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Prop Selection for 89 2655 with 5.7-gctid401399

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    Prop Selection for 89 2655 with 5.7-gctid401399

    I have a newly installed 5.7 that is now running very nicely. It idles beautifully a rides very nicely up until 3600 rpms. The problem is (and as of now the only engine problem) is that it won't spin any faster than that.

    I am sure that the timing advance is set correctly 28 btdc at 2800 rpm so I am not experiencing detonation. My fuel vent and pick up are both clear. My throttle is set properly. and am am getting full extension at the carb. I have no growth on the bottom of my boat (I trailer it), and no water in my hull.

    I's going to experiment with the prop to see if I get any improvement. I am currently running a 15.5 x 15. I just ordered a 15x13. Does anyone run with a 14" prop on a boat this heavy? Could it improve my situation. My cobra has 1.41 gears.



    I ran a ss 15 1/2 X 14 perfection! 3500 rpm @ 25 mph WOT 4200 at 31mph...6500lbs


      Thanks Chris. I just got a 15.5 x 13. I'm gonna see what that gets me. I hope to top out closer to the 4K range. I might be able to make up the difference with trim & bottom paint then.

      Going for a test run tomorrow. :-)



        Greg, as you know, a WOT RPM should have been verified early on, not now after your break-in period. I hope that you have not over-burdened this new engine.

        I've been following your threads fairly closely, and I've read where you have gone through your fuel system, checked throttle plate opening, checked for hull growth, etc. etc, and was on board with the Ignition TA suggestions. If I may; your 28* @ 2,800 RPM is a tad bit aggressive for a non-quench SBC, but is close. (I'd rather see that 28* hold off until 3.2K RPM)

        Please note that Detonation is not always as audible as is Pre-Ignition.

        Your final drive ratio of 1.41:1 seems rather tall to me (I don't know the OMC ratios), so the 13" pitch sounds like a good suggestion.

        However, being that the 1.41:1 is on the tall side of the spectrum, I'd be asking what other 2655 owners are using for a final drive ratio.

        Perhaps this question, along with prop suggestions, may get you closer to an answer.

        Ratio Rule # 101, there is no substitute for a correct final drive ratio for the engine, and the hull may have some influence on this.

        But let's say that the 1.41:1 is correct!

        Since all of your ducks appear to be in a row, and you still cannot reach WOT RPM, I'd ask if you have verified your tachometer!

        What if your 3,600 rpm is actually 4,200 rpm?

        What if your primary and secondary throttle plates are opening fully, but your secondary fuel metering is goofed up?

        Wish I had more to offer.

        Hang in there...... you'll get it figured out.

        Rick E. (aka RicardoMarine) 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

        Please, no PMs. Ask your questions on forum.


          Hi Rick,

          A carb rebuild is in my future. Thanks for that. I actually have access to another marine quadrajet. I'm going to do a rebuild on that.

          I don't think I would have done any damage as most of my time is spent in the lower rpm range. We normally travel a very short distance in the harbor and anchor to swim. I only have only been at WOT to test.

          Plus I have been checking my plugs periodically. I am not seeing anything unusual.

          I've posted on here a couple of times for others with 2655's to find out what prop sizes they are using. I haven't had many responses. Searching around though I see others with cruisers and similar set ups using 14" props.

          Also, I mistyped in the post above. I am seeing 28 degrees at 3000 not at 2800. I must have had 28 on my brain.



            Oh, I have verified my tach at lower rpms. I will check today.

            Thanks for your help on this Rick!!



              OK I am starting to think that my tach is screwed up. I went from a 15.5 x 15p to a 15.5 x 13p with only a tiny increase in RPMs (under 100rpm difference). From everything I've read, I should have picked up 400rpms.

              So i know i didn't change any wiring at the tach end, is it possible the tach was connected wrong at the motor end? Could it have a bad ground or be connected to the wrong side of the coil?