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Which RPM reading do you think is the closest to the truth?

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  • Which RPM reading do you think is the closest to the truth?

    My roadster has a 3.78 rear end, 6.0 head and a F150 4 speed trans.

    I have taken the RPMS with my Westech tach, a dwell meter and a digital timing light, and i am not sure about accuracy.

    700 475 500
    1500 1000 900
    2000 1310 1000
    2500 1850 1200
    3000 2000 2000
    3500 2500 3000
    These are the tools: gB2EAQYBCABEgJfKfD_BwE

  • #2
    None. If you graph all three data sets the lack of comparative linearity is telling.

    The West Tech tach and the Dwell Meter tach are both influenced by signal quality (your ignition system, both mechanical and electrical) as well as their own internal analog calibration. The digital timing light is influenced by the quality of a reflected signal and its own internal mapping of that signal against an internal signal generator that was calibrated how and to what standard? For the price I'm sure none of the three were individually calibrated.

    A bench signal generator with a traceable calibrated clock chip can be use to test all three devices. This will not, however, account for inconsistent input signal (electrical or light reflective) from your vehicle. That is something you need to address independently.

    I have no idea what your lead sentence has to do with the problem.


    • #3
      Thanks Mike. Lead sentence, I thought it would be important to shed some light on rpms that I have a 3.78 and not a 4.11 and a overdrive and not a standard A 3 speed trans.


      • #4
        I, like you, tried 3 different tachometer tools, and a cheapy accessory tach, so 4 different tools all together, with 4 different results. Finally gave up trying to figure out which was correct, and went with a meter I bought at HF the other day. Reasoning being, it's inductive (clamps around a plug wire) so not influence by primary ckt issues or dwell, and it's the newest, and I'm hoping they have learned how to make one that is pretty accurate. Now if it really will make a difference to what you are doing, you could always do the math. Mark a tire, roll the car and measure how far one revolution moves the car, use your GPS speedometer and your calculator to finger it out. Too difficult for this old mind...
        CB the Wonderful