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Re: Just to add

gary said:
The more power you make the higher the losses, as the saying goes you dont get something for nothing
How do RRs take this into account when estimating flywheel power? Surely the coast down losses would be the same for a 400bhp car as for a 700bhp car as long as they had the same transmission, wheels, tyres same tyre pressure. How does it know what losses to add to the wheel figures?

I agree that the losses when on load would be greater on the higher powered car but the transmission isn't loaded on coast down.
 

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Discussion Starter #42
for the benefit of the less well up on this subject could you explain, coast up and down, or in fact the process involved? anyone? or perhaps a new thread with this info, and put a reference in here to it? Is that a good idea or not?



Cheers,

Alex
 

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coast down

Not entirely sure exactly how it works but basically the operator gets the rollers turning with lower gears and then puts the car into (typically) 4th gear at low rpm (eg 2000) and the throttle is opened wide. The 'dyno' applies suitable resistance so that under full throttle the engine revs up at a controlled rate (eg over 5 - 10 seconds). The dyno knows from the resistance it had to apply and the speed of the rollers how much power is being generated at the tyres. This figure (ignoring any difference in rolling resistance of tyres on the rollers compared to a flat road) is the power that you're actually getting down onto the tarmac to propel the car along.

The next stage is to figure out how much actual engine power had been lost in the transmission and tyres. This is the bit I'm not so sure about. The operator disengages the clutch when the engine has reached the end of the power run (eg 8000rpm for most GTRs) and lets the rollers spin back down again with his/her foot planted firmly on the clutch. The system then tries to measure transmission drag. I'm not sure if it actually tries to drive the rollers (and thus test how much effort it takes to do so against the drag of the transmission) or whether it measures spin down time compared to what it would be if they were just spinning freely with no car on there. From this, it has an idea about how much power is lost in transmission drag.

It adds the two figures together to get an estimated flywheel figure. Other factors can be keyed in to the system to adjust the figures to compensate for ambient conditions. For example, if it is a really hot day, the operator keys in the temperature and then the RR software would add power on that it thinks would have been lost due to the high ambient temps so that you end up with a theoretical figure of what it would have produced on a standard day (20C?) etc.

My point was that
  1. the measured coast down losses wouldn't be any different if I had a tuned GTR or a stock one
  2. actual transmission losses WOULD be greater on a tuned one compared to stock because some of the losses are proportional to load
  3. if the above 2 points are correct, then the estimated flywheel figure from a RR is as much a guess as a calculation.
    [/list=1]

    I could be wrong about this, I'm trying to get confirmation by asking here but so far nobody has come up with a suitable explantion to clear up my doubts.

    Incidentally, if the operator has his/her foot slightly on the brake pedal during coast down, the system records a huge 'transmission loss' and gives you massive (false) power readings!
 

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Hi

James,
I do have comparisums of different chassis components but have not built a large enough data base on them yet so my reports would not be credible,
reference losses of larger hp cars, we cant predict the loss from wheels to flywheel, it is only a guestimate and the same value is applied regardless but not used by us, thats why i only judge improvements on at the wheels figures
regards
 

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Gary, thanks for clearing that up. Presumably you try to keep tyre pressures constant from run to run? As you know, tyre pressures can make a big difference to measured ATW figures.

While I'm on the topic, is there much difference in rolling resistance between a 245/45 R17 tyre (as per stock GTR 33) and a 265/35 R18 or 265/40 R18? I'm wondering how much I'd lose if I changed my wheels ...
 
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