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Thread: Odd results at rolling road today. Any thoughts please?

  1. #41
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    Quote Originally Posted by 59bhp View Post
    the SAE have always used power quoted at the flywheel as the standard unit of measurement, until they change that I don’t see why anybody should go about quoting fabricated acronyms such as "whp"
    The reason people quote "whp" when doing a dyno run is as said wheel horsepower is the measured and (in theory) known quantity at the time whereas flywheel horsepower is an estimate based on whp. The only way to accurately measure the flywheel horsepower is on an engine dyno.

    That and because its a bigger number!

  2. #42
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    I see. So we are saying here that a normal dyno works by effectively measuring WHEEL BHP, then applies a % to that figure to get the FLYWHEEL Bhp?

    I was under the assumption that it measures WBHP, then lets the engine run down to zero, which would measure the drag and therefore the drivetrain loss. Then add the WBHP to the drivetrain loss to get the FWBHP. Therefore accurately measuring the FWBHP.

    Then I was hoping to be presented with a pretty graph showing both accurate WBHP and FWBHP, from which I could check that there was not an abnormal drivetrain loss. That was kind of the whole point of the RR day. But we are now saying that the conversion from WBHP to FWBHP is just purely a % being added on? Kind of defeats the point doesn't it? I might as well just say... "its actually 650FWBhp"
    Last edited by jont; 16-07-2014 at 13:48.

  3. #43
    I <3 BBS LM Actual_Ben_Taylor's Avatar
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    The dyno measures torque and RPM if we're being pedantic

  4. #44
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    Quote Originally Posted by jont View Post
    I see. So we are saying here that a normal dyno works by effectively measuring WHEEL BHP, then applies a % to that figure to get the FLYWHEEL Bhp?

    I was under the assumption that it measures WBHP, then lets the engine run down to zero, which would measure the drag and therefore the drivetrain loss. Then add the WBHP to the drivetrain loss to get the FWBHP. Therefore accurately measuring the FWBHP.

    Then I was hoping to be presented with a pretty graph showing both accurate WBHP and FWBHP, from which I could check that there was not an abnormal drivetrain loss. That was kind of the whole point of the RR day. But we are now saying that the conversion from WBHP to FWBHP is just purely a % being added on? Kind of defeats the point doesn't it? I might as well just say... "its actually 650FWBhp"
    As far as I am aware they do do this but it would also have another loss in there of the inertia of the engine itself and the energy needed to compress air in the cylinders so this means by adding those 2 figures you would get the theoretical output of the engine which would equate to the same figure you would get purely by working out how much fuel is being burnt by the engine multiplied by the calorific value of the fuel. which is far different from the actual power output of the engine. But I am no means an expert on dyno's.

  5. #45
    Guest 59bhp's Avatar
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    Quote Originally Posted by jont View Post
    I see. So we are saying here that a normal dyno works by effectively measuring WHEEL BHP, then applies a % to that figure to get the FLYWHEEL Bhp?

    I was under the assumption that it measures WBHP, then lets the engine run down to zero, which would measure the drag and therefore the drivetrain loss. Then add the WBHP to the drivetrain loss to get the FWBHP. Therefore accurately measuring the FWBHP.
    thats exactly how it works

    http://www.emeraldm3d.com/rolling-road

    from there

    "BHP at your wheels

    A rolling road takes its power reading directly from the driven wheels of the car. This means involving gearboxes, drive shafts, differentials and tyres. A lot of people talk about bhp @ the wheels as being the only meaningful number to quote: "It’s what you race with" they will tell you. In a way that is correct, but then the gearbox gets in the way of the true picture. We measure the torque at the wheels but the rpm is measured at the road wheel roller. Put the car in a lower gear and the torque at the wheels increases – but the rpm of the roller is reduced. In theory the resulting bhp should be exactly the same – but it never is. The lower the gear that you run the car in, the higher the bhp at the wheels. This is because we have rolling losses (some call them transmission losses) that increase with increased roller (and hence road wheel) rpm. The biggest single rolling loss is the tyre.

    Remember that you have not one, but two contact patches on a rolling road. The tyre is compressed in two places and the faster it spins the more often it is compressed. The tyre construction, the diameter and the tyre pressure all have a direct influence on the rolling losses. As an experiment we measured the power at the wheels of a Golf GTi. Then we put another 10 psi into the tyres and checked the power at the wheels again. The power went up by 4bhp! Can you imagine what happens to the rolling losses when the tyre is compressed by several bodies sitting on the back of the car trying to find enough grip to prevent wheel-spin? As long as the bodies stay on the back during the run-down, which measures the rolling loss, you get the right result in the power graph. If the bodies all jump off when the car is knocked out of gear and allowed to run-down, you lose the tyre compression and the losses are less – distorting the resulting graph plot. In order to make any sense of rolling road power figures you must measure the rolling losses and add them to the power at the wheels. When you do that you can run in any gear and get the same result on the power graph – almost."

    thats written by Dave Walker, a man whos forgotten more about anythign automotive related then ill ever know.
    Last edited by 59bhp; 16-07-2014 at 14:20.

  6. #46
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    Gotcha

    Yeh that's interesting. So by the sounds of that, to stand a chance of getting a decent accurate data, you need to use a rolling road that runs up, then lets it run down, to calculate the roling losses.

    Hmmm wonder where I'm gunna find one of those then...

  7. #47
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    Ok Ive been thinking this through in my little pea brain.

    Based on 59's comment:

    And assuming "drivetrain loss" = energy loss from flywheel through the system to the roller surface.

    I did the run in 4th gear. This gave a larger torque force at the wheels than expected, because there was less drivetrain loss than if it was in 5th gear(i.e. bearings/tyres etc not spinning as fast. All of which saps energy). Bear in mind here that the RR software was expecting 5th gear.

    Then the RR software took the torque that it measured and added whatever drivetrain loss it was set up to add for a 5th gear run, and finally came to the calculated engine BHP figure.

    So there were two inaccuracies here. Firstly it measured the wheels with less actual drivetrain loss than expected. And then the addition of an overly high drivetrain loss value to calculate the final engine power.

    And hence we come out with a stupid 400bhp or whatever it was.

    As there is no way to know the true drivetrain energy loss of the car in 4th gear now, there is no way to really calculate decent data from what we have. Doh.

    So as above, the only way to get something decent is to start again on a dyno that can measure the drivetrain loss in the actual gear, and calculate the final data correctly.

    Anyone have an objection to this?


    EDIT - well at least it's been an educational experience
    Last edited by jont; 16-07-2014 at 18:23.

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    At least we met a real old character!

  9. #49
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    Dyno days are fun but like I've said to other people before hand, before you do many mods go to one dyno get your base power recorded then once the mods are done go back to same dyno and see how much you've gained. It's the only true way of getting accurate gains.

    Like I said though dyno days are fun personally enjoy them myself but seen mates to from one dyno to another and had as much as 40bhp difference

  10. #50
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    Quote Originally Posted by jont View Post
    I see. So we are saying here that a normal dyno works by effectively measuring WHEEL BHP, then applies a % to that figure to get the FLYWHEEL Bhp?

    I was under the assumption that it measures WBHP, then lets the engine run down to zero, which would measure the drag and therefore the drivetrain loss. Then add the WBHP to the drivetrain loss to get the FWBHP. Therefore accurately measuring the FWBHP.

    Then I was hoping to be presented with a pretty graph showing both accurate WBHP and FWBHP, from which I could check that there was not an abnormal drivetrain loss. That was kind of the whole point of the RR day. But we are now saying that the conversion from WBHP to FWBHP is just purely a % being added on? Kind of defeats the point doesn't it? I might as well just say... "its actually 650FWBhp"

    At least here they first measure the drivetrain loss and then do the power run.

  11. #51
    Flamethrower Jez's Avatar
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    Its really difficult to measure actual drive train loss on a chassis dyno. An estimate is the best you're going to get. I was going to go into a load of detail about this but I've had a few beers and can't be arsed.

  12. #52
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    ^^ the man has spoken! Haha have a couple for me dude!


    Sent from my iPhone using Tapatalk

  13. #53
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    Quote Originally Posted by Jez View Post
    Its really difficult to measure actual drive train loss on a chassis dyno. An estimate is the best you're going to get. I was going to go into a load of detail about this but I've had a few beers and can't be arsed.
    It's still a lot better than just adding say 20% across the chart...

  14. #54
    Flamethrower Jez's Avatar
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    Yeah, maybe. Losses dont go up with power, so a flat percentage figure isn't anywhere near right. Depending on the dyno they measure the deceleration of the rollers and car drive train. But the deceleration rate is dependent on the losses of the transmission, tires etc. But also the mass of the wheels, tyres, gearbox internals, diff etc.

    Heavy wheels, diff, gearbox components - apparently lower losses as the whole lot takes longer to slow down
    Flat tires, thick box/diff oil - more apparent losses.

    If you want to "win", get really heavy wheels etc and reduce frictional losses by pumping up your tyres, thin oils etc.

    Although having said that if you've got heavy drive train it'll absorb more energy on the run up the revs.
    Last edited by Jez; 16-07-2014 at 20:29.

  15. #55
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    I guess this is due to the heavy wheels etc. acting as a flywheel! What you said there has always baffled me. Why is it a percentage? Surely the power needed to over come the drivetrain losses is a constant and doesn't change just by adding more power!


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    This is why I prefer to go by wheel power instead of pub figures.

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    Quote Originally Posted by pippin89 View Post
    I guess this is due to the heavy wheels etc. acting as a flywheel! What you said there has always baffled me. Why is it a percentage? Surely the power needed to over come the drivetrain losses is a constant and doesn't change just by adding more power!
    Engine friction increases with engine speed and to a lesser degree load so the same would be true of a gearbox to some extent.

    If you apply more force or spin it faster then friction will increase logically. Doubtful it can be easily expressed as a percentage all the same.

  18. #58
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    Quote Originally Posted by pippin89 View Post
    I guess this is due to the heavy wheels etc. acting as a flywheel! What you said there has always baffled me. Why is it a percentage? Surely the power needed to over come the drivetrain losses is a constant and doesn't change just by adding more power!
    not quite, without going into mechanical dynamics, energy is lost through multiples of ways.

    Looking at a very simple (and albeit negligable loss) just look at noise and heat, the very fact that sound and heat is being produced means that energy is being lost (which power is a function of), the faster your engine spins, the more sound and heat is being produced (versus idle lets say).

    Like i said, over simplified and negligable, but it shows how it should be a percentage

  19. #59
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    Quote Originally Posted by jont View Post
    I don't think there are any other reputable RR's in Cambs-ish area that I know of?

    Thanks
    There (used) to be one at Gransden, went there one and IIRC it was a new machine (but the bloke couldn't tune carbs).

    If you need a hand driving down I'm sure someone will be willing to help

  20. #60
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    Quote Originally Posted by tokyo_black View Post
    At least we met a real old character!
    Was that your Ferrari?

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