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Thread: A little bit of maths

  1. #1
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    A little bit of maths

    Ok i want to know how to get the power figure of rear wheel horsepower/kilowatt into crankshaft/flywheel power

    i know how to change kw into hp (x the kw by 1.34)

    any help would be great guys
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  2. #2
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    you cant come up with a formula to compare flywheel and rear wheel power, it depends on the individual mechanics of each car, for instance, motor magazine tested the HSV GTS and FPV GT, the GT is down 10kw on power at the flywheel, yet was 3kw ahead of the GTS at the rear wheels, in the same test, motor i think got different readings for the Monaro CV8 and SS Commodore, which had the same engine
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  3. #3
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    An approximation can be come to by adding about 30% to the rear wheel figure. All drive lines will take up a different amount of power but that gives you something to work on. Otherwise you will need a engine dyno.
    "A string is approximately nine long."
    Egg Nogg 02-04-2005, 05:07 AM

  4. #4
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    30% is usually the worst it gets but, from RWD's with heavy duty drivelines and AWD/4WD drivelines, i think FWD usually gets around 15-20% drivetrain loss
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  5. #5
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    People often make mistakes in this area. First, as fpv says, there is no formula for this. Second, the drivetrain makes a massive difference. FWD cars typically experience losses of about 20%, although auto transmissions lose more than manuals. RWD lose (generally) between 20-25%, although some are more. 4WD losses are huge. For example a '96 WRX I was witnessing being tuned began with a rear wheel kilowatt figure of 95 kw. That's a power loss of over 40%.

    Also the wheel power figure will be hugely affected by the make of the dyno. For example a dynojet typically records higher figures than a Dyno dynamics. According to Zoom magazine the difference is about 8%, which sounds little, but if you consider a car with 300 kw, 324 kw is much more impressive.

    Basically unless you test two cars on exactly the same dyno, in exactly the same conditions (ambient temp, airflow, fueling etc) then two results are not comparable.
    That depends on what your definition of the word 'is' is - US President Bill Clinton

  6. #6
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    Quote Originally Posted by Nildo
    Basically unless you test two cars on exactly the same dyno, in exactly the same conditions (ambient temp, airflow, fueling etc) then two results are not comparable.
    If you want to be that fussy , you can not actually get a definitive reading. Only one that relates to a certain set of testing criteria. Whats the point.
    "A string is approximately nine long."
    Egg Nogg 02-04-2005, 05:07 AM

  7. #7
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    well yuor never going to really get that accurate going for power at the wheels to begin with crisis, if you want absolute accuracy, rip the engine out and test on the flywheel again
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    ahhhhhh that explains why some cars which have been dynoed are less than the manufactuer claims

    thanks for all the info guys
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    i hate math
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    yeah me too, but funny enough its my best subject
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    Quote Originally Posted by -datsun 1600-
    ahhhhhh that explains why some cars which have been dynoed are less than the manufactuer claims

    thanks for all the info guys
    Yeah, this makes sense. All cars will produce less power on a typical wheel-driven dynomometer than they will at the crankshaft. However, they will not produce "less than the manufacturer claims" because they manufacturer is only ever claiming power at the crank/flywheel.

  12. #12
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    hows this one then: the april 2002 wheels magazine's feature article was a grey import nissan skyline gtr v-spec 2 n1 vs an hsv gts coupe. thats 206kw vs 300kw. both cars were put on the dyno and they turned out at 214kw vs 230kw.

    wheels then went on to guess the flywheel output of the skyline to be over 300kw, as theyve tested a porsche 911 that has gotten less rear wheel power, with only 309kw at the flywheel

    performance times severly went the way of the skyline, with 0-100km/h at 4.4 and 5.8 seconds and the quarter mile at 13.9 and 12.7 seconds. braking distances from 100km/h went the skylines way as well, with distances of 35.3m and 40.9 being achieved. some people will probably dispute the skylines 0-100km/h time but wheels noted the N1 had a 1000rpm higher redline than a normal R34 as well as larger garret turbo's pumping out 16 psi, oil cooler and blueprinted engine
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    Quote Originally Posted by fpv_gtho
    hows this one then: the april 2002 wheels magazine's feature article was a grey import nissan skyline gtr v-spec 2 n1 vs an hsv gts coupe. thats 206kw vs 300kw. both cars were put on the dyno and they turned out at 214kw vs 230kw
    Thanks for this... it's good of you to bring up that what I said before is only true when assuming that the manufacturer's specifications are accurate. If so, the WHP will always be less than the said factory crank HP.

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    its still got to be substantially less to be believable though. would you believe a skyline had 206kw at the flywheel if it made 200kw at the wheels, or if it made about 150kw at the wheels
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  15. #15
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    Quote Originally Posted by fpv_gtho
    its still got to be substantially less to be believable though. would you believe a skyline had 206kw at the flywheel if it made 200kw at the wheels, or if it made about 150kw at the wheels
    You do realise I'm agreeing with you, right? I have been the whole time...

    I definately would not beleive that a Skyline making 200kw at the wheels would have so little as 206 crank hp, hence what I said about manufacturers claims. In the case of this Skyline, it seems that Nissan's claims would have to be much less than the truth. Heck, if you've got a national standard 280hp limit to deal with, that seems like the best way around it!

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