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Horsepower vs Torque Debate

  1. Dec 17, 2015 #1
    My dad and I are having a debate about HP and torque. He thinks that a 400hp diesel with 800ft lbs of torque will have more power and out pull and have better acceleration than a gas engine with 400 hp andf say 300 ftlbs of torque. As I understand it, the relationship between HP and torque is as follows

    HP = (Torque x RPM) / 5252

    So ultimately HP is what determines the power of a vehicle. Who is right?
     
  2. jcsd
  3. Dec 17, 2015 #2
    Hp is Hp. However, the same Hp with less lb.ft. implies greater rpm to make up for the lower torque output. Theoretically, it is torque that translates rotary force to linear force. By that, I would say that the lower torque machine must "rev up", which takes more time as well as greater gearing ratios to, once again, make up for the lower torque output. But, once again, power is power, no matter how you slice it. High torque engines are like mules- slower but more powerful. Low torque engines rev up higher and, going by what I've seen at the dragstrip and elsewhere, they rule in all accel challenges beyond the initial launch. But, hey, that's just an opinion.
     
  4. Dec 17, 2015 #3

    russ_watters

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    Staff: Mentor

    Disagree; most of what you said were facts. :-p

    So yeah, as you said, power is power, but the main difference might be that during acceleration you aren't just delivering power to the wheels, you are also delivering power to accelerate the drivetrain. So the vehicle with the simpler drivetrain - the higher rpm/lower torque one - should have slightly better acceleration.
     
  5. Dec 18, 2015 #4

    russ_watters

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    Staff: Mentor

    So... two people liked this post (thanks), but it looks to me like I said the conclusion backwards:
    No, the lower rpm motor does less accelerating of Tl itself and has a simpler (less ratio reduction) drivetrain.
     
  6. Dec 19, 2015 #5

    rcgldr

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    Homework Helper

    If the reduction is taken care of in the transmission, then the rest of the drivetrain rotates at the same rpm.

    I think the real issue is the shape of the torque curve. If low rpm torque is reasonably close to peak torque, then less clutch slippage is needed for the initial launch.
     
  7. Dec 22, 2015 #6
    Horsepower IS a unit of power. Torque, is not. So, by definition, the car with more horsepower has more power. 800 ft-lbs sounds like a lot, but anybody could make 800 ft-lbs. Hell, my cat could do it, given the right gearing. Lots of torque might help you, but it might not, that’s a matter of engineering. But in the end it’s the power at the wheels that matters, and for that you need power at the crank, not torque.
     
  8. Dec 22, 2015 #7

    cjl

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    Horsepower is the more important parameter, though you don't just want to look at peak horsepower. Ideally, you want high horsepower over a relatively wide range of engine speeds - an engine that makes >300hp from 3500 to 8000RPM with a peak of 350 will probably feel more powerful and usable than one that peaks at 400 but is below 300 at all engine speeds except a narrow peak from 7000-8000.
     
  9. Dec 28, 2015 #8
    Everyone seems to dance around the bush on this topic, so ill address it directly.

    first truth: acceleration = power./(mass x velocity).
    this means at any speed, who ever has the most HP will have the greatest acceleration.
    2nd truth. average Hp will determine the average acceleration.
    so, if you are comparing two 400hp engines and both have the same HPcurve shape, (even though one might have 1000f-lbs and the other 500ft-lbs max, both cars will accelerate the same.

    so,you have two trucks, both with 400hp and one with 1000ftlbs of torque (diesel ) and the other with 500ftlbs of torque (gas)
    generally even though the diesel might have a flatter HP curve than the gas engine... if both can dump their clutches at the same HP spot on the HPcurve, both will produce the same acceleration when the tires hook up (if the are at the same HP level at that point.. this becomes another discussion so lets just look at the point at which they both have traction) . if there is any difference, this can be made up with gear box changes with closer gears to optimize the power available.

    the acid test is this. if we have infinitely variable gearboxes, where would the engines operate at... max torque or max HP???? thats rigiht, max HP. HP is the rate of doing work, so the faster you do work, the greater the acceleration. same HP, same acceleration. as long as the curves are the same shape , or the gear box allows for maximization of the HP curve.


     
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