Changing Rear Ratio for Track

Discussion in 'Drivetrain, Exhaust and Electrical' started by docs302, Dec 5, 2018.

  1. docs302

    docs302 TMO Race

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    I have noticed quite a bit of slop and clunking in the rear axle at low speeds and this might be a good time to swap the ring and pinion for a lower ratio. At a few of the tracks/hills that I run regularly I am getting into 5th gear near the end of the front straights. I'm considering a drop to 3:55 or 3:31 but don't know if they are going to be enough to keep me out of 5th. I also don't want to kill my acceleration out of the slower sections as the 3:73 works well with the MT-82 ratios. Can anyone comment on their experiences? I also plan on dropping in a T2R at the same time.
     
  2. Coz

    Coz TMO Race

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    With a 27" tire and your current gears you'll hit around 7300 rpm in 4th. If you're running at NJMP or especially Pocono, I can see why you're going into 5th. If you drop to a 3.55 you'll be able to get to about 130 mph at 7500 rpm. With 3.31 probably around 135 at 7500 rpm.

    Are you needing to drop into 2nd often? That may be more of a reason to switch ratios than the front straight.

    Out of curiousity, why are you thinking of going from your Boss T2 Torsen to the T2R. Bias ratio? What are you encountering that is driving the potential switch?
     
  3. docs302

    docs302 TMO Race

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    Coz, I see you are in my area so if you know NJMP you know what I mean. On both Lightning and Thunderbolt main straights I am needing 5th gear at half to 3/4 length which really kills the acceleration. Same thing happens at Pocono (probably can't prevent that one) and on the fastest hillclimb I run in Reading which again kills progress. So if I can stay in 4th I will be in a much better position when it comes time to brake. I don't remember going down to 2nd so I don't think that's an issue.

    As for the T2R, maybe I am not understanding the bias difference between the Boss Torsen and the FR500 but additional torque to the weighted tire sounds like a better traction situation, especially with a 315 width. But if my interpretation is off please weigh in.
     
  4. Coz

    Coz TMO Race

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    I don't profess to know a lot about the differences in bias ratios. I know the Boss T2 is 2.7:1 and the T2R is 4:1. I was actually hoping that your experience would help me. Based on the link below, and other discussions I've been under the impression that the T2R is better suited for autocross.

    https://torsen.com/ask-torsen-type-2-vs-t-2r/
    (next to the last paragraph is key)

    If you ever take your car to the drag strip, it is my understanding that the Boss T2 (same as in the 13-14 TP GT500 and GT) is stronger in that situation than the T2R.

    I would recommend you make a table of the various rpms a combo of tire size, gear selection, and gear ratio. Then, if you have any videos with a data overlay on them at NJMP, see what mph you're at in key areas on the track and then see what rpms you'd be in (and hence gear selection) with either the 3.55s or 3.31s. See if they will make you drop to a lower gear than you do now in order to get you into you're powerband. If so, you'll have to assess how often that may happen and if it is worth eliminating the extra shift on the straights that you have now.

    Something like this
    upload_2018-12-5_22-36-5.png
     
    Last edited: Dec 5, 2018
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  5. Mad Hatter

    Mad Hatter Gotta go Faster

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    Remember that if you have 315/30-18 slicks or similar you will be at 25.6" tire diameter.... Thats why I changed to a 3.31 rear end for three reasons..... 1) higher top speed on the straight 2) Useful 2nd gear for the tight curves. 3) 5th gear is the weakest in the MT82, tired of killing the syncros in 5th. Now I get 131 mph in fourth at 7750 rpm

    And its all Grants fault :)
     
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  6. docs302

    docs302 TMO Race

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    Coz, thank you much for the Torsen discussion and tables. I have read many but that one makes the most sense. I do both hillclimbs (closer to autocross) and track so there is a tradeoff. I will have to think about this a little bit more. I believe the TruTrac diff has a bias between the Boss and T2R Torsens so maybe that's the best compromise. I have some video of Thunderbolt so the tables you provided will really help.

    Mad Hatter, I'm right with you on all your points. I didn't think the 3:55 would be low enough and you seem to confirm my thoughts.
     
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  7. docs302

    docs302 TMO Race

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    Yeah he's a bad influence ;)
     
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  8. Coz

    Coz TMO Race

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  9. docs302

    docs302 TMO Race

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    I try shift by 7500 but often hit the rev limiter
     
  10. Coz

    Coz TMO Race

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    Lol. I hope you have billet OPGs.
     
  11. docs302

    docs302 TMO Race

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    Not yet.....but someday I will :)
     
  12. Mad Hatter

    Mad Hatter Gotta go Faster

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    Sneak in a pair of LM intake cams when you do ;)
     
  13. Coz

    Coz TMO Race

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    Did you still have the stock Boss 302 exhaust cams or do you have the 302R exhaust cams?
     
  14. docs302

    docs302 TMO Race

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    Why not!
     
  15. docs302

    docs302 TMO Race

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    Stock motor. Only changes are Kooks LTH's. A CAI and dyno tune will be completed over the winter. Otherwise it stays that way until I kill it.
     
  16. Coz

    Coz TMO Race

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    Sorry, I meant the question about the cams for @Mad Hatter . He recently installed the L&M intake cams and seems very happy with them.

    I bought the Cobra Jet oil pump with the billet gears, a Boundary crank sprocket, MMR timing chain, guides, and tensioners and will be going with the L&M cams and Boss 302 springs while I'm in there. Just wondering what to do about the exhausts. L&M offers a "paired" exhaust set but it's another $600. The stock Boss 302 cams are much cheaper. Also going with the 2018 intake manifold.
     
  17. Mad Hatter

    Mad Hatter Gotta go Faster

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    Stock Boss exhaust cams.... Looks like no need to change them.
     
    Last edited: Dec 6, 2018
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  18. Mad Hatter

    Mad Hatter Gotta go Faster

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    thats what I said.... But things have a way of sneaking up on you... Should get a tune with the LTH... Shaun from AED chewed me out for not having one.... the Kooks are a great upgrade, gained 30hp and lots of bottom end (with a tune)
     
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  19. Mad Hatter

    Mad Hatter Gotta go Faster

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    Delighted with the intake only cams.... Can't wait to get some time on the dyno, but the seat of the pants meter says something very good happend! My co-driver in a recent 3 hour race has a 430rwhp S550.... He was impressed how much more power/torque I have....
     
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  20. Grant 302

    Grant 302 OPM Spent: $587,402 Moderator

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    Just FYI the Boss Torsen is also a T2R...just with the 2.7 bias as noted. From inference, the Boss unit is a stronger/updated design.

    As always ;) :D
     
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