Why do I make more torque in 4th than 3rd?

Discussion in 'General Subaru Discussion' started by SurlyOldManMN, Nov 17, 2011.

  1. SurlyOldManMN
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    SurlyOldManMN Omdat fok jou Staff Member

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    Crossposting from lgt.com forums because this is really, really pissing me off...

    n00b question, I know...

    Everything I understand about torque says I should have more mechanical advantage (and subsequently more torque) in lower gears. Whereas gear decreases in size as we row from 1st-6th, mechanically speaking I should see progressively less torque. In preschool terms, it's easier to use a big gear to spin a little gear than it is to use a little gear to spin a little gear.

    What else am I not considering here? This has been bothering the piss out of me ever since my car was dyno'd in 4th (should be 3rd even though it's a 6 speed, due to gearing) and put down higher numbers than 3rd.

    I'm sure it's something that's going to cause a massive, epic facepalm, but I just can't wrap my head around this.
     
  2. Mr.Tran
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    Mr.Tran Well-Known Member

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    What's was the difference between the two? Heat soak may have been the culprit.
     
  3. Nuke
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    Nuke Well-Known Member

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    I lol'd
     
  4. SurlyOldManMN
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    SurlyOldManMN Omdat fok jou Staff Member

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    315wtq/250whp in 3rd
    336wtq/263whp in 4th

    Dude, don't leave me hangin here! :frantic:
     
  5. xluben
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    xluben Well-Known Member

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    The dyno compensates for the added mechanical advantage of the gear. The calculation it uses should take gearing out of the equation (references it all back to 1:1). It can tell the gearing based on the rpm and the rate at which the rollers are being turned. Your car is definitely making more actual torque in a lower gear, but that's not what the dyno is trying to measure.

    Likely the reason you made more torque (after all the calculations are taken into account) is because the taller gear resulted in a slower pull. The added time allows your turbo to spool, etc.
     
  6. SurlyOldManMN
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    SurlyOldManMN Omdat fok jou Staff Member

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    This makes sense, but then I expect to see the same numbers between gears.

    It's a VF54 mang, I'm on my way down by 3k... ;)

    Here's the plot for reference:

    [​IMG]
     
  7. idget
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    idget Want to pokéman? PM ShortytheFirefighter Staff Member

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    Same reason the dinosaurs died out. Because you touch yourself at night.
     
    curly2k3, dustin8511 and carl like this.
  8. Chux
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    Chux Well-Known Member

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    This. Look how much sooner your turbo spools on the graph. The peak is only .1 higher, but it happens quite a bit sooner (which is where your torque peak is). Also the average boost pressure goes from 13.6 to 14.1.
     
  9. xluben
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    xluben Well-Known Member

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    Yeah, you can see that it spools much sooner (rpm-wise) when you are in a taller gear and the pull takes longer.

    The rest is probably just normal variation (heat soak, warm headers, slight AFR changes, etc, etc) and probably has nothing to do with the change in gear.

    Hell, my car had something like 40ft-lbs of torque difference between the two pulls. Absolutely no changes were made, but the car spooled quicker and boosted higher the second pull.
     
  10. SurlyOldManMN
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    SurlyOldManMN Omdat fok jou Staff Member

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    Hmmmm...

    So far I think it's even money between these two:

    336wtq/263whp it is!

    I didn't think of the earlier boost translating to higher peak torque. I think I can find the answer to that one with some turbo dynamics learnin'.
     
  11. WRX1
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    WRX1 _ Staff Member

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    The load on the motor is different. You should see some of the crazy graphs that 4eat's make because you are holding the gear longer. If you were to log both of those runs, you are more than likely hitting different load cells within the map, giving you little different timing and boost numbers (or target numbers). Couple that with temps and how the car/turbo comes online (which also effects target load cell numbers), its not a big shock to see a difference in numbers. You could probably do a pull in 5th and generate a bit more tq.

    Russ
     
  12. Nuke
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    Nuke Well-Known Member

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    Lol everyone expect to hit the same psi in 1st gear compare to 3/4th