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Torque in different car gears?

  1. Feb 22, 2016 #1
    Recently I have been researching torque and horsepower.

    I know that the lower the car gear, the more the torque.

    However, whenever I see specifications of a car, it says :

    X lb-ft of torque at Y rpm.

    Now, shouldn't they specify the gear that was measured in? Or is this the torque the engine makes, measured before the gearbox?

    I am a little confused, I would be happy if you could help me with some explanations.

    Thank you!
     
  2. jcsd
  3. Feb 22, 2016 #2

    billy_joule

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    Engine torque values are referenced to the crankshaft.

    Compare the output plots of a test stand dynamometer and a rolling road dynometer.
    They both reference the engine RPM, even though the rolling road measures the torque at the wheels.
    Why? Engine RPM is universal so comparisons are straight forward. wheel RPM depends of vehicle speed, gear, wheel size, diff ratio etc so most comparisons are essentially meaningless (or at least would take more thought to interpret correctly).
     
  4. Feb 22, 2016 #3

    rcgldr

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    Most chassis / road type dynamometers measure the torque to drive the dynamometer and the speed at the driven surface of the dynamometer, allowing them to calculate power, without the need to have engine RPM as an input. When the engine RPM is input then they can calculate the overall effective gear ratio by noting engine rpm versus surface speed of the dynamometer, allowing them to calculate the engine torque after drive train lasses. A test stand dynamometer is connected to a stand alone engine, perhaps with some of the components such as a water pump or alternator connected, and directly measure engine torque and rpm at the output shaft.
     
  5. Feb 22, 2016 #4

    cjl

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    As you correctly guessed, the torque specifications for cars are measured at the engine, prior to the gearbox. The torque at the wheels will definitely depend on which gear you are in, as well as on the efficiency of the gearbox and drivetrain.
     
  6. Feb 22, 2016 #5

    billy_joule

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    Science Advisor

    Yes, and the engine torque has no use if it's shown against wheel rpm. My point was, using crank rpm gives a consistent x axis scale to compare torque (and more importantly, power) output between engines. I've never seen a rolling road use wheel rpm on it's plots x axis.
     
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