• Welcome to SVTPerformance!

RWHP vs BHP

Discussion in 'Road Side Pub' started by Jameeson, Nov 30, 2019.

  1. Jameeson

    Jameeson silver is the fastest color Established Member

    Messages:
    470
    Joined:
    Apr 20, 2016
    Location:
    Houston
    Well I’ve have had too much time on my hands recently so here is a thought

    Suppose car is RWD, manual trans, and has 500 hp.Consensus being 15-20% drivetrain loss
    15%, being loss of 75 hp

    Now car has a big shot of n20

    makes 800hp
    15% loss now 120hp

    Help me understand why it would it take another 45hp to turn the same mechanical parts..transmission, axles, driveshaft, etc that it did at 500hp?
     
  2. MFE

    MFE Well-Known Member Established Member

    Messages:
    1,769
    Joined:
    Oct 29, 2005
    Location:
    Phoenix
    Because the losses mount with acceleration and loading.
     
  3. ON D BIT

    ON D BIT Finish First Established Member

    Messages:
    14,607
    Joined:
    Jul 7, 2003
    Location:
    Currently in Sonoma County
    Do you have an example with numbers?
    Crank hp then same motor with n2o crank hp on engine dyno.
    Then taking the same motor and run it again on a chassis dyno n/a and n2o.

    Someone was trying to tell me that as you go up in power the percentage loss goes down. Yet he could never give me an example.
     
    Jameeson likes this.
  4. _Snake_

    _Snake_ Well-Known Member Established Member

    Messages:
    2,644
    Joined:
    Jan 21, 2014
    Location:
    Flo-Rida
    Because inertia and friction.
     
  5. CobraBob

    CobraBob Authorized Vendor Premium Member Established Member

    Messages:
    89,363
    Joined:
    Nov 17, 2002
    Location:
    Cheshire, CT
    Jameeson, Lambeau and _Snake_ like this.
  6. _Snake_

    _Snake_ Well-Known Member Established Member

    Messages:
    2,644
    Joined:
    Jan 21, 2014
    Location:
    Flo-Rida
    Great article.
     
    Last edited: Nov 30, 2019
  7. ON D BIT

    ON D BIT Finish First Established Member

    Messages:
    14,607
    Joined:
    Jul 7, 2003
    Location:
    Currently in Sonoma County
    The only way to ever know true drivetrain loss is with a engine dyno and then immediate chassis dyno.
     
    me32 and 03Sssnake like this.
  8. tt335ci03cobra

    tt335ci03cobra Well-Known Member Established Member

    Messages:
    6,214
    Joined:
    Feb 15, 2008
    Location:
    USA
    Too many variables. Rotational mass, height and rolling resistance of wheels and tires alone can distort any data. A sticky 28x12.50 drag slick at 15psi on a heavy old steel wheel will dyno far lower than a thin lean hard wall 24x8 economy all season tire on a carbon fiber rim. Put a 44” mud bogger on a 67hp del sol and it might just throw a rod.

    Now consider internal issues like heat management. 50° changes the molecular state of aluminum or steel enough so that traces of loss bring to happen known as thermal loss. Very minor fusion is happening basically. Too cold to fully meld, but there’s some spit swapping of electrons at play... animals.

    How about inertia loss. An object wants to remain at its current state so motivating it to do otherwise more tq. As rpm rises, there is additional resistance to spin that needs to be countered. As time to spin to rpm is reduced, additional motivation is required. Spinning a given car from 60-160mph in 4 seconds vs 15 seconds in 4th gear brings up many many variables besides just power needed. Assuming all variables are the same, simple math says you’ll just need x more tq/hp to do so, but in reality it’s almost always about 2-5% more than that. Inertia and thermal drag play into effect.

    In general, I like to use what it lost stock vs engine hp (assuming healthy testing on a 70° day with low humidity) so let’s say 50hp/50tq and add on an extra 10% of that for every 100tq added.

    Take a stock 6000rpm redline 400/400 car like a 2006 gto. They make 350/350wheel.

    So it’s using about 50/50. Rpm is low on that car and it balances out well.

    Take that car to 900whp/900wtq on a stockish redline of 6000rpm, and I’d personally assume 50/50 loss like stock and an addition 10% per 100tq so we are up 550tq vs stock. 10% of the original 50 loss at stock power level is 5tq. Multiplied 5.5 times is 27.5 Added to the original 50 I get 77.5tq loss at 900wtq so I’d guess the engine is actually about a 980/980 engine on an engine dyno fully dressed with accessories. This is all assuming everything is healthy and optimal regarding heat management, fluids and so on, wheels and tires are the same, drivetrain weight is same or lower, clutch is better etc.

    I do this because builds aren’t done in a vacuum. We don’t just add power, we improve the half shafts, get better clutches etc and lighter driveshafts etc. lighter wheels etc.

    I don’t buy the 15% garbage. It’s lazy math that’s as valid as saying I must have a 15% slower 60yd dash than someone of similar composition who is 15% taller. False.
     
    Last edited: Dec 1, 2019
    Jameeson likes this.
  9. GNBRETT

    GNBRETT Well-Known Member Established Member

    Messages:
    2,125
    Joined:
    Oct 14, 2010
    Location:
    Middle Earth
    Ur not losing 15% thru a manual transmission. More like 10%. Nonetheless its more Parasitic loss. The more power from the engine the more energy required to turn the wheels. Timing changes with Nitrous tho. Lotta variables including clutch, car weight, gearing, wheels, Brakes, Trans, gas used etc.....

    The track will see more load than a dyno will so unless u take it out and see what ur mph is ur just guessing.
     
  10. Jameeson

    Jameeson silver is the fastest color Established Member

    Messages:
    470
    Joined:
    Apr 20, 2016
    Location:
    Houston
    agreed,

    Is a good read

    Tt335 had great explanation as well and all of that makes sense to me. But for the sake of discussion - 2 identical cars - Only difference being n20 .

    I get more pressures, friction, etc, does that stay at a constant rate?
    . I guess to simply my original question -

    if xx% loss is standard with xx car, would the percentage stay consistent if the only factor added was HP? OR does it take a certain HP/TQ to power the drivetrain alone, and then a much large percentage of the power made by the motor is making it to the wheels, and the percentage adjusts.
    So In my example an 800 hp car is no longer seeing 15% drivetrain loss, like the 500 hp car, but more like 10-11% since the only thing that was added was n20??

    I think I more or less got the answer to my question

    What prompted this I was talking with someone about dyno numbers and they were very quick to say -
    “yeah you make 840whp .... that’s like 960 at the motor”....well that assumes my car is still having an assumed ~15% loss at these power levelS....I was not convinced that was 100% accurate

    anyway I think I get It overall

    Thanks for the replies gentlemen
     
    _Snake_ likes this.
  11. ON D BIT

    ON D BIT Finish First Established Member

    Messages:
    14,607
    Joined:
    Jul 7, 2003
    Location:
    Currently in Sonoma County
    Does a engine that makes 500hp crank n/a at 5.5k rpm with a loss of 15% still lose 15% when the engine is only turning at 2k rpms and making 200hp crank?

    would the loss be greater since it still needs to run and work/turn everything it does at 5.5k rpm?
    75/500 is .15
    75/200 is .375
     
  12. Sinister04L

    Sinister04L RIP Kane Established Member

    Messages:
    29,141
    Joined:
    Feb 24, 2003
    Location:
    Houston, TX
    A dyno is a tool, nothing more.
     
  13. 08mojo

    08mojo ... Established Member

    Messages:
    2,509
    Joined:
    Feb 9, 2012
    Location:
    Atlanta, GA
    At steady state (which is how I read your post), meaning the rpms are being held at 5.5k and then again at 2k, the losses are from friction. Friction has a coefficient associated with it, so the loss is a constant percentage. If the loss is 15%, the power robbed from 500hp is 75hp and the power robbed from 200hp is 30hp.
     
    ON D BIT and Rubenk like this.
  14. 1 Alibi 2

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

    Messages:
    2,553
    Joined:
    Aug 27, 2011
    Location:
    Hackettstown, N.J.
    2011, - ( 550 ), baseline 481.4, - 12.5 % drive-train loss.
    2014, - ( 662 ), baseline 585.4, - 11.6 % drive-train loss..
     
  15. Lemmiwinks

    Lemmiwinks Active Member Established Member

    Messages:
    908
    Joined:
    Mar 24, 2012
    Location:
    Sweden
    15% is a rule of thumb.
    It's not applicable to any real-world situations.
    For real world numbers you will need to exactly measure your drivetrain losses. And why would we ever want to do that?
     
  16. MFE

    MFE Well-Known Member Established Member

    Messages:
    1,769
    Joined:
    Oct 29, 2005
    Location:
    Phoenix
    And yet it's amazing how closely the real world claimed horsepower vs chassis dyno horsepower represents roughly 15% across manufacturers and platforms. [shrug]
     

Share This Page