalom ski every now and then. I never suspected the 4.3 was in too bad of shape. It tended to perform worse each time I took it out. The last time was real bad. Once I got home I ran a compression check. It was all over the place, 0-130. Could have been just valves, but I couldn't see any point in dumping any money into a little V6. I found a complete 350 on craigslist for $1000. 1996 vortec block with pre-vortec heads, edlebrock performer intake with a Holley 600. Engine had low miles and real good compression. I originally assumed I could just swap out the engines and be done.
    Well that's what I was going to do until I talked to a few people in the business. I was so confused from all the B.S. on the internet. Some posts said GM marine engines come from GM with different cranks, pistons, ect. I looked up specs on both, couldn't find any major differences. I found nothing that said what cam or if they came with stainless steel head gaskets or not. I contacted Summit racing to suggest a cam. This engine has hydraulic roller lifters, which should be standard for marine engines. I sent heads out to have stainless steel valves, seats, and viton seals. This isn't necessary, but well worth it. Second most common failure in marine engines is valves, 1st would most likely be exhaust manifold unless you're wealthy enough for stainless steel exhaust. Don't forget water pump. They look the same on the outside, but the inside is different. Also I found that my accessory brackets won't work with the automotive water pump. One important note: If you plan on running a closed cooling system, you could probably get away without stainless steel head gaskets and marine water pump. I recommend a marine carb with spark arrestor, marine starter and marine alternator, which I already have. If I had to do it all over again, I would have opted for a marine shortblock. But this way is more interesting. plus I know exactly what I have. I will let you know how it works.
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Do you have the marine flywheel? It should be matched to the harmonic balancer. The fuel pump has to be marine and have the proper pipe to the carburetor; no cheap automotive hose. The carburetor must have the overflow J-pipe so the fuel falls into the throat and not outside.
     
  3. WestVanHan
    Joined: Aug 2009
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    WestVanHan Not a Senior Member

    And what about the drive?
    They're rated for a certain maximum torque,overloading them will reduce their life in several ways.
    Too much overloading and there will be pieces of shrapnel in and sometimes outside the case
     
  4. PAR
    Joined: Nov 2003
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    Location: Eustis, FL

    PAR Yacht Designer/Builder

    To bad about trying to rebuild the '86 - '95 heads as they're not very good. The iron Vortec heads are way better and can be had from the junk yard pretty cheap. You'll want the Vortec truck heads, found in all full size after '96, before they switched to the LS architecture. They flow better than '86 - '95 aftermarket heads (aluminum or iron), right out of the box.

    The Vortec should be a roller cam. Switching out the stock rockers with aftermarket rollers, will give a 15 HP bump on a stock 250 Vortec and it'll rev up much better too, plus keep the valves a bit cooler. They also do well with porting, polishing and flow straightening, though serious grinding needs to be done by someone that's capable of matching flow, for the performance envelop you desire.

    There are several items to marinize this engine and most of this can't be upgraded from automotive units, particularly fuel delivery. There are several cam grinds available for this engine, ranging from 210 to huge HP. You can easily ask 300 HP with the stock pieces, though polylock the stock rockers, if you do toss this much as it. Anything more and you'll want to consider significant upgrades to it's inerds. Valves aren't necessary the weak link with this engine, assuming a clean and properly thermostated cooling system is installed.
     
  5. JohnnyDude
    Joined: Nov 2014
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    JohnnyDude Junior Member

    I have the fuel system from my V6 and it works great.
     
  6. JohnnyDude
    Joined: Nov 2014
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    Location: Orland, FL.

    JohnnyDude Junior Member

    I have the fuel system from my V6 and already tested it on the V8, works great.
     
  7. JohnnyDude
    Joined: Nov 2014
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    Location: Orland, FL.

    JohnnyDude Junior Member

    Not sure about the flywheel..I'm going to do some research.
     
  8. JohnnyDude
    Joined: Nov 2014
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    Location: Orland, FL.

    JohnnyDude Junior Member

    I've been talking with the guy that owns Hall's prop shop. He gave me a ton of good and helpful information. Not like most of the B.S. on the net. So many misinformed people.
     
  9. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    It will handle the torque, but will have to be re-geared from 1.65 to 1.50.
     
  10. JohnnyDude
    Joined: Nov 2014
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    Location: Orland, FL.

    JohnnyDude Junior Member

    I kind of agree with you. That was a big concern of mine when I first started this project. Mike at Hall's said once I get the engine in and finally put her in the water to watch my rpms to speed ratio compared to how it used to run. Especially on the bottom and top ends. If it seems like I have too many rpms, he will take a look at my prop to see what pitch it is and make necessary adjustments.
     
  11. JohnnyDude
    Joined: Nov 2014
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    Location: Orland, FL.

    JohnnyDude Junior Member

    Do you live in Florida ? I used to know a guy that went by Gonzo.
     
  12. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    That is me. I am living in the frozen North now.
     

  13. sdowney717
    Joined: Nov 2010
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    Location: Newport News VA

    sdowney717 Senior Member

    I changed out a v6 to a chevy 350 v8 on an old Glastron with an old OMC stringer. I upgraded the upper gears to a v8 capacity. Had to cut space on the OMC bell housing to fit the starter.

    Still the engine over reved, too much power, not enough prop.
    Took prop to prop shop and they put a cup to the prop.

    Was perfect afterwards. Coming back more than once people were surprised how that boat flew, would get up to the upper 40's mph. One guy said he could barley keep up and he was used to passing all. And boy were they surprised when the saw how old (from the 60's ) it all was.
     
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