Gears do affect rwhp

Discussion in 'Tech Articles, How-To's & Write Ups' started by blackfang, Jan 21, 2006.

  1. blackfang

    blackfang Active Member

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    Just some info I thought I would put out for you guys and gal.

    A car having 3:27 rear end gears is placed on a DynoJet chassis dyno and
    has a dyno performed. Then this same car is taken off the DynoJet
    chassis dyno and has the rear end gears changed to 4:10. The car is then
    placed back on the DynoJet chassis dyno and another dyno is performed.
    Will the second dyno show a loss of horse power caused by the 3:27 to
    4:10 gear change?

    ANSWER:
    Yes. This has been a long standing debate among car enthusiast, so I
    recently emailed DynoJet and asked this exact question. Below is the
    response I received.

    "The 4:10 gear will show less horsepower than the 3:27. The reason is
    due to rate of acceleration changes. The rate of acceleration is quicker
    with the 4:10 because of torque multiplication being increased to the
    rear wheel. The horsepower will show less because the targeted RPM is
    met before the horsepower has a chance to overcome the rotational mass
    (dyno, drive line, etc.) or moment of inertia in speed. Because the
    speed is decreased and the RPM is met faster, the horsepower never has a
    chance to catch up with itself, so to speak. The overall ratio of 1:1
    will always produce the most horsepower on the chassis dyno. Having said
    this, a similar problem can occur with horsepower loss when the rear
    gear is too high. The horsepower is being absorbed in just trying to
    keep the rotational mass spinning. Please keep in mind that your
    engine's horsepower never changes but what gets to the dyno or drive
    surface does.
     
  2. wayne94mustang

    wayne94mustang Active Member

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    makes perfect sense
     
  3. Matt94GT

    Matt94GT Legend SN95 Supporter Retired Staff

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    thanks man
     
  4. blackfang

    blackfang Active Member

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    No Problem, just adding some more tech.
     
  5. Outlaw97GT

    Outlaw97GT Active Member

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    Good write-up... :thumb:

    Some how much HP loss will there be? any idea's...
     
  6. NITROUSINJECTED

    NITROUSINJECTED Active Member

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    id think that it would just move your horsepower numbers around not so much add hp
     
  7. Lightning Struck

    Lightning Struck Legend

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    hmmmm and most dyno shops ive talked too says gears dont have effect on hp numbers. :rollinglaugh:
     
  8. 91lxcoupe

    91lxcoupe New Member

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    Makes perfect sense. The only reason dyno shops don't think gears effect hp ratings are because they dont effect hp at the flywheel. It technically changes rwhp but when you take your car to the dyno they load it up differently depending on ratios.
     
  9. Downshift

    Downshift Well-Known Member

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    So by doing that would the torque be increased as the hp is decreased?
     
  10. Monsterbishi

    Monsterbishi Member

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    Bingo.
     
  11. Adam

    Adam Member

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    The torque multiplication is increased, not the actual torque. Your rear gears can only increase the multiplication factor of the cars torque output, not the actual torque number that the car produces. For example, you have a bolt thats stuck, you can't break it loose, so you get a longer breaker bar to break it loose, your not any stronger, your just using a tool to multiply the amount of torque that your arms can produce....same exact thing :)


    And for the record Dynojets are all over the place when it comes to dyno numbers. A true load bearing dyno like dynapaks are much more accurate for tuning. As people spend more time involved with cars/dynos/racing etc they see how unimportant dyno numbers are, except in your internet sig..lol You would be surprised how much my car doesn't make. My 1st 12.92 pass was 247rwhp/271rwtq. Now i am around 260rwhp/285rwtq.
    Adam
     
  12. 4.6snake

    4.6snake New Member

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    you have to input what gears you have into the dyno before you run so the machine can compensate for it.
     
  13. 97gtconv.

    97gtconv. Member

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    ditto, did that before and after from my 3.08 to 4.10's no difference
     
  14. nicely put
     
  15. 19mustang95

    19mustang95 Legend

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    good analogy....and i love breaker bars, makes life so much easier. ;D
     
  16. Knuckles281

    Knuckles281 Active Member

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    that's a good post
     
  17. Venom351R

    Venom351R Legend

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    exactly
     
  18. realitygt

    realitygt Legend

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    wow, did you strip it and put massive slicks on stick rims? that's a sick time for just 247rwhp.
     
  19. LaserRedSVT96

    LaserRedSVT96 New Member

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    then does that make my 3.73s the culprite for my lower than average dyno numbers? maybe i just need a tune. i dunno.
     
  20. Venom351R

    Venom351R Legend

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    no, that would have more to do w/ altitude, temp, plus every dyno is different due to those reasons and others.