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Swapped out 5.0 for 5.7, cobra gear ratio question-gctid343756

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    Swapped out 5.0 for 5.7, cobra gear ratio question-gctid343756

    New gm 350 block installed on stock 305. Now I realize the upper gear ratios are different. 21/17 should be stock on the 5.0, 21/16 was used for 5.7.

    Is there a prop pitch work-around for the ratio difference or do I need the 21/16 gear set?

    Thanks in advance.

    Jarrett

    #2
    Your WOT RPM range will be very similar between the two engines, but your torque potential will be much different.

    The tooth count differential between the two sets of upper gears is one thing.

    You may want to consider what this does to the over-all ratio. I.E., when considering the lower unit gear reduction as well.

    The sum of the two gives you the over-all reduction ratio.

    If the over-all ratio ends up being not all that different, I'd sure give things a try before going to the work and expense.

    Your prop selection may change a bit.

    Consider this:

    AQ series Volvo Penta 5.0L and 5.7L

    Each produces a different horse power and torque.

    Both requrie the 1.61:1 over-all ratio, and no one has any issues.

    But do speak with an OMC expert.... they may be able to share something.

    .
    Rick E. Gresham, Oregon
    2850 Bounty Sedan Flybridge model
    Twin 280 HP 5.7's w/ Closed Cooling
    Volvo Penta DuoProp Drives
    Kohler 4 CZ Gen Set

    Comment


      #3
      I would try it first. See what your maximum RPMS are at WOT. You might find that you need more pitch on your propeller.

      Keep in mind that the boat behaves differently when it's heavily loaded, or empty. Sometimes, a skipper runs a special prop

      for special needs. Example: A low pitch prop to slow the boat for Salmon trolling, or to give the boat a better hole shot for skiing.

      Of course, when doing this, you need to be mindful of your WOT rpms. I deliberately throttle back when running my trolling prop.

      If you're putting the 5.7 in your 2160, it will behave like a high performance ski boat. I had a 2160 with a 3.0. It was adequate and got

      3 mpg.

      Comment


        #4
        I'm currently running a 15x15 four blade stainless as the tired 5.0 was having a hard time getting near recoomended wot with a three blade 15x17. I also troll all the time and run heavy. I thought I'd have to increase pitch for wot purposes with the change but perhaps the 15 pitch will work out.

        Am I right that the current 21/17 gear set will spin slower than the 21/16 which came with the 5.7 from the factory? I take it the 21/17 would be restrictive on the 5.7 for wot purposes and thus lowering prop pitch would compensate.

        Thanks for your responses.

        Comment


          #5
          What hull are we talking about ?

          The difference in those ratios is not enough the worry about, if anything, you will be better off with the slightly lower ratio (more torque), if you hadn't seen the ratio tag on the housing you would never know the difference.

          The ratios in the lower are the same for both apps. so there will be no issue there.

          At the most you can play with prop pitch but depending on what hull you have, with that engine and overall ratio, you will able to tweak the prop. pitch and be good.
          " WET EVER "
          1989 2459 TROPHY OFFSHORE 5.8L COBRA / SX
          mmsi 338108404
          mmsi 338124956
          "I started with nothing and still have most of it left"

          Comment


            #6
            A change occured after 1988 in the gear ratio.

            I would test it as is before changing gears out. Test the boat as you run it most often.
            Phil, Vicky, Ashleigh & Sydney
            1998 3055 Ciera
            (yes, a 1998)
            Previous boat: 1993 3055
            Dream boat: 70' Azimut or Astondoa 72
            Sea Doo XP
            Sea Doo GTI SE
            Life is short. Boats are cool.
            The family that plays together stays together.
            Vice Commodore: Bellevue Yacht Club

            Comment


              #7
              itsabowtime2 wrote:
              A change occured after 1988 in the gear ratio.
              The Cobra was introduced into the market in 1987, and by 1989 there was a ratio change? That may be note worthy.

              If those numbers are accurate;

              5.0L would require the 21/17 tooth count

              5.7L would require the 21/16 tooth count

              Lessor hp/lessor torque (5.0L) would = a numerically higher ratio, or an increased reduction.

              More hp/more torque (5.7L) would = a numerically lower ratio, or a decreased reduction.

              If you did change the upper gear set from 21/17 to 21/16, you may actually be able to use the same propeller.

              You'll just need to experiment a bit with the 21/17 and a few props, and see what happens.

              .
              Rick E. Gresham, Oregon
              2850 Bounty Sedan Flybridge model
              Twin 280 HP 5.7's w/ Closed Cooling
              Volvo Penta DuoProp Drives
              Kohler 4 CZ Gen Set

              Comment


                #8
                Here's what I have in the 1988 OMC factory shop manual:

                5.0 gear ratio overall: 1.50:1 upper ratio: 21:17

                5.7 gear ratio overall: 1.41:1 lower ratio: 21:16

                Lower ratios are the same at 14:26

                I agree with the other posters, I'd try it out first and see what you get. The max rpm range given for both the 5.0 and 5.7 is 4200-4600. You may be closer to the upper end but even if your max rpms were 4800 I would not worry about that and you might well wind up with better out of the hole performance.
                88 Four Winns 200 Horizon 4.3 OMC
                98 Jeep Grand Cherokee 4.0/Selectrac
                07 Jeep Grand Cherokee 5.7 Hemi/Quadradrive II

                Long Island Sound Region

                Comment


                  #9
                  Thanks gents I'll give it shot with the 21/17 and see what I can get out of the combo. i'll report back on the results. Gotta be better than the 23 year old 5.0 with health issues.

                  Jarrett

                  Comment


                    #10
                    Reel Power wrote:
                    Thanks gents I'll give it shot with the 21/17 and see what I can get out of the combo. i'll report back on the results. Gotta be better than the 23 year old 5.0 with health issues.

                    Jarrett
                    Think of this as though your 5.0L started eating better, started working out at the local health club, and gained strength.

                    It's certainly worth trying.

                    .
                    Rick E. Gresham, Oregon
                    2850 Bounty Sedan Flybridge model
                    Twin 280 HP 5.7's w/ Closed Cooling
                    Volvo Penta DuoProp Drives
                    Kohler 4 CZ Gen Set

                    Comment


                      #11
                      i have a 89 2755 with a 5.8 cobra drive. the drive has a tag that says 5.0/5.7/5.8. the three engines output are so similar you should have no problem with the swap. its not like your mixing a v6 or big block in it.

                      Comment

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