Outboard conversion

Discussion in 'Outboards' started by pwillie, Apr 21, 2016.

  1. pwillie
    Joined: Jul 2014
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    pwillie Junior Member

    what is the outboard conversion from 2 stroke to 4 stroke?...Will a 90 4 stroke pseform like a 110 2 stroke?
     
  2. upchurchmr
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    upchurchmr Senior Member

    Isn't 90 horsepower less than 110 horsepower?
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    It certainly will use a lot less fuel, but with the less advertised horses and greater engine weight, you are going to lose some speed and acceleration. Might be detrimental for pulling skiers etc, but for normal cruising, unless the boat is already marginally powered, should not be too much of a problem.
     
  4. PAR
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    PAR Yacht Designer/Builder

    Yeah, HP is HP, though a modest weight penalty in 4 strokes, they'll still produce the bollard pull they're rated for.

    More directly, if you have a 1 ton boat with a 90 HP outboard, it'll run in the 37 MPH range, given a reasonably shaped hull form. The same hull with a 110 HP outboard will get to about 40 MPH.
     
  5. pwillie
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    pwillie Junior Member

    I read some place that the new 4 stroke engines weigh less than some of the old 2 stroke engines...the 110hp that I have now is a 1988 and the newer 90 is a 2012...? Also, would the 4 stroke have more torque?
     
  6. Mr Efficiency
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    Mr Efficiency Senior Member

    That sounds like an old cross-flow V4 Johnson or Evinrude, which weighed a touch over 300 lbs from memory. And not renowned for temperate "drinking" habits. I'd say if the boat pops on to plane easily enough now, and you are not a thrash-artist, 90 hp 4 stroke will do the job, provided that job is just normal cruising. Judiciously used, you might cut the fuel bill in half.
     
  7. pwillie
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    pwillie Junior Member

    24 years difference in technology could make it work?
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    Horsepower has not changed. The 110hp would be rated at the prop for that engine, ditto your 90 hp 2012 engine. What is the boat used for ?
     
  9. pwillie
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    pwillie Junior Member

    17 ft center console,used for fishing... I think a 110 is rated at the top for this hull...
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Should be good.
     
  11. PAR
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    PAR Yacht Designer/Builder

    If the boat was designed before 4 strokes really came to be, there may not be enough volume aft to support a max HP 4 stroke, so consider dropping to 100 HP or being okay with the 90.
     
  12. Mr Efficiency
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    Mr Efficiency Senior Member

    The extra weight of a 90hp 4-stroke would be negligible. Just keep thinking of the fuel savings.....those old V4 tuned-exhaust versions sucked gas something awful.
     
  13. ondarvr
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    ondarvr Senior Member

    When you compare 2 strokes to 4 strokes in the same HP rating the 2 stroke will typically feel more lively and responsive, especially at lower RPMs, 4 strokes can feel sluggish when getting up on plane. Top speed on the two would be similar though.

    While an old 110 hp cross flow isn't the best performing two stroke, if it's in good running condition it may feel much more powerful than a 90hp 4 stroke.

    I would much rather use an EFI 4 stroke though, the difference in technology makes the newer motor a dream to use.

    The weights have been coming down on the 4 strokes, but they still may weigh a 100 lbs more than an old school 2 stroke depending on the hp range.
     
  14. theoldwizard1
    Joined: Sep 2016
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    theoldwizard1 New Member

    Check some recent Yamaha "big" 4 cylinder 4 strokes ! They are amazingly light.

    The Yamaha "big bore" 4.2L V6 engines uses a plasma applied coating directly on the aluminum cylinder walls, (no liner) to further reduce weight.
     

  15. Barry
    Joined: Mar 2002
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    Barry Senior Member


    I am wondering if in fact the engines are rated at the prop as compared to the power head.

    Years ago, late 50's early 60's, the (Kiekhaefer) mercs had two designations. The 60 hp (at the prop) was supposedly a 75 hp at the power head. And the additional designation for a 60hp merc was "Mark 75 A"

    The other manufacturers used a power head only rating and had less at the prop.

    I cannot confirm this but was told this by an old Merc mechanic who bled quicksilver when cut

    But today, is the rating at the prop or power head??
     
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