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120 hp Lower unit gear ratio

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  • 120 hp Lower unit gear ratio

    I only know enough to be dangerous. The local prop guy doesn't allow testing a prop before buying so I have to have a pretty good idea when I start out. I have a 1986 Johnson 120 hp and have the a prop that came with the boat plus one that I bought and have had repitched. The orignal is a 14-1/2x19 (unknown brand) that ran at 5300 WOT and the newer is a 15x17 Michigan that ran at 5500 rpm then repitched to 15x17 and ran 5600 rpm. The boat speed (gps) was 41 and 43. So I know the limiter is a factor and to calculate the theoretical speed I need to know the gear ratio.Thanks!


  • #2
    Re: 120 hp Lower unit gear ratio

    Here is a list that points towards 2.0 : http://www.boat-props.com/info/prop_...ge=rpm_johnson

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    • #3
      Re: 120 hp Lower unit gear ratio

      It appears that you must have the larger style gearcase. That ratio is .44 or 12:27.You should look for a performance center or another prop shop. This guy is not concerned in getting you propped out correctly, he just wants to sell a prop.

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      • #4
        Re: 120 hp Lower unit gear ratio

        He only wants to sell and he's the only one within 100 miles (at least). I don't plan on buying any more from him but now it's potluck on ordering a brand at higher $.I do have the larger style gearcase and from my number crunching it looks like it's working better than I thought. Thoeretically I could get better speed out of the 14-1/2x19 at lower rpms but then they drop below the proper range for me to be operating (5400-6000) plus I must be getting more slip (right term?) since my actual speed is worse. Thansk for the help! At least now I know I'm somewhere in the ballpark even if its not optimized.

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        • #5
          Re: 120 hp Lower unit gear ratio

          You have the same motor as me. It is a 2.0:1 ratio. You are not far off with the MI Wheel, these motors should not be pushed much over 6000. In fact I believe they have a limiter at 6200.How is the holeshot with the MI? You may find that the 19P is a bit too much. What is the type and weight of your boat?

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          • #6
            Re: 120 hp Lower unit gear ratio

            Those motors came with 2 styles of gearcases. Since he is running the larger diameter he must have the larger one. It does not hurt those motors to run over 6000. Some had rev limiters at 6700.

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            • #7
              Re: 120 hp Lower unit gear ratio

              Mark, the hole shot isn't too good but it will get up on plane. Its a 18 ft quantum bass boat and I guess it would weigh about 2700 loaded. From my experience I won't ever have to worry about hitting 6000 rpm! lol

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              • #8
                Re: 120 hp Lower unit gear ratio

                Hey wolfb2,,I believe that you can get a prop from ******* or Bass Pro and try it,,just keep far away from shore,do not hit anything in the water,like stumps or logs etc,,and they will return them,if they still look brand new.It will probably cost you 15-20 bucks,shipping both ways.I have done this before with no problems,but I was very careful not to scratch or mark up prop.I also have a 1993 Johnny 120 that I am trying to get propped right,it is on a 19 foot aluminum Spectrum.I am considering trying a 19 pitch Ballistic.My motor has the smaller gearcase.I tried a 12 3/4 by 21 pitch stainless(brand unknown) and was running 43-44 at 5200 rpm's. I am hoping the Ballistic will give me a few more miles per hour,,,need to outrun brother.Post back your results,and I will post mine when I get some more testing done.

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                • #9
                  Re: 120 hp Lower unit gear ratio

                  You realy want to prop it so the WOT is about 5200-6000 RPM. It seems that is what you have right now. I think if you go with a steep er pitch you will have about the same top speed but a worse holeshot. Did you try putting on one of those fins (Stikray etc.) I have one and they help the holeshot tremendously.One thing that may help is to play th the motor height. Rainsing the motor can help get higher RPM but you must be carefull of the water pressure. If you want to do this get a water pressure gauge, they are about $30. The pressure should be between 20-25 PSI at WOT

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                  • #10
                    Re: 120 hp Lower unit gear ratio

                    Adding anything below the waterline (DoelFin, Stingray or any) will increase drag and cost top speed.You're pretty much closing in on the aspects as you're evaluating performance/rpm/pitch.You get good advice here and combined with your experience, and a good prop shop, you're bound to succeed. Do remember to consider the obvious:Boat weight (remove any unneeded load, redistribute)Motor tune upMounting heightTilt angleHull shape & surfaceBTW: If you see something moving extremely fast on the water, it's someone who took Dhadley's advice.

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                    • #11
                      Re: 120 hp Lower unit gear ratio

                      Thanks all. I appreciate the help!

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