4.3 to 5.7

Discussion in 'Powerboats' started by easyc, Nov 16, 2002.

  1. easyc
    Joined: Nov 2002
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    easyc New Member

    I'm currently considering swapping out a 4.3 for a 5.7. A local machanic said I couldn't do it, something about the gearing in the outdrive would blow the engine. My boat is a Renken 2050 with an OMC outdrive. I did the search and found nothing. Any input would be appreciated.
     
  2. curtis73
    Joined: Mar 2002
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    curtis73 Junior Member

    It should be do-able. I'm not a boat expert by far, but I do know autos. I keep cruising the forums waiting for someone to need my pathetic amount of knowledge.

    As far as the swap to a 5.7, it should be easy provided there is enough room. The 4.3 is a SBC with two cylinders missing so the 5.7 adds approximately 5" fore of the 4.3. The motor mounts should bolt up as-is. I know that marine mounts are different, but the blocks share common bolt holes.

    Here's where I get hazy on boat facts, but your 4.3 probably revs really low unless its a modified screamer. A 5.7 would most likely be geared very similarly, and both engines have a very similar powerband in stock marine form. A 5.7 revving at 4000 rpms is no less reliable than a 4.3 revving at 4000 rpms. (actually the 5.7 will most likely last longer because of the offset cranks found in the 4.3)

    Your mechanic is most likely referring to the fact that a 200 hp 5.7 will spin the same gears and prop much faster than a 150 hp 4.3. Someone else can maybe help here, but couldn't the difference be made up with prop selection? It may make it a little less than optimal acceleration, but I might experiment with a larger diameter prop with a couple more inches of pitch???

    Even if that fails, wouldn't there be a fine selection of lower unit ratios offered for that drive?
     
  3. yipster
    Joined: Oct 2002
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    yipster designer

    easy…… ?

    apart from whats beeing said here some more guesses.. by refitting you might need lower/higher motor mounts that are for mercruiser off the shelf available but the transoms weight will probably increase on your 2050 so much that you also need exhaust raisers.. a solution than might be adding a buoyant solid swim platform as frankyfranky somewhere described on this forum…

    yipster
     
  4. yipster
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    yipster designer

    forgot to mention that by using raisers the engine hatch might become squeezed, but yes, think it can be done
     
  5. easyc
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    easyc New Member

    thanks for the info. I had the same idea on the prop., I found a gear set for $500. can't decide which to try first.
     
  6. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    I have swapped many engines and what your mechanic said about gear ratios is correct. The 4.3 has a ratio of 1.65:1 and the 5.7 a 1.50:1. It is possible to compensate with a higher pitch propeller but the efficiency suffers. You won't need different risers, the weight difference is not so excesive. The motor mount position is further forward; so you need to check how the stringer are built. It may need some wood and fiberglass work. All the front brackets, pumps, pulleys, etc. are identical. The ideal solution is to re-gear your drive to the correct ratio. Any marine mechanic ( and yours should have offered) will be able to do the job. Also you could trade in yours for a rebuilt or used unit.
     
  7. Guest

    Guest Guest

    Engine conversion

    Wow - looks like a goldmine of info. Since you guys seem to know your engines, I've been thinking of a swap - 3.0L to a 4.3L. I heard the bellhousing will bolt right up. If that's the case, the swap wouldn't be too bad, especially since it uses the same outdrive. Can anyone comment on this?
    thanx!
     
  8. gonzo
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    gonzo Senior Member

    The 3.0 has a gear ratio of 1.85 or 1.95:1 . That means you need to regear or swap upper units in your drive. Also, the four cylinder has an exhaust collector with one side only pipe. It needs to be changed to a double pipe. They have the same bolt pattern. The 4.3 has mounts on the sides instead of forward. Check your stringers for engine mount pads; they may need to be built. I have lots of new, rebuilt and used stuff. Let me know if you need to buy anything.
     
  9. tdno11
    Joined: Apr 2015
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    tdno11 New Member

    I was reading and I see on here that gonzo says that going from 4.3 to 5.7 has different motor mount positions. What about going from the 4.3 to 5.0? Are the motor mount positions similar or would I need to do fiberglass and wood work?

    Thanks
     
  10. rasorinc
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    rasorinc Senior Member

    A 5.0 should match up to a 5.7. Same block V-8, 4 cylinders each side. A 4.3 is a V-6 and won't match up to the others
     
  11. Eagle1
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    Eagle1 Junior Member

    So I join the forum cause I see some interesting point here. So I'm currently changing my volvo penta 3.0 to a small block chevy marine crate engine I bought from summit racing, so so I'm outfitting it with volvo penta accessories. My 3.0 is front mounted and I'm planning to make side mounts for this new application. So you guys are saying my 3.0 won't work?
     
  12. gonzo
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    gonzo Senior Member

    The rear mounts are the same. The front mounts are different. You will need to build up the stringers to make mounts for the sides. You will also need to change the gear ratio as you are increasing the HP.
     
  13. Eagle1
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    Eagle1 Junior Member

    Yea so how do I change the gear ratio?
     
  14. gonzo
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    gonzo Senior Member


  15. Eagle1
    Joined: Dec 2022
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    Eagle1 Junior Member

    Like a higher pitch prop couldn't compensate that instead of changing the gear unit?
     
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