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recirc button, higher air volume

Discussion in '80-Series Tech' started by floydturbo, Jul 14, 2005.

  1. floydturbo

    floydturbo

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    I have searched and cannot find an answer. On my 96 TLC when using the a/c in the recirc mode, the air volume out of the vents is say 10/10. When I push the recirc button to fresh air, the air volume drops to about say 8/10. Anyone know why, does the fan speed actually decrease or is the air going elsewhere, or is the intake partially plugged?????
     
  2. cary

    cary

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    My 80 does this also, as have other cars I have owned. I believe it is because the air does not have to be drawn through as many vents so there is less resistance.
     
  3. Scamper

    Scamper

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    Have to agree with Cary. In the fresh air mode, the air encounters a lot more distance and resistance thru the system than it does when in recirculate mode.

    There may also be a perception that the air flow is less since the noise is lower in fresh air mode. How do you really know it's 8/10 vs 10/10? Did you use some air flow meter?
     
  4. e9999

    e9999 You want to do what...? Moderator

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    ??
    when I push the recirc button on either my 80 or DD or the 100, the air flow rate greatly *increases*! Always thought that to be normal...
     
  5. Koffer

    Koffer

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    When you put it on fresh you are now taking air from the outside and pushing it into a somewhat sealed cab and it needs to be forced out of the rear vents and other small openings .
    This pressureizes (SP?) the inside slowing down the amout of air flowing in .
     
  6. MrZumma

    MrZumma

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    Think of the passenger compartment like an air tank. When you have fresh air selected with all windows closed, it is trying to pressurize the passenger compartment. When you have it on re-circulate, it is just moving the existing air in the passenger compartment around.

    I would imagine if you had an anemometer and held it up to a vent while in fresh air mode and opened a window; it would read the same as if you had it in recirculation mode on the same vent.
     
  7. e9999

    e9999 You want to do what...? Moderator

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    OK, here goes, went to check it:


    With truck parked, engine on, fan on highest setting, door open, I get:

    - RHS most of the 2 middle of the dash vents gave
    15mph with recirc and 13mph with fresh
    - the LHS most of the middle vents gave
    19mph with recirc and 17mph with fresh

    not as big a difference as I thought based on sound, but measurably there...

    Of course, when moving, the "fresh" setting might be helped by the RAM effect but I still found it to be less air than recirc on casual observation when driving in the past...
     
  8. OZCAL

    OZCAL

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    That's always been my take on it.
     
  9. e9999

    e9999 You want to do what...? Moderator

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    you guys are killin' me.... :)


    went back out again.

    In recirc mode, engine running, fan on Hi, LHS middle vent:

    - 1 door open: 17.4 mph
    - all doors and windows closed: 15.2 mph

    interestingy, the flow rate decreases instantaneously when the cab is sealed. Maybe 1 or secs lag, that's all. So much for air compressibilty effects....

    so, yes, the sealing of the cab makes a difference!
     
  10. OZCAL

    OZCAL

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    Whad'ya have, a propellor beanie wired up to a tachometer :flipoff2:
     
  11. e9999

    e9999 You want to do what...? Moderator

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    nope, the propeller has a gear that activates a tiny hammer that pounds on my skull. I just have to count the number of hits in a minute and do the calcs... (after I come back to) ;)
     
  12. OZCAL

    OZCAL

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    Count the knocks in ten seconds, then multiply by six. Saves $$ in aspirin. :grinpimp: