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Easy way to work out a more sensible torque fig from the crazy 'derived' dyno results

Discussion in 'Dyno Figures' started by kahlnz, Jan 31, 2013.

  1. kahlnz

    kahlnz New Member

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    I know loads of people dislike the crazy 'derived' rear wheel torque figures that we get on dyno results so here's a way to make sense of it...

    1. find the rear wheel hp figure at 5,250rpm (i.e. 245rwkw/329rwhp)
    2. given hp=ft lb (torque) at this rev, 5,250rpm, you know you have 329 ft lb
    3. find the 'derived' rear wheel torque figure at 5,250rpm (i.e. 735nm/545 ft lb)
    4. divide the figure in step 3, 545 ft lb by figure from step 2, 329 ft lb and you will get the derived factor, in this case 545/329=1.66 (this is the fixed multiplication factor the derived figures use when considering final drive gear, converter slip, dyno roller diameter, etc)
    5. simply divide all of your torque results by 1.66 (use the figure you got in step 4) and it'll give you a way more sensible torque figure.

    After I performance tuned my VE SSV and added an OTR my dyno results were 244rwkw/800nm (derived, at the wheels). Although these are true, the torque figure never made sense and I couldn't think about it practically but when you apply the method above you get 244rwkw/478nm which looks spot on. test it one step further and factor in a drivetrain loss of 23% (auto) and flywheel figures of 317kw/620nm sound just right when the stock one was 270kw/530nm.

    I hope this helps some people to better appreciate their dyno results.
     
  2. acarmody

    acarmody Donati..Whoa Green

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    So using this method my car (VX V6 Ecotec) actually makes 207nm of torque, which I will apply the drivetrain loss (your 23%) makes 255nm of torque.

    Wish someone would have told me my car will only make an extra 25nm of torque after installing a cam, higher compression, intake, exhaust and various other smaller stuff.

    I MUST HAVE GOT RIPPED. Or this method doesn't work. One or the other.
     
  3. kahlnz

    kahlnz New Member

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    What is your rwkw and derived rwnm at 5,250rpm? Also what is your peak derived rwnm? Cheers
     
  4. monkeys437

    monkeys437 New Member

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    Torque = (9880xPower)/(RPM) with torque in Nm and Power in kW.

    If you want a real engine torque figure I think the beast estimate is using estimated driveline losses to work out your engine power in kW, then using the above formula.

    However what you've come up with seems like a pretty decent estimate too, maybe just over thinking it a bit
     
  5. acarmody

    acarmody Donati..Whoa Green

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    Peak torque 350nm.

    At 5250rpm, 140rwkw and 320nm
     
  6. kahlnz

    kahlnz New Member

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    Sounds like you might be working with dud dyno stats, with the mods you've done I'd expect a fair bit more than 140rwkw anyway. With my calcs, 140rwkw/255nm and peak of 278nm seems to sound right from a ratio perspective so maybe the dyno was well under, is it auto or manual? diff or wheel mods?
     
  7. Not_An_Abba_Fan

    Not_An_Abba_Fan Exhaust Guru

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    You are working it backwards. A dyno measures torque, real torque, not derived, not estimated, not formulized (is that even a word?) then it applies a formula to obtain a power figure. Power is the estimate. Torque is measured. All engines will make their peak torque at different RPM, and that will even change in different gears. To have a true benchmark, torque should be measured at direct 1:1 drive, so whichever gears gives 1:1 drive then that is the gear to measure torque in. Then it will give a calculated power figure. The power figure is the one that is "derived" or "calculated", not the torque.
     
  8. Bomber R8

    Bomber R8 New Member

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    Exactly. Very well said!
     
  9. kahlnz

    kahlnz New Member

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    thanks for that Not_An_Abba_Fan,

    so basically, just worry about what's happening at the rear wheel? so my dyno results of 328kw and 910nm at the wheels is all I need to worry about? and the 1/4 mile results. I guess I was overanalysing it so I could get an idea of a nm rating at the flywheel but who cares?! What's your thoughts on dyno results for later model autos where you can't get a 1:1 drive? my results were measured in 4th gear (1.15:1).

    Cheers
     
  10. Not_An_Abba_Fan

    Not_An_Abba_Fan Exhaust Guru

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    You got 910nm of torque? What was your RPM? I worked it out to about 3500rpm, you must have a big engine to produce that much torque that low in the revs and only get 328kW.

    Or did the dyno give you "motive effort"?
     
  11. kahlnz

    kahlnz New Member

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    yeah 900+nm, it's from 4,500-5,500rpm, sounds high hence why I was initially determined to work this backwards to a more 'realistic' figure which I thought (using my formula I posted originally) was about 545rwnm. The dyno was a DynoTorque and the tuner said this is the true torque measured through the wheels, no other way to measure it. Still confusing but they are the experts so won't argue with them, now I know why all these turbo's etc can pull 1500+nm of torque through a very wide rev range.
     
  12. Not_An_Abba_Fan

    Not_An_Abba_Fan Exhaust Guru

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    If you got 910 nm of torque at 4500rpm, you will have 780kW. At 5500 rpm it is 950kW. Something is wrong. Or you were in 2nd gear.

    Even 545 nm is 466 kW.
     
  13. kahlnz

    kahlnz New Member

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    I agree these are misleading results which is why I originally started this thread. with the calculations I stated at the beginning you can get a better idea of real torque versus the torque exerted against the dyno rollers (910nm). The formula I proposed in this thread estimates my peak not to be 910nm but rather 545rwnm and this is fairly closely confirmed with the standard metric formula where power (kw) = torque (nm) x RPM / 9549. This gives an answer of 525rwnm. Work this back through a 28% DT loss and you get a flywheel result of 455kw/730nm (610hp/540ft lb).

    This is all within logic given it started out with flywheel figures of 270kw/530nm. OTR and Tune gave it 340kw/630nm then Cam, PnP Heads, Headers and Exhaust gave it 455kw/730nm.
     
  14. Not_An_Abba_Fan

    Not_An_Abba_Fan Exhaust Guru

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    The torque exerted against the dyno IS real torque. Thats how a dyno works, it measures rear wheel torque exerted on the rollers. The in built software takes into account the roller diameter, as long as the inputs are accurate, eg tyre size, which gear it runs in, tyre pressure etc, then the result is the torque you have at your rear wheels. Then the software converts that figure to power.

    Generally, what ever the kW figure calculated for the rear wheels, is about your HP figure at the flywheel.

    There should be nothing misleading about a dyno's torque readout. There are other ways dyno's measure though, "tractive effort" or "motive effort" are one, but the main thing to remember is that a dyno is a tuning tool, software has been built in to calculate power, but all that is is wank factor. But if it's a number you need to brag about, then all those that you are comparing it to, should all get measured on the same dyno, on the same day, by the same operator, as each dyno will give different results as consistency is the key for "shoot outs".
     
  15. Tsunamix

    Tsunamix Active Member

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    Yeah - few glaring holes in stuff above -

    Abba is that the right way round ? 280HP at rear wheels for me, thats 209kW at the wheels. I reckon I got a damn site more than 209HP at the flywheel. Thats only 177kW :)

    The other way round is still misleading - 280hp at wheels may be closer to 280kW at the engine.

    i think everyone is making assumptions on losses scaling. They don't.

    Percentages can not work in calculating losses. Percentiles are proportional, lossess are fixed plus a small amount of variation due to things like oil hysteresis etc, Friction losses remain roughly fixed. Your alternator may take 2HP to drive on a 200kW engine. If you use the same alternator on a 300HP engine it still takes 2HP to drive it - not 1% which is 3HP.

    Follow me ?

    ANyway the only way you can use % is as a cross check for relative losses.

    Stock Ecotec
    147kW Flywheel
    Approx 119 at wheels
    Losses = 28Kw (19%)

    Flywheel Torque = 304Nm at 3600
    Rear wheel derived torque = 559 at 3600, which gives a calculated flywheel equivalent of 242Nm or losses of 62Nm or 20.4%.

    Losses are relatively close so the numbers are relatively accurate.

    Looking at my Dyno results - attached, you can see that the RWKW = 209 (or 280HP) my driveline looses will be similair to stock, maybe a little less as gearbox is modified, so expecting losses of 25kW, my engine kW islikeley to be 234kw.

    Apply same losses calc to torque, remembering 3,7 gear ratio, means flywheel equivalent torque, before losses is about 487Nm. (1351 /3, * 4 , / 3.7). Add about 55nM torque for losses and actual engine torque is likeley to be 542Nm.

    I Estimate my engine results as 234kW and peak torque of 540Nm. Seems pretty fair really.
     

    Attached Files:

  16. Not_An_Abba_Fan

    Not_An_Abba_Fan Exhaust Guru

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    I'd say your readout is pumped up due to the fact that it was 3rd gear with 3.7's. 3rd gear in a VT is not 1:1. What you have is an accurate measure of torque in 3rd gear, do it again in 4th and see what it reads. It will be different in every gear, but for a true comparison, if you were to quote your figures showing your car's output, then it really needs to be done at 1:1.

    I can chuck my 3.5L Rangie on a dyno, put it in low range and 2nd gear, I bet it will pull huge torque figures.

    But that aside, using the dyno as a tuning tool, it doesn't really matter what gear you are in, you just need to load the engine up and see how it performs.

    And yes, sorry, other way around, whatever the HP at the wheels is roughly what the kW are at the flywheel.
     
  17. Decicrate

    Decicrate Member

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    Everything I have read on a 4l60e shows that 3rd gear is 1:1 ratio and forth is 0.696:1. Straight out of the Holden workshop manual it states the 3rd gear ratio is 1:1.
     
  18. Not_An_Abba_Fan

    Not_An_Abba_Fan Exhaust Guru

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    Am assuming his is manual, but if it's auto, then all good.
     
  19. Decicrate

    Decicrate Member

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    It is an auto.
     
  20. Tsunamix

    Tsunamix Active Member

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    I have seen a heap of dyno results in the dyno thread that aren't run in a 1:1 ratio.

    Luckily i gone my work done by someone with a brain - its an auto 4L60E.

    My numbers are probably low on the power side, but if they are low on the power side they are also low on the torque side. I only claim the same losses as a stock engine :)

    Maybe - just maybe I have a bit more.
     

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