Cars We Drive!

Discussion in 'Fine Living, Home, Design & Auto' started by Bert1568, Jul 18, 2006.

  1. HRoi

    HRoi Senior member

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    yes sir - e90 M3, not M5
     


  2. KurtS90

    KurtS90 Senior member

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    I think thats an M3 sedan?

    Edit: Failed to check next page...
     
    Last edited: Apr 15, 2012


  3. GreenFrog

    GreenFrog Senior member

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    guys that's a 328i

    :sarcasm:
     
    Last edited: Apr 15, 2012


  4. Sir F

    Sir F Senior member

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    Nice, so what did you change the rims?
     


  5. Kookz

    Kookz Senior member

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    Can't tell if trolling or serious...
    All the torque in the world means nothing about how fast or quick a car is.
     


  6. Arthur PE

    Arthur PE Senior member

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    it's an e92 (sorry e90, e92 is the coupe) M3 sedan
    and very nice
    ~420 HP (DIN) and 295 lb-ft torque
    revs to 8400 or so
    50/50 wt distribution
     
    Last edited: Apr 15, 2012


  7. Arthur PE

    Arthur PE Senior member

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    torque is all that matters as far as acceleration, HP is associated with constant top speed, but really torque is doing the work
    torque is the force which drives/propels the car, an instantaneous primary quantity
    power is the rate of application of said torque, a derived/secondary quantity, P = T x w (w =2 x Pi x rev/sec) in our system P = T x rpm/5252
    5252 = 60 sec/min x 550 lb ft/sec (HP conversion) / 2 Pi

    F = ma
    F in this case is thrust, which is axle torque divided by wheel radius (torque = force x radius, it's a vector, so the cross product, some call torque moment)

    so if gearing is say 6 (tranny x diff) and engine torque is 300 axle torque is 1800 lb ft, if we say 15% losses ~1530 net
    if the tire radius is 12" or 1 ft thrust = 1530 lb-ft/1ft = 1530 lb (force)

    so a = F/m, so the more torque or the lower the weight the faster it accelerates
    as far as top speed, work = f x distance, so it is still torque, which not only moves the car against gravity/inertia, but against wind resistance and mechanical resistance (gear/engine friction, tire slip/friction, etc.)

     
    Last edited: Apr 15, 2012


  8. A Y

    A Y Senior member

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    I'm not sure how to respond to this. The entire post you quoted is serious and correct. Your use of torque without reference to RPMs (or gearing) is meaningless.

    To move a car requires force. That force is torque, so torque is what matters when you're trying to figure out how fast is a car. Torque is affected by RPMs because for equal amounts of torque generated at two different RPMs, torque at the higher RPM is more useful because it can be geared down more, multiplying its effect. Gears do the same thing, too. To accelerate a car quickly requires high torque in the RPM range in which you're accelerating.

    Power only comes into it as an expression of the torque at a particular RPM (horsepower = torque*RPM/5252). In cars, it is an abstract quantity that is indirectly related to the driving experience. It's a good shorthand for expressing a certain quality of a car, but unless you drive around near redline, with the throttle on the floor, it isn't easily relatable to the everyday driving experience of a car.
     


  9. Kookz

    Kookz Senior member

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    This is my whole point in quoting your post. The use of torque without reference to RPM and gearing is meaningless, but torque WITH reference to those IS the very definition of (horse)power.

    Everything you mention in your second paragraph is horsepower, you just seem to keep wanting to call it "torque at higher RPM".

    Power is work over time, torque has no reference to time and thus has no bearing on the quickness of a car.


    Code:
                                                    Volkswagen GTI            Volkswagen Golf TDI
    0-30 (sec):                                      2.8                               3.0
    0-45 (sec):                                      4.7                               5.3
    0-60 (sec):                                      7.0                               8.7
    0-75 (sec):                                      9.7                               13.2
    1/4 Mile (sec @ mph):              15.0 @ 95.2                16.4 @ 83.7
    0-60 With 1-ft Rollout (sec):             6.6                               8.3
    30-0 (ft):                                          34                                31
    60-0 (ft):                                         130                              121
    Skid Pad Lateral Acceleration (g):    0.87                             0.86
    Slalom                                             65.5                             69.2
    
     
    Last edited: Apr 15, 2012


  10. Arthur PE

    Arthur PE Senior member

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    force (or torque) has a time relative component

    F = mass x accel
    mass = kg (or slug)
    accel = m/sec^2 (or ft/sec^2)

    it is the second derivative of position/displacement relative to time (d2s/dt2) times mass

    if a car weighing 2000 lbs and with 2000 lbs of thrust (net axle torque / tire radius) it will accelerate at 1 g (ignoring tire slip/friction, drag, etc.)
    32 ft/sec-sec or 32 ft/sec^2
    if linear and 1 gear gets us to 60 mph (88 ft/sec, btw velocity is the first derivative of displacement) it will take ~ 88 ft/sec / 32 ft/sec-sec ~ 2.6 sec (units check, ft cancels, 1 set of sec cancel, leaving sec)

    I size and select engines for various applications and always deal in torque

    btw : engine torque = V x mep / 4Pi
    V = displacement
    mep, mean effective pressure ~ compression ratio x atm pressure x vol eff (x boost ratio (net) if forced induction)
    4 = after 3 and before 5, base 10
    Pi = 3.141596....not as in apple
    :D
     
    Last edited: Apr 15, 2012


  11. Kookz

    Kookz Senior member

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    Arthur, you mention axle torque which is absolutely correct, but that is a product of the power, not torque.

    In a fixed gear, a car will accelerate fastest at peak torque. However, a car will always accelerate fastest at peak HP given the ability to change gearing (which is why we have transmissions) because peak HP means maximum wheel torque, and has no relation to engine torque. An F1 engine makes less torque than a VW Golf TDI.
     


  12. Kookz

    Kookz Senior member

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    I take it that the PE means what I think it means (I'm a CivE) but you are telling me that torque is force, and that is incorrect. Pounds are a unit of force, and newtons are a unit of force, but ft-lbs and N/m are not units of force.
     


  13. Arthur PE

    Arthur PE Senior member

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    axle torque = crank torque x gearing (torque multiplication)

    power = torque x rotational speed = T x w (w = 2 Pi (a radian/rev) x n = rev/sec)

    power is (kind of) torque per unit time ~ torque x 1/sec
    or the rate of torque application

    if an F1 engine makes 800 HP at 18000 then T = 5252/18000 x 800 ~ 250 lb ft? (don't feel like getting a slide rule)


     
    Last edited: Apr 15, 2012


  14. Arthur PE

    Arthur PE Senior member

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    torque is a force, a rotational force (T = F x r) a moment
    lb-ft is torque (or force about a moment)
    ft-lbs is energy or work as in distance x force or feet x pounds

    wiki
    Torque, moment or moment of force, is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist to an object.
    Loosely speaking, torque is a measure of the turning force on an object such as a bolt or a flywheel. For example, pushing or pulling the handle of a wrench connected to a nut or bolt produces a torque (turning force) that loosens or tightens the nut or bolt.

    it's not a linear force, like thrust, but thrust x radius = torque




     
    Last edited: Apr 15, 2012


  15. Kookz

    Kookz Senior member

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    Which is exactly my point, so once again: torque does not matter when it comes to how fast a car is. That's not to say a car with a broad powerband (low end torque) doesn't make for a better drive, but that's because of power, not torque.
     


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