recommended rpm

Discussion in 'General Motoring' started by mtb Dad, Jan 5, 2006.

  1. mtb Dad

    mtb Dad Guest

    Can one do damage driving at low rpm's? My wife likes to drive our 86
    740 turbo at sub 2000 rpm in OD, I suspect using the turbo to
    compensate. It bugs me, but can it really do harm?

    It's a second hand car, so I don't have a manual.
     
    mtb Dad, Jan 5, 2006
    #1
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  2. mtb Dad

    jg Guest

    It should carbon up, use more fuel and possibly overheat. Oil won't pump
    with enough pressure for the work being asked of the motor, so it won't last
    as long. But it's hard to prove all that to someone who doesn't understand
    and God protects fools, women and drunks.
     
    jg, Jan 5, 2006
    #2
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  3. mtb Dad

    James Sweet Guest


    I usually try to keep it under 2000 RPM cruising, though that's not
    always possible. It's best to just drive it by feel, if it's lugging or
    vibrating excessively then downshift to raise the RPM.
     
    James Sweet, Jan 6, 2006
    #3
  4. mtb Dad

    The Visitor Guest

    So long as your engine isn't pinging or rattling. If so you should be
    using a better grade of gas. Cars with an ecm should make the fuel
    richer to prevent detonation. What does it hurt?
     
    The Visitor, Jan 6, 2006
    #4
  5. mtb Dad

    mtb Dad Guest

    It does go into the yellow zone on the turbo dial, which worries me
    because I've heard overuse of the turbo can wreck the engine. The
    previous owner says his teenage kids blew two motors! But what's the
    difference between a teenaged boy driving it hard, and my wife
    low-rpming it up the hills with the turbo in yellow?
     
    mtb Dad, Jan 6, 2006
    #5

  6. Boy Racers (of whatever age) tend to rev the engine high and
    long, and this is harder on turbos than having to operate at low
    speeds and higher pressures. Do these engines even increase the
    turbo pressure that much under low-speed load...?
    --







    http://freevision.org/michael/index.html
     
    Michael Cerkowski, Jan 6, 2006
    #6
  7. mtb Dad

    mtb Dad Guest

    How could reving high be hard on the turbo if the turbo dial is showing
    a lower reading for the same speed? I assume the turbo dial is
    pressure, but I don't know.

    The particular circumstance is driving on the highway around 90km per
    hour, in OD, around 1800 rpm, then not gearing down for hills, and the
    turbo going way into the yellow until back on the flat.

    When I drive, I gear down for hills, the engine revs are higher
    (2500ish) but the turbo reading is much lower, well below the yellow.
     
    mtb Dad, Jan 6, 2006
    #7
  8. mtb Dad

    James Sweet Guest


    Using the turbo a lot will consume more fuel and cause more wear on the
    turbo, but you really have to try in order to blow a Volvo motor. I
    don't know what the teenagers could have been doing other than possibly
    constant redlining the RPM or messing with the boost. I drive mine
    fairly hard and have yet to damage even a high mileage motor.

    Generally you want to keep the needle in the black unless you need the
    power, boost won't hurt it but it won't help the economy any since the
    ECU richens the mixture under boost.
     
    James Sweet, Jan 7, 2006
    #8
  9. mtb Dad

    James Sweet Guest

    The turbo pressure will increase to the full 8.5 PSI at any speed if you
    give it enough gas. The pressure is always whatever the guage shows
    regardless of RPM, though the guage is not calibrated. The difference is
    that the turbo will be spinning faster to maintain any given pressure at
    a higher RPM as more air will be flowing through the motor.
     
    James Sweet, Jan 7, 2006
    #9
  10. mtb Dad

    The Visitor Guest

    Yes if it is going that high I imagine it could hurt the engine.
    Especially on that year I am not sure there is anything to enrichen the
    mixture to protect the engine. I don't know. Using premium gas will help
    to protect the motor. If she scraps the motor, you will really have
    something on her for a long time!
     
    The Visitor, Jan 7, 2006
    #10
  11. For a given pressure, higher revs result in more heat and more wear
    on the turbo. I doubt it glows dull red when run under load at low
    speeds like it can at high speeds.


    As noted elsewhere, if it isn't pinging, she probably isn't damaging
    it.
    You may want to see if you can adjust the wastegate to limit the boost.
    --







    http://freevision.org/michael/index.html
     
    Michael Cerkowski, Jan 7, 2006
    #11
  12. FWIW, the 740 series water cooled turbos are remarkably robust. I couldn't
    convince my wife (who was the primary driver of our '85 765T for more than a
    decade) not to rev the engine when she started it or to let the turbo cool
    down when pulling off the freeway into a gas station. I could hear the turbo
    spinning down as she got out of the car. At nearly 240K miles it is still
    going, and the engine has never been apart beyond timing belt replacements.

    Your wife is more valuable than the car, so relax and enjoy the ride.

    Mike
     
    Michael Pardee, Jan 7, 2006
    #12
  13. mtb Dad

    James Sweet Guest


    Because the boost guage is showing just that, pressure. If the engine is
    revving higher the turbo has to spin faster to maintain the same boost.
    Really though either scenario will not hurt the turbo, it should last a
    long time regardless of how it's driven as long as it's well maintained
    and doesn't get extreme abuse.
     
    James Sweet, Jan 7, 2006
    #13
  14. mtb Dad

    The Visitor Guest

    Ultimately, turbo or not, anytime you pull to much hp at too low of an
    rpm, out of an engine, the likelyhood of detonation increases. (Also the
    temperature rise of the intake air aggrevates this situation.) That will
    damage an engine. Modern cars enrichen the mixture to decrease this from
    happening. I don't know what sort of hills your driving on. If you hear
    pinging or a rattling sound during this event you are damaging the
    engine. The exhaust gas temperature also goes way up and can cook older
    turbochargers. I don't know what is in your car. I do suspect though,
    she is asking a bit too much to be in overdrive going uphill. Especially
    some hills I have in mind.
     
    The Visitor, Jan 7, 2006
    #14
  15. mtb Dad

    Boris Mohar Guest

    Maybe she needs this: http://www.viatrack.ca/



    Regards,

    Boris Mohar

    Got Knock? - see:
    Viatrack Printed Circuit Designs (among other things) http://www.viatrack.ca

    void _-void-_ in the obvious place
     
    Boris Mohar, Jan 7, 2006
    #15
  16. mtb Dad

    AB Guest

    In general high revs wear the top end of the engine (rings, cams and valves)
    whereas low revs wear the bottom end of the engine (bearings). As long as
    the engine isn't lugging it should be ok although personally I tend to keep
    the revs between 2000-3000 for general driving (S70 T5).
     
    AB, Jan 13, 2006
    #16
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