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8cyl to 6cyl fac tach

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Tkeduc8r, Oct 6, 2002.

  1. Tkeduc8r

    Tkeduc8r Registered Member

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    found a tach dash out of a 74 C60 and would like to put it in my 74 c10 driver.. tach is for a v8 and i would like to know what i need to do to alter the incoming pulse from the coil... oh and i have an HEI in it out of a 79... thanks fellas...
     
  2. dyeager535

    dyeager535 1 ton status Premium Member

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    If I read your post right, your truck is a V6, your tach is a V8. Never seen anything that will "cut out" 2 of the 8 cylinder firing events.

    GM used different printed circuit boards on Chevy 6/8 cylinder tachs, even though the face was the same. I haven't heard of anyone converting tachs either way, you just have to start out with the right number of cyliners in the first place.
     
  3. Blue85

    Blue85 Troll Premium Member

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    What is a C60?

    There may be a way to modify the tach circuit board to work with 6-cylinder, but chances are you would need to look at a V-8 to know what changes to make anyway. This would be the most straightforward approach, but I don't know how to get the info. Anybody have factory tach schematics?

    Otherwise, to do what you describe would require a frequency converter. I'm not sure where you would find one. If you are good with electronics, it's not that difficult to design.

    I recommend swapping in an 8-cylinder engine to match your tach. /forums/images/graemlins/grin.gif

    Most aftermarket tachs allow you to select the number of cylinders. I think it's basically a frequency to voltage converter with an adjustment on the divider. What I did was use the guts from an aftermarket tach on a factory faceplate. The difference is that I had to design my own scale and put it over an exising fuel gauge scale because I didn't have a factory tach to begin with. You would reuse the needle that is on your tach now to retain the factory look. For easiest work, get an aftermarket tach that has a pin sticking out of the front to mount the needle, similar to the factory-type. Some have the needle sandwiched between two endpoints, making it very difficult to adapt a new one.

    The whole trick is to match the scale. All of the older factory tachs I've seen go to 7000rpm. You need to find one that goes to 7000rpm in the rotation (# of degrees) as the factory tach. To run one in the "wrong" mode (4-cylinder or 8-cylinder mode) would require a full-scale of 4667rpm or 9333rpm, so you're pretty much out of luck there.
     
  4. imiceman44

    imiceman44 1 ton status

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    Well I used to have one of those 4-6-8 tachs, where you select the right combo, and I opened it up it had a series of resistors and capacitors to do it.
    I don't have them now, but if someone has them and can understand electronics, they can help you do it.
    If by the time I get one and you still didn't figure it out, I will help you do it.
    IceMan
     
  5. tipoil

    tipoil 1/2 ton status

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    maybe u could use the electronic part of a factory tach from a 78-87 oldsmobile (they have a switch on the back 6 or 8) or from a chevy ,pont or buick .

    or just see what it does so u can do the same to ur tach
     
  6. dyeager535

    dyeager535 1 ton status Premium Member

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    Camaro stuff probably bolts right up, (and is more common) the Olds tachs are quite small, Camaro stuff is almost as big as the trucks.
     
  7. tipoil

    tipoil 1/2 ton status

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    maybe but here the camaro stuff are long gone ond the olds too!! most junkyard dont have them anymore .

    the 3 big junkyard around here dont have a single blazer in stock!!(execpt for a few s-10... a few!!!).
    wish i was in the usa!! /forums/images/graemlins/usaflag.gif /forums/images/graemlins/usaflag.gif
     
  8. m j

    m j 1/2 ton status

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    the big yards dont keep much older then 5 years.
    you will have to find a rural old fashioned yard
     

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