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New Definition derivative for Horsepower

  1. Sep 16, 2016 #1
    There has been SO much talk of HP and HP vs torque, it can make your head spin. Ive been trying to help with some clarification for those that seem to be very confused of the physics and concepts that i came up with a new "definition" to augment what is commonly read as the definition of HP. In my work in the auto Racing field, there are many enamored with the term "torque", often confusing it with the actual torque they care about, that of what is found at the rear tires (after the gear box).
    Because HP is the rate of doing work (basically means acceleration of a mas at any particular speed) or the rate of change of KE. ( again, acceleration of a mass). HP, should be synonymous or equivilant to saying, its " a body's capacity or ability to accelerate at any speed).. I thought of this when i remembered the definition of energy being a body's capacity to do work. Well, how fast you do that work is power, (rate of KE change) so hence the new definition.

    This way, it takes all the debate out of what HP really is... .Its not just a calculation. torque is not the "star" in most peoples minds for a vehicles ability to accelerate, it's power. Saying torque is the 'star" is like looking at work and saying the lever/distance is the star or weight/force is the star. Both are needed to know anything relevant.

    so, Horsepower IS " A measure of a vehicle's capacity to accelerate"

  2. jcsd
  3. Sep 16, 2016 #2


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    What's wrong with James?
  4. Sep 16, 2016 #3


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    Most scientists wouldn't be happy defining horse power that way. For example if one car can accelerate twice as fast as another that certainly does not mean it has twice the horsepower. The expression "a measure" is far too loose.

    In my opinion there is nothing wrong with existing definitions of power.
  5. Sep 17, 2016 #4
    Its easy. Remove the word horse. Then use the units of kilowatts.

    People think of 'a horsepower' as a phyical entity rather than a quantity. Then 'torque' as an entirely different entity. It annoys me.

    Rather than changing definitions, change the people that use them incorrectly.
  6. Sep 17, 2016 #5


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  7. Sep 25, 2016 #6
    This makes so much sense now, I always wondered why some engines were able to produce more TQ than HP.
  8. Oct 1, 2016 #7

    Randy Beikmann

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    Like the OP, I dislike the "Torque is a measurement, horsepower is a calculation" explanation. It does nothing to clarify things for me. Plus, torque itself is "measured" on a dyno by measuring strain in a force transducer, calculating the force from that, and then multiplying by the lever arm distance to the force transducer.

    Torque and horsepower are both physical quantities, and we can use them as two different ways of looking at the same situation, and in the end, come to the same conclusions. Usually one way is simpler than the other. One major difference in using them is that power, as you've said, is not tied to rotation (it's more general). It can be either force X velocity, or torque X angular velocity. So as you say, you can find the force available at the tire patch from F=P/v. Once you subtract the force to overcome drag, you can use a=Fnet/m. But even after that, remember there are other factors.

    Three things can limit acceleration in a car: 1) the amount of traction at the driving tires, 2) wheel stand, and 3) power. The acceleration you can achieve is the lowest of these three. A lot of space is used to discuss this in my book Physics for Gearheads, including the effects of weight distribution, load transfer, and traction coefficients. The idea is to bring in the design and tuning choices made in the vehicle.

    To summarize, I wouldn't quite say that power is the ability to accelerate. At low speeds, traction and wheel stand resistance determine that. At higher speeds, power does determine the limit.

    Isn't this fascinating stuff?
  9. Oct 1, 2016 #8


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    The question, "Which matters more, power or torque?" is a false dichotomy. I don't feel they are two ways of looking at the same thing except in special situations.
  10. Oct 1, 2016 #9


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    Power and torque cannot be directly compared because they have different dimensions.
    Torque must be multiplied by angular velocity before any comparison can be done.
  11. Oct 1, 2016 #10

    Randy Beikmann

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    I'm not sure what you mean by their not being two ways of looking at the same situation. If you want to use engine torque, you need to know the torque (which is often unknown), go through a few gear ratios and driveline efficiency, and know the tire radius. But if you know the vehicle speed, mass, and acceleration, you can calculate the power to the ground from the straight-line motion. It all depends on what is known in the situation.

    I agree that the "torque vs. horsepower" is overblown, at least from a theoretical viewpoint. With the same driveline efficiency, horsepower is what matters. BUT, for the same horsepower, lower-torque engines need more gear reduction (at a given vehicle speed), likely increasing the effective mass of the engine/flywheel inertia that is added to the mass of the car. And typically they are more difficult to drive, having narrow power bands.
  12. Oct 1, 2016 #11


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    I mean they aren't the same for the reason balancore stated. It's like comparing wealth and power. Different things that can sometimes be related but scientifically are quite different.
  13. Oct 1, 2016 #12

    Randy Beikmann

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    I think we actually agree, but I'm not sure. ;-)
  14. Apr 4, 2017 #13
    actualy, since HP is the rate of change of KE, it would be impossible for two identical cars with all the same conditions , to NOT accelerate at the same rate if the HP for both were the same. KE start - KE end = X Joules X Joules/time =power.
    power is directly proportioanl to acceleration at any same vehicle speed (same mass car (and all other conditions) comparison)
    acceleration = power/(mass x velocity) is true, right? so you cant have half the power and ever accelerate at a greater rate than another car.

    yes, i agree.... im all about rate of change of KE.... joules! watt-seconds or kw-seconds or even HP-seconds. its what you want to optimize on a race car to make it faster than your competition.

    this is also a pet pieve of mine. torque EVER mentioned in the same sentence when using them to be compared to one another. they are not related other than multplying one to get the other. torque can be in Nm or in-oz, and power in KW and the curves might never cross and could not be compared numerically. you cant have "more torque than HP" it doesnt make sense.. ............though i know what you mean... and because we have so much data, yes, a engine with high "numerical" levels of torque in "Lb-Ft" vs HP will generally be a long stroke comparatively or larger displacement engine. but it doesnt do anything any better than a little tiny engine with little torque and the same HP if hey are both geared optimally.

    but saying all the lmiting factors are the same for two cars, its HP or power than determines its rate of acceleration at any vehicle speed.. thats why i like the new definition to be , HP or power is a vehicles ability to accelerate. torque , engine torque that is, is meaningless. rear wheel forces, and rear wheel torque, after the gear box is relevant, but needs much more factors to be determined to tell anything of its potential. ratios, wheel diameters, etc. HP says it all. AND, a dyno will often times just measure the rate of change of KE........ that IS power!!!! the drums go from one RPM to another in 1 second, and the KE change is X Joules you just take the X Joules and muliply x 746 to get HP. simple!
  15. Apr 4, 2017 #14
    not really.. Torque is completely usless in most cases , where HP or (power) tells you your acceleration potential or ability or utilization at any vehicle speed.. torque obviously is the root of all the force to accelerate but it doesnt tell the entire story comparatively, until you do many more calculations.

    i remember a thread i started a long time ago, where you were getting frustrated with me regarding this. i think now that i understand your objections, saying that HP is a vehicles capasity to accelerate at any speed is good, because it doesnt discount the real cause of acceleration , force (or torque), But gives a very clean way to compare vehicles performance with no other factors known (as long as they are the same in the comparison)


    actually, now the ligher, lower torque vehicles ar very easy to drive. in some cases a higher numerical value torque car might be more forgiving if you are in the wrong gear, but if you drive them both correctly, there are as many advantages as disadvantages given the same HP. lower torque , higher RPM engines can have lighter rotating components, closer ratio gears, lighter weight engines, to counter act any perceived advantages. this is why the cars that are winning most of the races these days are cars powered by smaller higher reving engines. Like F1, power king.. the rest is just optimization. ;)
  16. Apr 4, 2017 #15

    Randy Beikmann

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    When you say "lighter, lower torque vehicles," you are changing two things: mass and torque. Given the same overall vehicle mass, and two engines with the same power, but one having 2/3 the torque, the one with less torque has to run 50% faster to produce the same HP and vehicle acceleration. The two engines may both produce the same effect, but many/most people would prefer the torquier/slower engine because there's less fuss. I remember when the Honda S2000 came out everyone raved about the naturally aspirated 2.0 L engine with 240 HP (quite a feat), but many people didn't like managing the engine, having to keep speeds in the 7000-8000 RPM range for performance. They bumped the engine to 2.2 L, keeping the power the same, but increasing torque 6%. I think it became more enjoyable to drive then.

    In absolute terms, both 240 HP engines should (giving the right gearing) provide the same performance, but people tend to prefer torquier, easier to drive engines. As you say, though, race drivers prefer whatever is fastest, as long as its fussiness doesn't distract them from the rest of their driving duties.
  17. Apr 4, 2017 #16
    Randy, yes, i meant lighter lower torque vehicles that have the same HP/weight , but i should have just stayed with the same platform mass. the difference being, like you noted, a wider range of usable HP , in some cases. now a days, the gear ratios are matched to the HP usable range, so vs the "high torque " brothers in the same weight car, (racing viper vs racing porscheGT3RS) the weight isnt much different .. the HP isnt much different, the torque is about half , and both cars accelerate the same at any speed anywhere on the track.........even though there are some "torquies" that still think torque is important, and it isnt in an absolute sense. sure around town, the bigger grunt of the viper down low, can allow you to use 4th gear from a stand stilll almost . ;) but driving the car as designed, both cars get to use about 92% of their available HP all the time over any race track. (shift to shift)
  18. Apr 5, 2017 #17
    A vehicle that produces more torque at a given RPM than another vehicle at that same RPM will have more horsepower at that given RPM.

    Torque is the measure of the magnitude of a twisting force that tends to cause rotation. You can apply torque to something without having it move. Think about using a wrench on a bolt that refuses to budge. You are imparting a twisting force onto the bolt, but not moving it.

    Horsepower is merely a unit of work done over a given period of time. To make one horsepower, you need to do 550 ft-lbs of work in one minute. That means you can lift 1lb 550 feet, you can lift 550lbs 1 foot, or any factor pair equaling 550 ft-lbs. Think of torque as the amount of weight that you are lifting, and horsepower as a function of how much work you can do in one minute.

    Lets say there are 3 guys hauling sand with their shovels. Guy #1 can scoop and haul 50lbs of sand in one trip, but he can only make 3 trips per minute. Guy #2 can scoop and haul 10lbs of sand the same distance as guy #1, but he can do the trip 15 times per minute. Then there is guy #3. Guy #3 can scoop and haul 50lbs of sand the same distance as the other guys, but he can do it 15 times per minute. Guys 1 & 2 have the exact same amount of horsepower. Guy #3 has 5 times the horsepower of the other two guys.

    Here is the basic equation for calculating horsepower. HP = (Torque X RPM) / 5252
  19. Apr 5, 2017 #18
    No argument with you when referring to engines or vehicles with manual transmissions. A vehicle with an automatic transmission will have it's RPM, HP and torque relationship determined by the choice of torque convertor.
  20. Apr 5, 2017 #19
    As an agent for a European auto co. in the 90's I met and worked a lot with Jim Thornton. He's credited as the inventor of the Funny Car and torque cancelling chassis along with being the driver for Chrysler's Ramcharger Team and Head of their racing division. We spent a fair share of time discussing how engineers from the different companies get along and share with each other and how outright stupid customers can get. It drove Jim nuts because people expected him to see only Chrysler like they did. He never said this but I got the impression it's why he left Chrysler. People can relate to numbers and they depend on numbers to justify decisions they make. Whether they can define what the numbers mean or not there needed to justify opinions and choices. Overall there is not a whole lot of difference in cars made to meet the same price points and purposes. Most men though will argue numbers before they argue the aesthetics of the vinyl used on their trucks dashboard.
  21. Apr 6, 2017 #20
    RogueOne , If you read the comparison, we were talking about same HP engines. (one with more torque than the other). Horsepower is the rate of doing work, and this by definition is a capacity to accelerate. a rate of change of KE is a HP as well, and thats how dynos measure HP. the point is, torque is the force that causes the rotation, but if its rated at the engine its relativity meaningless, unless you have a bunch more data points ( RPM, gear rations, tire diameters, etc) but, if you have a HP value you have a much better indication of its acceleration potential than if you only have a engine torque value. sure, if you had rear wheel torque values after the gear ratios, this would give you the exact same information . while two cars may make different HP at the same RPM with different torque values, the one with the most HP can produce the greatest forces at the rear wheels at any same speed. that might be at a higher RPM and a very low torque value.............HP is a great indicator, much better than engine torque values , simply because it contains more information.
  22. Apr 6, 2017 #21
    This is a nonsense thread. Zanick your premise was to redefine 'Horsepower' as a vehicles capacity for acceleration based on the premise "how fast you do that work is power, (rate of KE change) so hence the new definition."


    How is that, in any way, a redefinition? Infact, it's a tautology. You are simply restating THE definition power is rate of work done. The work in this case is accelerating the mass of the car around a track. If you live in an abstract world, where you just want a quick calculation of how fast you can go fine.

    My objection to threads like this is the inevitable broad statement that "engine torque is totally meaningless". Engine torque defines the maximum possible loading down the driveline. If you actually want to build the damn thing in real life, you need to know and consider torque at points down the drive line because we don't live in an abstract world. Real components made of real materials break if you overload them. A car with a broken gearbox, or broken driveshafts has zero acceleration.
  23. Apr 6, 2017 #22
    If you elaborated on that idea more, I could tell you why its false. As of right now, I don't even know what you are trying to say. I'm trying to understand why you would think this unless you are measuring torque at the crank, and measuring horsepower at the rear wheels without measuring torque at the rear wheels. (not possible, btw) and then referencing it by the crankshaft's RPM rather than the roller's.

    Are we all clear that dynamometers measure torque, and that the engine speed at which that amount of torque is produced is how horsepower is made? I am trying to find out how exactly you guys think horsepower and torque are measured at a given RPM. The engine with more torque at a given RPM is always the engine that has more HP at that particular RPM. The vehicle that produces more torque at the rear wheels at a given wheel speed is always the one that makes more horsepower at that wheel speed.
    Last edited: Apr 6, 2017
  24. Apr 6, 2017 #23

    jack action

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    This is not what "engine torque is totally meaningless" means. It means that power alone gives more info about the capabilities of an engine than torque alone. Actually, torque alone says nothing about the capabilities of an engine, thus the use of the word "meaningless".

    If you have an engine having a certain power-rpm relationship, connected to a driveline, you will have a certain torque-rpm relationship along that driveline.

    If you put another engine, having the same power but at half the rpm, you just need to connect a 1:2 gearbox between the engine and the driveline to obtain the exact same torque-rpm relationship everywhere on that driveline.

    If, instead of engine power, you try to match the engine torque curve for both engines that are at different rpms, no gearbox will be able to match the torque-rpm relationship of both drivelines.
  25. Apr 7, 2017 #24
    Thread becoming a normal torque vs horsepower 'debate'
  26. Apr 7, 2017 #25
    Sorry, stall speed and rotational equilibrium. Accepting a steady RPM without a change in acceleration as rotational equilibrium where torque is zero.
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