5.0 to 5.7 l mercruiser help?

Discussion in 'Powerboats' started by Automech, Mar 31, 2010.

  1. Automech
    Joined: Mar 2010
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    Location: florida

    Automech New Member

    I was thinking of switching out my 5.0l factory mercruiser for a 5.7 l mercrusier long block with 300 hp . ,
    I seen some info claiming to have better fuel mileage?

    Swapping the engine would be easy for me .
    However i was wondering if my outdrive can handle 300 hp It is an alpha 1 Gen 2 with a 1.62 ratio and factory prop. ?
    Also Is it true that i would save fuel?

    boat is 23 feet long and weighs 4500lbs with a deep v hull ..
  2. rasorinc
    Joined: Nov 2007
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    Location: OREGON

    rasorinc Senior Member

    Call a merc dealer on that as you want to be 100% right. I think the 1 was limited to 250 hp. If the new engine is fuel injected you will probably save.
  3. CDK
    Joined: Aug 2007
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    Location: Adriatic sea

    CDK retired engineer

    I've done exactly the same thing many years ago.
    A Carlson CV-23 with an Alpha-1.

    I treated the engine very gently during break-in, then changed oil and wanted to know what the engine was capable of at full throttle.
    The vertical shaft in the drive snapped almost immediately, the upper part milled around and ruined its bearing, seal and a piece of the upper gear case.

    Need I say more?
  4. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    If you are changing to a larger engine it will use more fuel at full throttle. That is why it produces more power.
  5. erikgreen
    Joined: Mar 2008
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    Location: MN, USA

    erikgreen Junior Member

    Very common

    This is the most common engine mod for older Mercruiser boats. It's pretty much drop-in provided you have the original 305 bolt-ons and mounts, and provided the engine you're installing has marine gaskets and parts.

    The original Alpha one topped out at about 300 horsepower.. but, there were actually a few 454 engines sold with Alpha Is before Mercruiser developed the Bravo series, and some of those are still running.

    The ultimate life of the sterndrive isn't so much a matter of engine size but a matter of how the owner treats it. Don't ram the throttle from 0 to full as fast as you can and an Alpha I can last a long time with a 454 pushing it. If you go to a 300 hp 5.7 you'll have to take the same care.

    I have done this upgrade myself and it works well.. my pre-alpha drive handles the 350's power just fine, even if I go wide open to get on plane. My engine develops a bit more power than a stock carbed 350 due to a hot cam, FYI.

    You won't get better fuel mileage, you'll get worse. My 350 uses about 20 gallons per hour wide open. If using less fuel is important, put in a V6 engine with vortec heads and a good carb... you'll get about the same power with a bit less fuel. I wouldn't go any smaller than that on a boat that size.


  6. Angelberto
    Joined: May 2020
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    Location: Aruba

    Angelberto New Member

    Hi, how are you doing. I have a 5.0l mercruiser mfi. I'd like to buy a 350 5.7l engine, with carburetor. I'm asking if I can put the fuel inject system of the 5.0l on the 350 5.7l . And reprogram the computer to 5.7l . It is possible?? And what about the crank sensor. Thanks for your tips.
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