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    Tach calibration questions

    Did some testing today to check out how close my tachs are. Used an inexpensive electronic tach to read off reflector on damper pulley. Here’s what I got using PH tachs as reference
    port engine:
    PH 550. FB 540. Actual 615
    PH 1000. FB 990. Actual 1052
    PH 1500. FB 1510 Actual 1478
    PH 2000. FB 2010 Actual 1936
    PH 2500. FB 2510 Actual 2510
    PH 3000. FB 2990 Actual 2982

    sb engine:
    PH. 650. FB 560. Actual 622
    PH. 1000 FB 895. Actual 970
    PH. 1500 FB 1410 Actual 1411
    PH. 2000 FB 1890 Actual 1861
    PH. 2500 FB 2380 Actual 2446
    PH. 3000 FB 2900 Actual 2919

    observations: it appears the tachs read higher than actual except for at idle. The differences are not linear. I’m curious if others have noticed this. Do these variances seem reasonable? I’m considering new props to enable me to reach full rpm + 3-5% under load. Tach calibration measurements were done at dock with transmissions in neutral. There are some puzzling outliers. Comments?
    2000 4788 w Cummins 370's, underhulls, swim step hull extension
    12' Rendova center console with 40HP Yamaha
    MV Kia Orana
    Currently Alameda CA

    #2
    Steve,
    Your results were a lot closer than mine. My old Faria tachs were mostly accurate up to about 1300 rpm then they diverged wildly from the laser tach. After several discussions with Faria reps and adjusting the pots on the back of the gauges with no lasting benefit I installed Aetna digital tachs on the FB since that’s the only location I drive from. Good rpm info now!
    Jim Gandee
    1989 3888
    Hino 175's
    Fire Escape
    Fyrflyer@ca.rr.com

    Comment


    • Woodsea
      Woodsea commented
      Editing a comment
      Thanks Jim!
      I expected variation. But I guess I though it would be linear. What puzzles me is the somewhat randomness of the data. I’m wondering if my measurement is suspect?

    #3
    You could try adjusting the tachs Steve. Mine had a high and low pot accessible via the back of the tachs. I repeatedly adjusted them and could get kind of close then two weeks later they were back to their old tricks.
    Jim Gandee
    1989 3888
    Hino 175's
    Fire Escape
    Fyrflyer@ca.rr.com

    Comment


      #4
      Jim,
      There is one pot on the back of the tachs used for setting number of teeth on the flywheel. I did move that back and forth to clean corrosion, but no noticeable change. I rechecked my on engine measurements. The 2000 rpm indicated on S engine PH tach came up to 1925 rpm actual. That makes more sense. All the other readings were pretty close to my measurements in the first post. The photo tach I use requires a black background so the reflector tape really stands out. Gorilla tape solved the issue. The actual results are at least now repeatable.

      Now I know that my worst case reads 80 rpm low at 3000 helps me selecting new props!
      2000 4788 w Cummins 370's, underhulls, swim step hull extension
      12' Rendova center console with 40HP Yamaha
      MV Kia Orana
      Currently Alameda CA

      Comment


        #5
        Steve,
        for analog tachs I think you’re about as close as you’re ever going to get, and glad to hear it! For prop adjustment I’d personally would not rely on the analog tach but on you ur strobe tach reading which you’re probably already planning anyways!
        Jim Gandee
        1989 3888
        Hino 175's
        Fire Escape
        Fyrflyer@ca.rr.com

        Comment

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