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Optimum RPM range for '82 70HP Evinrude?

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  • Optimum RPM range for '82 70HP Evinrude?

    Hi Guys,

    I'm all wired up and ready to install my NOS Faria tach. Looks great, thank you to Sixmark for the help.

    I'm running a 15.5' tri hull with a 1982 70 HP Evinrude. The combo runs great. Planes fast and I can back it off quite a bit from WOT to conserve fuel.

    Assuming the tach install goes fine, I'm expecting WOT RPM to be around 5000 - 5500.

    Being able to back off throttle quite a bit and still plane nicely - how low is too low RPM criusing range? Where, RPM-wise, does this motor run best?

    Thx


  • #2
    Re: Optimum RPM range for '82 70HP Evinrude?

    The factory recommended rpm range is 4500-5500. I'd prop it so that you make sure to hit the top end of that rpm range. You will max out your power and fuel efficiency if it runs at 5500.

    Comment



    • #3
      Re: Optimum RPM range for '82 70HP Evinrude?

      Really? 5500 RPM will give me max fuel efficiency?

      I'm not looking for max power but max fuel while still running the boat on plane. Would 4000 be too low?

      I'm concerned with not damaging the motor while still getting greatest distance on a tank.

      Comment



      • #4
        Re: Optimum RPM range for '82 70HP Evinrude?

        Originally posted by lazersnake View Post
        Really? 5500 RPM will give me max fuel efficiency?

        I'm not looking for max power but max fuel while still running the boat on plane. Would 4000 be too low?

        I'm concerned with not damaging the motor while still getting greatest distance on a tank.
        The 5500 (I go 5800) is the maximum RPM that you should be able to reach at WOT with an average load. You prop for that so that you don't lug your engine and coke it up.

        You cruise at something well below maximum for greater fuel efficiency.

        Comment



        • #5
          Re: Optimum RPM range for '82 70HP Evinrude?

          That's my question. How low is too low. What's the lugging range?

          Comment



          • #6
            Re: Optimum RPM range for '82 70HP Evinrude?

            4500 rpm is considered lugging if that is the max rpm the engine will run with the prop you have installed. If you want max fuel efficiency, prop it to run 5500-5700, then pull the throttle back to 3500-4000. It will pull best out of the hole with that rpm setup, too.

            Comment



            • #7
              Re: Optimum RPM range for '82 70HP Evinrude?

              Lugging is generally caused by having a propeller with too high a pitch so that you cannot reach top of the full throttle range. It's similar to driving a car uphill in high gear.

              The full throttle range for your 1982 70HP engine is 4500 to 5500 RPM. The motor will be most fuel efficient at an RPM that will just comfortably keep the boat on plane.

              Here's a link to a well documented discussion of the subject: http://members.iinet.net.au/~pauldaw...25_economy.pdf

              Comment



              • #8
                Re: Optimum RPM range for '82 70HP Evinrude?

                i think you could back the throttle down untill it just about loses its plane, then give it a tab more, so its a tad above the planing threshold.... i would say that would be the optma crusing speed

                but hey im new to boating just makes sense to me
                "Those who hammer their guns into plows will plow for those who do not."Thomas Jefferson.
                "When the people fear their government, there is tyranny when the government fears the people, there is liberty."Thomas Jefferson.

                Comment



                • #9
                  Re: Optimum RPM range for '82 70HP Evinrude?

                  Tach wired up and resonding! Cold idle (in water-filled bucket) is 1000 RPM. Quick rev up to 2500 RPM and back down. Looks good. Now to install in the dash and take 'er out this weekend.

                  I'll post WOT and cruising RPM speeds on return.

                  Any suggestions or tips would be appreciated.

                  Thx,

                  Jack

                  Comment



                  • #10
                    Re: Optimum RPM range for '82 70HP Evinrude?

                    WOOHOO! Tach works great! WOT @ 5400 RPM running 33 MPH. Throttle back to 4000 RPM running 23 MPH. Hole shot is nice and quick, too. I think I hav the right prop!

                    One problem: My trim gauge no longer works. I jumped power, ground and lights off of it to my tach so I know those are making a conncection. Assuming the signal wire is not loose (I'll check that first), how do I check for trim signal? Light tester, ohm meter. etc.

                    Is is normally fused?

                    Also, Is this the trim signal sender on my motor. If it is, can I thumb manually it in lieu of raising and lowering the motor?

                    Any help is, of course, greatly appreciated.

                    Thx,

                    Jack
                    Attached Files

                    Comment



                    • #11
                      Re: Optimum RPM range for '82 70HP Evinrude?

                      The trim gauge should work within the 15 degrees of the trim range, not the whole tilt range. It's a variable resistor, so as the lever moves, more or less shows on the gauge. You can move the lever manually to test it.

                      The 1982 gauge and sender are usually wired such that the positive voltage comes from the ignition switch and the ground is from the relay box.

                      There is normally no need for a fuse as the ignition switch is fused at the engine end of the harness.

                      Comment



                      • #12
                        Re: Optimum RPM range for '82 70HP Evinrude?

                        Let me make sure I understand.

                        No fuse to check due to ignition being fused and I'm looking for a change in resistance thru an ohm meter when manually moving trim lever (ignition on).

                        I'll give it a go!

                        Thx,

                        Jack

                        Comment


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