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Altitude Adjust and speedo adapters

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by CV202, Nov 29, 2000.

  1. CV202

    CV202 1/2 ton status

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    Wondering if anyone has any info of adjusting a 1990 TBI 350 for altitude. I runs great at sea level, but has been a pig ever since I moved into the Mountains. I'm also looking for info on Adapters for the speedo. I run 33's right now, and my speedo is off by a bit. I'm told this is also the reason the ABS and cruise control don't work. HELP!

    Thanx for any help.

    Why do people keep calling it a Bronco!!!!!
     
  2. BurbinOR

    BurbinOR 3/4 ton status

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    Can't help you on the VSS (vehicle speed sensor), though I think you need to "update" the chip to compensate for the larger tires, etc. Don't quote me, as my VSS is disabled on my TPI setup.

    As far as the altitude adjustment, I am reasonably certain [​IMG] that any computer controlled motor/computer automatically adjusts for the increase/decrease in altitude with its' sensors. Or so I think.............someone will add to/correct this assumption.

    <font color=black>'79 - 406 TPI -<font color=orange> K5#5 - <font color=blue>See it at---&gt;http://www.blazzinor.coloradok5.com
     
  3. HarryH3

    HarryH3 1 ton status Author

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    Your engine management computer will automatically adjust for the change in altitude. It measures it via the Manifold Absolute Pressure sensor. The problem is that as you increase altitude there is less air pressure available to force air into your cylinders. The 14.7 PSI of ambient air pressure drops as you get higher. As a result, combustion pressures get lower as the truck gets higher (effectively lowering the compression ratio).

    Since less air is entering the engine, the computer (or a correctly calibrated carb on older vehicles) dispenses less fuel, to keep the mixture correct. I live at about 7,000 feet above sea level in Colorado and power here is down about 25% from what it would be at sea level. [​IMG]

    The only types of "altitude adjusters" that I'm aware of are turbo chargers and super chargers. They force feed air into the engine, thus filling the cylinders quite well. I seem to recall reading that they were orignally invented for piston-engine fighter planes, since power loss was such a concern for them at high altitudes.

    Nitrous oxide is another way to introduce more oxygen into the combustion process, but it gets expensive if you use it much. (And very expensive if it malfunctions. The tops of the pistons get ventilated in a matter of seconds if the mixture leans out too much). [​IMG]

    <font color=black>HarryH3 - '75 K5</font color=black>
    http://ThunderTruck.ColoradoK5.com
     
  4. realsquash

    realsquash 1/2 ton status

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  5. realsquash

    realsquash 1/2 ton status

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    Any closed-loop system adjusts for altitude changed on-the-fly. It is built into the math the ECM uses.

    Your cruise control should work fine, also. Tire size/gears don't affect it.

    Squash
    http://www.trailrunners4x4.org/users/realsquash
     
  6. shum1

    shum1 1/2 ton status

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    How do you adjust a spedometer on the 241 t-case? Its an electric sender and I have not been able to find any info for my 1990 rig.
     
  7. HarryH3

    HarryH3 1 ton status Author

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