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Discussion in 'Time Slip Bar' started by blue 07, Nov 1, 2018.

  1. blue 07

    blue 07 Active Member Established Member

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    Can someone out there explain to me the real calculations on how to get real numbers on horsepower at the motor (crank) based on rear wheel horsepower. For example, my Shelby GT500 is at 702 rwhp (730 torque if it matters) and I hear numbers from 810 to 825 for hp at the crank, what is the real number ? Seems you ask 10 people and you get 10 different answers. I was just using my car for an example, but if you were at 500 at the wheels or 576 at the wheels or whatever it might be, how is it calculated ?
     
  2. DSG2003Mach1

    DSG2003Mach1 Well-Known Member Premium Member Established Member

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    you get different numbers because it's all just a shot in the dark. I think most people add 10-12% to rwhp to guestimate flywheel hp with a manual trans. Most people dont get to dyno their motor on a stand and then in the car.
     
  3. Shadow Grey 03

    Shadow Grey 03 Well-Known Member Established Member

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    With the newer, more efficient drive lines, the 10-12% is relatively close and a good place to have an idea of hp at the flywheel.
     
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  4. DSG2003Mach1

    DSG2003Mach1 Well-Known Member Premium Member Established Member

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    Personally I feel like this inflates the flywheel numbers though. Lets say I have a turbo car and on low boost it makes 500 at the tires, that'd be 550 flywheel. We're saying it takes 50hp to turn the driveline. If I turn the boost up and make 800, now we're saying 880 flywheel for a loss of 80 hp. Does the power absorbed by the driveline really change or are we just artificially inflating flywheel power?
     
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  5. Shadow Grey 03

    Shadow Grey 03 Well-Known Member Established Member

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    That's why we estimate. If we run our stick car at the track we can use weight and trap speed to estimate hp numbers as well. These are estimates based off of many, many data points, but there are still variances in those results too.
    At the end of the day, I like to have dyno numbers with a time slip. That way I can try and see if there is much left in the car based on what it put down. This is using the same dyno and getting results from more than one track.
     
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  6. DSG2003Mach1

    DSG2003Mach1 Well-Known Member Premium Member Established Member

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    If I ever hit the lotto Im gonna dyno a motor on the stand at a couple different boost levels then drop it in a car and dyno again at the same boost levels...ya know, for science
     
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  7. StrayBullitt

    StrayBullitt Well-Known Member Established Member

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    I had always thought this too, always thought it would be more of a fixed amount of power to turn the drive train, got bored it read up on it a bit, it is said that the more force or more load that is put into the gears ect. the more that is wasted and so the % holds true.

    FWIW From what I am seeing it looks like a 13/14 GT500 can dyno stock within about 8% of the factory 662 rating.
     
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  8. Soap

    Soap Well-Known Member Established Member

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    The only problem with that is factory ratings don't mean squat. Most are "adjusted" based on insurance rates, gas guzzler taxes etc.

    03-04 Cobra's were factory rated at 390hp but consistently dyno around 370 bone stock. That doesn't mean the driveline is so efficient it only has a 5% loss.

    --Joe
     
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  9. StrayBullitt

    StrayBullitt Well-Known Member Established Member

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    I get it.. and that is why I say take it for what it's worth. But at about a 45hp loss through the Drivetrain stock on a GT500.. I could see that being pretty close.
     
  10. tvspower

    tvspower Member Established Member

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    Wow what a find on this subject.
    Some may know, but I blew up motor last June
    Pistons rings n bent rod
    102k kms car ran very good, BUT I f,,,,up.

    Present
    Engine n drivetrain removed by Wicked Garage
    Engine at Custom Engines
    Engine to be dyno'd n tuned at Internationaldyno
    Wicked Garage to install engine n drivetrain
    Engine in car then chassis Dyno'd n tunned
    I hope during this process eliminates ANY ISSUSES
     
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  11. 1 Alibi 2

    1 Alibi 2 If not today, when ?????? Established Member

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    I always looked @ what my car base-lined @, used / added that loss to the rwhp after modding.................Same dyno used for both.
    OEM advertised - 662
    Baseline - 585
    Net loss - 11.7 %
    modded rw - 780
    Est. crank - 871
     
  12. Shadow Grey 03

    Shadow Grey 03 Well-Known Member Established Member

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    The dyno is a tuning tool that a lot of people live and die by. I've gone into tuning sessions with certain expectations and have come up short a few times. I wouldnt get too caught up in the numbers unless my track times didnt get me what I was estimating. I wont lie and say that I wasnt disappointed at times, but my mood would always change once I hit the track.
    People would always tell me I was lying about my dyno numbers after making a pass. They failed to take driver and conditions into the equation, as well as dyno variations.
    I am curious to see the engine dyno, then chassis dyno numbers from the guy above.
     
  13. LaserRed94

    LaserRed94 Member Established Member

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    To get your 871 you multiplied 780 X 11.7 = 91. Then added it to your 780. Or maybe just multiplied 780 by 1.117 and got 871. But you should actually divide 780 by .883. If your net loss was 15% you would divide by .85. Not much difference, but the difference is in your favor. 585 /.883 is 662. 585 X 1.117 = 653. Hope that makes sense.
     

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