V8 tach to I-6

Discussion in 'The Projects Forum' started by ken thompson, Apr 22, 2008.

  1. ken thompson

    Thread Starter New Member

    Apr 22, 2008
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    How do I recaliberate a V8 1976 Ford Mustang II tachometer to be used in a car with an Inline 6?
    Thanks,
    Ken
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Are you sure you need to? That is, is the sensor on one lead, the distributor rotor, or on the main lead? If it is on the rotor or just one lead, there shouldn't be much difference. That is, 1 pulse for every 2 revolutions. Can you tell us more about the tach? John
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    John,
    Can't tell you about the newer electronic ignitions, but all of the older ones put out a tach signal that pulsed each time a cylinder fired.

    So in that case, every time three pulses came from the 6-cyl ignition tach signal, the 8-cyl tach itself would need four evenly-spaced input pulses.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    You are right. The modern ones work off the main wire. I was thinking of the add-on to my '62 Ford Falcon.

    Here are 4 options I came up with:
    1) Buy a new face for the instrument with revised numbers on it. Magic marker may work in a pinch, but the new face would look much better. :)

    2) Buy a converter. I searched ebay for "tach* conver*" and got several hits, but not for Ford.

    3) The circuit is probably very simple and not digital in 1976. Here is one from a Chevy that I ran across on the Internet. You could open yours up and see if it looks similar. A different timing cap and resistor is all it needs, according to the Corvair/Corsica site.

    4) You may get lucky. According to the second link below, some indicators have a built-in switch for setting the cylinder number.

    Annotated references:

    http://www.tachman.com/ford.htm (Offers tach rebuilds and conversions)

    http://www.fordmuscle.com/forums/mustang-pages-1979-1995/472695-converting-4-cylinder-tach.html (Discusses a switch in the indicator that can be set to the correct cylinder number)

    http://corvaircenter.com/phorum/read.php?1,121872,121982,quote=1 (Discusses the Corvair conversion )

    John
     
  5. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Doen´t the ratio of the gearbox matter too? And what about the size of tires?
    Edit: from the schematic I see you could set it with the potentiometer., If you have a simalar tacho.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Vehicle speed varies with gearbox ration and tire diameter, but engine RPM is a direct measure of how many times it goes around in one minute.

    Yes, if there is enough adjustment range, using a test tach could calibrate the other one.
     
  7. ken thompson

    Thread Starter New Member

    Apr 22, 2008
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    Thank's for your responses, I found Dakota Digital offers a tach recalibration unit, ~80.00. I am going to look at adding a potentiometer first.
    Thanks again,
    Ken
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Actually, you could probably make a calibrator yourself. All you would need is a stable timebase.

    If you took a 12vac 60hz signal from the output of a transformer, and squared it up using a CMOS IC with a Schmitt input (like a 4093 or 40106) then you would have 60 output pulses per second, 3600 output pulses per minute. A 4-stroke engine cylinder fires every other downstroke. So, divide 3600 by the number of cylinders, and then multiply by 2 to obtain what the tach should read; in this case 3600 / 6 * 2 = 1200 RPM. You could check to see how close it is now by 3600 / 8 * 2 = 900 RPM.
     
  9. kubeek

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    Sep 20, 2005
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    But it is supposed to be tachometer, not a RPM meter?
    I thought that speed should be measured on wheels and not on the motor.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    An automotive tachometer is for measuring the speed of the engine in RPM.
    A speedometer is used to indirectly measure the speed of the vehicle by directly measuring the average rotational speed of a pair of wheels.
     
  11. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Ok, sorry for the confusion then, my dictionary said that tachometer=speedometer.
     
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