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Torque RPM at Shifts.

  1. Nov 11, 2009 #1
    Hi everyone
    I am damn confused about torque and rpm variation.Please tell me:

    1.> When we shift from lower to higher gear, definitely RPM at wheels increase, but does the torque at drive wheel increases or decreases?

    2.> I read yesterday that a car accelerates fastest at an RPM corresponding to peak power because it transfers max torque to drive wheels. Then what is the significance of peak torque because i thought car would acclerelate fastest at peak torque.
     
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  3. Nov 11, 2009 #2
    Torque will DECREASE as you shift into higher gears. You are trading torque for RPM. Try it in your car. Floor it in 1st gear and floor it in a higher gear (at the same RPM).

    For second question, at any given speed, a car will accelerate fastest at its peak power. However, in any given gear, the car will accelerate fastest at its peak torque.

    At first these might seem in conflict with each other, but I assure you they're not. It might take some mental wrestling you get you mind around this (at least it did for me), but I suggest you deal with it because that's how it works. Took me a while to grasp it, and it felt enlightening when I finally did....

    ...at least I hope I'm not the only one...
     
  4. Nov 11, 2009 #3
    Your answer to my second ques is right and i am trying to wrestle in my mind.
    But for my first question first of all i didn't talk of engine torque but torque at wheels. What will happen to torque at wheels as I shift? Please reconsider or if i'm wrong notify me!
     
  5. Nov 11, 2009 #4
    The torque at the wheels will drop. It is greatest in 1st gear (easily in the thousands of lb-ft) and decreases with each shift, as you trade torque for RPM. If you could keep the engine RPM and torque constant, as with a continuously variable transmission (CVT), you would see that the torque at the wheels would be constantly dropping with acceleration.

    On that note, in order to accelerate fastest, a car with a CVT should keep its engine at its POWER PEAK. Even if the engine could produce 1000lb-ft, and 100hp...keep it at its power peak for greatest acceleration.

    To help your mind wrestle, consider going to get some groceries with this dyno graph. Lets say you're in 3th gear, and that the RPM at the bottom also correspond to speed (6000RPM=60mph, 7500RPM=75mph). Imagine you're at 6500RPM (65mph) and you gently press floor it. Your acceleration will rise with the torque curve, will peak at around 7500rpm (75mph), and then will slowly drop. So there...in 3rd gear, your acceleration is greatest at 7500RPM (75mph). BUT....keep flooring it, because you're still not at the power peak. The power peak will arrive at 8300RPM (83mph).

    Now you will be accelerating a bit slower than you were at 75mph. However, it's important to realize that even though 3rd gear gives the best acceleration at 75mph, you could have theoretically accelerated even faster at 75mph, if you had a different gear ratio for example that would put you closer to the power peak. At 83mph...that's all you got. You are wringing everything you can out of the car, and any change in gear ratio would just slow you down.

    http://advancedstreetperformance.com/img/dyno_S2000-oem-vs-asp.jpg [Broken]

    I hope I didn't confuse you...
     
    Last edited by a moderator: May 4, 2017
  6. Nov 11, 2009 #5
    This means max accleration can be achieved only in first gear.
     
  7. Nov 11, 2009 #6
    Right. And that's the way it is.
     
  8. Nov 11, 2009 #7

    Ranger Mike

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    for purposes of the basic discussion, yes...if one is racing a stock grocery getter ( honda , subaru , prius?? 4 cylinder ) in reality, maximum acceleration may occur in second or third gear... let us review- at launch the vehicle is stationary...one can count on a lot of tire slippage when the clutch is engaged..wheel spin kills acceleration..on muscle cars one had to feather the gas pedal in first gear to keep from burning the tires ...so second gear provided the maximum acceleration in many cases. it depends upon the cars configuration
     
  9. Nov 11, 2009 #8
    But in higher gears we get higher rpm at the expense of less acceleration as compared to first gear i.e. in higher gears the torque provided to accelerate per unit speed is less as compared to small gears but that torque persists over a long range to increase the speed overall. Is it so?
     
  10. Nov 11, 2009 #9

    Ranger Mike

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    Look at the graph by Lsos
    think or torque as low end horsepower or
    in my opinion ..it is easier to think of
    horse power as high end torque...
    the torque curve is relatively flat and maxs out at 136 ft pounds..only 10 ft pounds over the 3000 rpm value..so torque launches the vehicle and horsepower ramps up very nicely
    from 3000 RPM to 8000 RPM...what must be done to take advantage of this very linear curve is to match the gearing to make max use of the curves
    you will get acceleration over the entire RPM range when this is done
     
  11. Nov 11, 2009 #10
    Lsos
    in higher gears we get higher rpm at the expense of less acceleration as compared to first gear i.e. in higher gears the torque provided to accelerate per unit speed is less as compared to small gears but that torque persists over a long range to increase the speed overall. Is it so?
     
  12. Nov 11, 2009 #11

    Ranger Mike

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    GXP at the drag strip...

    --------------------------------------------------------------------------------



    Car Setup: 2000 miles on the car. Still stock when this run was made.

    Conditions:
    Temperature:...................91 degrees F
    Humidity:.........................75%
    Barometer:......................28.93 in. HgA
    Track (MoKan Dragway):...First racing after 21" of rainfall...poor traction

    Best result:
    60' ET..............................2.0954
    330' ET............................6.0196
    1/8 ET.............................9.2810
    1/8 Trap.........................75.40
    1000' ET.........................12.0436
    1000' Trap......................87.63
    1/4 ET............................14.4059
    1/4 Trap.........................94.28
    __________________
    2007 Mysterious 5-speed purchased 05/04/2007


    note the times at the 60 foot mark, 330 ft mark 670 ft mark , 1000 ft mark and 1340 mark..
    this is increasing acceleration..

    it took 2 seconds to run 60 ft.
    6 sec to reach 330 ft but only another 3.28 seconds to go another 330 ft and only 2.76 seconds to go another 330 ft and only 2.36 second to go the last 340 feet!
     
  13. Nov 11, 2009 #12
    Ranger Mike, I'm not convinced your conclusion of "this is increasing acceleration" is correct. Just a quick glance at your speeds...0-75.4mph is 9.28 seconds. 0-94.28mph is 14.4 seconds.

    It took just about 9 seconds to get to 75mph, but it took another whole 6 seconds just to go 20mph faster. The speed is increasing as you get further down the track, so obviously the distances will go by faster. But while speed is increasing, your acceleration is still dropping. Fast.

    Besides, with that 1/4 mile time, you should have absolutely no problem with traction, except for a small part of 1st gear (depending on front/ rear wheel drive?). After that the acceleration should drop...as it does.

    Here's how a typical car's acceleration curve looks like. Notice the change in slope after each gear shift. Road and Track magazine publishes these for alot of cars....

    http://www.p914.com/p914-resources/p914-numbers/p914-numbers-accel-comparison.gif [Broken]
     
    Last edited by a moderator: May 4, 2017
  14. Nov 11, 2009 #13
    Here is a little physics:

    F = ma (force, Newtons)

    F dx = E (work, joules)

    F dx/dt = F v (force times velocity = power (watts) or HP x 746)

    So HP = m a v/746 =mass x acceleration x speed / 746 (in mks units)

    So at higher speeds, acceleration is less.

    Bob S
     
  15. Nov 12, 2009 #14
    Its easy to get confused about this as more torque should net more acceleration (at that instant). And in fact, this is actually true in any given gear. If you measure instantaneous acceleration in every gear, the fastest acceleration in each gear will occur at the torque peak. Where this becomes more clear is when you relate it to a vehicle's speed. Most cars on the road use fixed sets of gears. But if you chose a particular speed (say 60 mph) and calculated 2 gear ratios where one would place your RPMs at the torque peak at that speed and another that placed the RPMs at the HP peak, what you would find is that the acceleration at that instant would be faster at 60 mph with the gear that placed the RPM's at peak HP. And there would actually be more torque at the wheels as well. Why? Because the higher RPM operation provides more leverage to the available torque.

    Consider:

    HP = (Torque x RPM) / 5252

    To save myself some time, I'm going to paste something I wrote-up elsewhere so please forgive me if it seems out of context:

    "If you take 2 different motors that both produce 300 HP. One does so at 4000 RPM (Motor A) and the other at 8000 RPM (Motor B). If you do the calculation, the Motor A will be putting out 393.9 ft-lbs of torque at 4000 RPM. Motor B will output 196.95 ft-lbs at 8000 RPM. That's exactly half the torque output of Motor A, but at twice the speed.

    Now if you take Motor A and directly output to the wheels (1:1 gearing), then the wheels will be putting out 393.9 ft-lbs of torque when the wheels are spinning 4000 RPM. Now take motor B and gear it 2:1 to the wheels. When the engine gets to 8000 RPM, the wheels will be turning at 4000 RPM. Same speed as the car with motor A. And get this, since the gearing will double the torque, the car with motor B will be putting out 393.9 ft-lbs of torque at the wheels. Same as the car with Motor A. HP makes it easy to make a more direct comparison of output as it shows the true potential.

    As you can see, having more RPM basically gives you a bigger lever. Torque is simply how much weight you put on the lever. HP is the measure of how much can be lifted based on the weight and lever arm length."


    To add to this, the reason Peak HP provides the most torque to the wheels is because it represents the point where the engine has the most leverage for the amount of torque it puts out at the corresponding RPM. Its hard to see in cars with traditional transmissions, but if you consider a CVT, where a car can accelerate while its engine can be held at a specific RPM, you'll find that for a given vehicle speed, operating at peak HP will yield more torque at the wheels than operating at peak torque.

    As for the significance of peak torque (as published for consumers by vehicle manufacturers), ultimately it has very little meaning. At best it might provide some insight into the shape of the torque curve (flatness/broadness) and its potential uses (trucks tend to need plenty of low-end torque to help get things moving and keep things comfortable when towing/hauling loads; imagine having to climb a hill with a trailer at 6000 RPM the entire way; gas mileage would be terrible as well). But unless a consumer knows what they are looking for and/or how to interpret it, its just a pretty number manufacturers flash in front of our eyes. Higher RPM motors are becoming more prevalant in sports cars and these tend to boast relatively low amounts of torque compared to the HP they have. Haven't followed F1 for a while, but those motors are pretty small (2.6L?) but can achieve some astronomical HP levels (700ish last I remember) without any turbos or supercharging (granted I think they run methanol so a direct comparison is not entirely possible). They rev over some 15,000 RPM (I wanna say 18,000 was about as fast as they got at some point) and probably make less torque than most consumer V-8's. 700 HP at 15,000 RPM equates to 245 ft-lbs of torque...
     
  16. Nov 12, 2009 #15

    Ranger Mike

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    [/URL]
    i think you are mixing apples and oranges...
    Speed x Time = Distance and comparing speed from 0 MPH is not an indication of Acceleration. If acceleration stopped and a constant velocity was established the times at the 330, 670 , 1000 and 1340 feet marks would be the same...the car continues to accelerate as it covers the 1/4 mile
     
    Last edited by a moderator: May 4, 2017
  17. Nov 12, 2009 #16

    Mech_Engineer

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    Lsos is correct, ideally a car's maximum acceleration is in first gear, and it can be seen in the graph he provided.

    http://www.p914.com/p914-resources/p914-numbers/p914-numbers-accel-comparison.gif [Broken]

    This graph is velocity versus time, which means acceleration is defined as its slope (derivative). The slope is highest off the line in first gear, and decreases as the car accelerates and shifts into higher gears.

    This has to do with the fact that higher gear ratios in the transmission transmit lower torque to the wheels, but also in the fact that kinetic energy is related to the square of the velocity. For a car accelerating from 0-60 mi/hr, it takes 11 times as much energy to accelerate from 50-60 mi/hr as it does from 0-10 mi/hr. It takes 21 times as much energy to accelerate from 100-110 mph.
     
    Last edited by a moderator: May 4, 2017
  18. Nov 12, 2009 #17
    I'm avoiding this bloody discussion again like the plague.

    EDIT: It's seems I just can't resist temptation on a good torque vs power thread *sigh*
     
    Last edited: Nov 12, 2009
  19. Nov 12, 2009 #18
    For your original question:
    You need to know 1 equation for this.

    POWER = Torque*RPM.
    all measured at rear wheel

    The engine only puts out so much power so this figure MUST stay the same. If you gear up for torque, RPM must go ddown. You gear for higher RPM, torque must decrease.

    That's really all there is to know.

    At any given speed you will accelerate fastest at peak power. In any given gear you will accelerate fastest at peak torque.

    This is becuase gears allow mechanical advantage (they multiply torque). Peak power occurs at a higher ENGNIE RPM than peak torque. Meaning that to get the same wheel RPM and therefore same speed the wheels you can multiply the torque more.

    eg. Wheel speed needs to be 1000rpm.

    Peak torque at 4000rpm (engine) 10nm torque ourput at this speed
    Peak power at 6000rpm (engine) 8nm torque output at this speed

    Ratios required:
    Peak torque: 4:1 reduction
    Peak power 6:1 reduction.

    So torque at wheels:
    Peak engine torque = 40nm torque at wheels
    Peak engine power = 48nm torque at wheels.

    NOTE: Do NOT confuse wheel speed with engine RPM
     
    Last edited: Nov 12, 2009
  20. Nov 12, 2009 #19
    You're right, the car does continue to accelerate as it covers the 1/4 mile. But, earlier you wrote "this is increasing acceleration", which it is not. Perhaps you meant "this is increasing speed".

    The car might be accelerating, but it is doing so at a decreasing rate, not increasing.
     
  21. Nov 12, 2009 #20

    Ranger Mike

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    I agree..was not correct in the wording but ..this is what the forum is good for..
     
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