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Little question about torque vs bhp

Discussion in 'Chit Chat Room' started by ssidrdw, Feb 23, 2014.

  1. ssidrdw

    ssidrdw Racer

    Hi all
    As far as I understand, if you have for example the same torque between 3000 to 5000 rpm, you should accelerate at the same rate, for example from 60mph at 3000rpm@4 gear and 60mph at 4000rpm@3 gear.
    If this is correct - torque tells you about the power of the engine, whereas you could have more BHP on an engine with the same torque\rpm graph, but higher max rpm, because bhp=torqueXrpm. So that engine won't make you faster in any way.
    Yet people mainly talk about BHP even at professional car shows, although it is not the parameter that defines the engine power at all.
    So, what am I getting wrong ? :D
     

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  3. bondyboy

    bondyboy Alien

    what's your question again?

    torque will rarely be a constant through a 2000rpm range, it'll slightly different (then shows a print of mine where it's constant between 6500 and 7500 doh!)

    [​IMG]
     
    Backmarker likes this.
  4. ssidrdw

    ssidrdw Racer

    what's the point of commenting not to the topic if you even admit you didn't understand the question?
     
  5. Akis Kev

    Akis Kev Alien

    That's what you got wrong. Torque at the wheels depends on the gear you have.

    [​IMG]


    On the other hand, you can directly compare the hp output between the gears because the values of this unit are not affected by gearing (I'm not getting in many details but you get the point I hope!).

    [​IMG]

    For maximum acceleration you have to be constantly near the revs that the engine outputs the most hp.
     
    Last edited: Feb 24, 2014
    djkostas and Queequeg like this.
  6. ssidrdw

    ssidrdw Racer

    But first, your graph shows exactly the opposite- you don't get anywhere near maximum torque in the wheels on the maximum rpm (highest speed for the gear),
    Second, I really don't understand how it is related? If you compare 2 engines, the difference between their power is the same whether they are connected to the wheels or are held in the air ... How does the fact that HP is the same across different gears and torque varies by gear ration, means that HP is the best parameter to compare engines acceleration ? The contrary, the fact is that you get less acceleration the higher the gear is, the torque graph shows that, and the HP graph doesn't (it doesn't represent acceleration).
     
  7. Akis Kev

    Akis Kev Alien

    The graphs are equivalent.
    The best way to compare accelerations are the 0-100 km/h, 0-160km/h, 0-400m, 0-1000m metrics. You just need these graphs to know where to shift.
    You shift at the point the curves intersect. If they don't intersect, you shift just before the rev limiter! ez
     
  8. Stereo

    Stereo Alien

    Ok, what you actually want for best acceleration is the highest torque you can get at a given speed (MPH).

    If you consider it as the torque curve, you need to be taking rpms and ratio into account:
    An engine that has, say, 100Nm torque at 2000 rpm, 200 Nm torque at 3000 rpm, 200 Nm torque at 4000 rpm, 100 Nm torque at 5000 rpm.
    Say that a ratio of 1.0 gets you to 100km/h at 2000 rpm. Then you have 100Nm in that gearing. To be doing 3000 rpm you have a ratio of 1.5, which is more advantageous - the 200 Nm becomes 300 Nm at the wheels. For 4000 rpm it's even better, a ratio of 2.0 - 400 Nm at the wheels. For 5000 rpm, you need a ratio of 2.5, which gives you 250Nm.
    From this type of calculation you can find out that 4000rpm is the most powerful rpm when you're at 100km/h.

    Alternatively, you can simply multiply torque by rpm, and then divide by a constant to make the numbers reasonable - 2000 rpm * 100 Nm = 200,000 'horse power', 3000 rpm * 200 Nm = 600,000 'horse power', 4000 rpm * 200 Nm = 800,000 'horse power', 5000 rpm * 100 Nm = 500,000 'horse power'. These are exactly the same figures as you would get by working out the comparative ratios - by comparing 'horse power' numbers, you know what rpm is optimal without having to take into consideration the difference in gear ratios each necessitates.

    So peak wheel horsepower is actually a better indicator of acceleration than torque - if you have 500 Nm torque at 1000 rpm, you need to gear it down much more than 500 Nm torque at 5000 rpm, so the end result acceleration is worse. Horsepower + weight figures lets you compare two cars' acceleration much more directly.
     
    horstmannhid, mangal and djkostas like this.
  9. ssidrdw

    ssidrdw Racer

    ohhhh ok that's an awesome explanation. I completely neglected the effect of gearbox multiplying the torque and the advantage of higher rpm.
     
  10. IDLH_

    IDLH_ Simracer

    You want the most torque that your tires can apply to the tarmac, period. That translates to mathematical "power" by comparing torque to time, in this case the revolutions of crank per minute.

    Remember power is generated at the crank and this value is what is typically advertised. It's then multiplied by the gear and further by the final drive ratio, finally by the circumference of your tire. Dynos measure torque and then mathematically compare it to your cars RPM (they clamp on a tach pickup, or they hook into your OBDII port). Just remember, the higher the RPM is, the more revolutions the torque has to accelerate the car per unit time, like adding water mill paddles to the same size wheel...
     
  11. bondyboy

    bondyboy Alien

    I was asking you to clarify the question
     
  12. Warlock

    Warlock Hardcore Simmer

    In everyday life, it is torque who drives cars. On race track, HP is the main thing.
    But on the other hand, HP is what sells cars, not torque...

    I know that many will have complains on my answer, but it is simple.
    If you wanna race, torque is not what you are looking at, except if you have many curves and can't get high speed but you need to take care of tire spin, for fast laps, all you think of is how you'll get most of HP to the tires through transmission.

    Here is simple example, it is not a great sim, but game was OK, namely GT6.
    Since there is all made in those fabulous PPs, I was looking for the car that had similar setup but one with more torque, and I came to the BMWs, 120i and 120d.

    120i E87 has:
    2l l4 natural aspirated engine
    150hp @ 6200rpm
    200Nm @ 3600rpm
    redline 6500rpm
    1260 kg
    max theoretical speed in 6th gear is ~260km/h

    120d E87 has:
    2l l4 turbo charged engine
    163hp @ 4000rpm
    340Nm @ 2000rpm
    redline 4700rpm
    1340kg
    max theoretical speed in 6th gear is ~240km/h

    ...it is obvious what car will win, or is it not?

    So what I did, changed max power until I got same PP, and changed gear ratios until I got same max speed.
    Diesel one had like 30hp less then the petrol one, but had more kg and managed to get still faster laps...

    So, HP is important in racing, but torque does also make difference, but at some point, it is hard to control cars if they have really high torque. New modern diesel cars have really high torque, but 90% of those cars come with 4WD. Good example is BMWs N57S engine, the 3.0 TriTurbo Diesel with 381HP and 740Nm, it only comes with 4WD tranny... why? How to handle so big torque in everyday drive on rear axle only?!?
    I know GT6 is not a sim, but it gives some directions...

    Hope this helps a bit :)

    Cheers...
     
    Last edited: Jun 12, 2014
  13. gigelis

    gigelis Gamer

    hp is how fast you hit the wall torque is how far you take the wall with you :)
     
    offcorsa, iVG and Nasos Charalampou like this.
  14. bondyboy

    bondyboy Alien

    someone's been watching top gear
     
  15. iVG

    iVG Alien

    Best. Explanation. EVER.
     
  16. GeraArg

    GeraArg Simracer




    Also, that youtube channel is quite cool. ;)
     
    Last edited: Jun 14, 2014

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