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VLSD final drive ratios for auto-x?

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  • VLSD final drive ratios for auto-x?

    Ok, I'm fabricating another VLSD that will be put into my 86 2x2 NA, which has been dedicated to auto-x. I have the ratios 3.54:1 and 3.7:1, maybe 3.9:1, to work with here. Which should I choose? I know this is a "trial and error" and "what feels best" scenerio, but I'm also looking for other peoples expert opinion in the matter so that it can help me make the right decision the first time.
    The Z will be turbocharged later down the road too.
    [strike:ff0tp92h]1984 300ZXT[/strike:ff0tp92h]
    [strike:ff0tp92h]1986 300ZX 2x2 NA2T[/strike:ff0tp92h]
    2000 Porsche Boxster
    2007 Toyota Yaris

  • #2
    I would go with a 3:9:1. I tried one in my turbo car and loved the response. I think it would work out good for you being NA and having a lot of torque on "tap" in an auto-x.


    • #3
      If it's going to be turbocharged down the road, stick with the 3.7 and put lower profile tires on the stock wheels. This will give you a gearing advantage you can easily change around with the nest set of tires, in addition to lowering the cars center of gravity. If you go for a 3.9 you may end up only giving yourself traction issues once the car has more power, negating some of the benefit of the shorter final gearing anyway.

      I spend most of the time in second gear and a little bit of third during a typical autocross run for WMR SCCA events.


      • #4

        If you turbocharged your auto-x Z31, the 3.7:1 will be great with this LSD from Motorsport Auto...

        MSA Limited Slip Ass'y, 84-85 ZX Turbo, 86-89 ZX With R-200

        or this

        Quaife ATB Limited Slip Assembly, R-200 Differential
        84' 300ZX 2+2 NA (Hardtop)


        • #5
          quiafe is the torque-biasing or helical style diff.... I'm told it's not so hot for auto-x but better for long sweepers, so by that logic you'd be better off with clutch pack types ...

          but since the question is about the vlsd we don't have to go much further on topic


          • #6
            I ran the 3.7 LSD in my 2+2 and then switched it to my slicktop. I live in second gear in autoX events. It is very rare that I have to go to third. If you use the 3.9 you will be on the rev limiter all the time. Jason's suggestion about the lower profile tires is the best way to go. My 2+2 had terminal understeer. The slicktop is a much better autoX car.
            ...and how much HP do you have at 0 boost


            • #7
              raidon84300zx wrote: "what feels best"
              Definitely not the thing to do when it comes to the rear end. A high ratio rear (ex. 4.10) would *feel* fast but when it comes to the numbers, a lower ratio would end up actually being faster even though it doesn't feel like it.

              A lower ratio tends to be good for turbo motors since they produce a lot of low and mid range torque. The lower ratio allows you put more of that torque to the ground over an extended period compared to the higher ratio giving you only short bursts so to speak.

              N/A engines with a narrow power band do better with the higher ratios since it keeps them in the power band on the shift.


              • #8
                I vote 3.9 geared LSD.

                I loved the 3.9 that I swapped the 3.54 out for in my AE. No one was safe once I put that sucker in.
                It is not recommended to confirm proper installation by driving into walls or other barriers as this could cause personal injury or damage to the vehicle.


                • #9
                  I have quite a bit of autox experience, so hopefully my words will cary a little weight. For autox, I would use the 3.54, even though I use the 3.7 on my cars. There have been too many times when I had to waste time shifting to third for just a second.

                  Also, a torque sensing diff, with the proper bias, is MUCH better than any clutch type unit. I have a torsen t2r in one of my cars, and it is the best diff I have ever raced on.
                  Chuck Stong
                  300+ Parts and Performance owner
                  2002 ZCOT president and always active member


                  • #10
                    does the torsen help bring the back around more than say suspension changes?

                    reason I ask is some people try to get rid of toe-in to make the rear more twitchy and more likely to spin around at full weight over, the torsen will send the torque to the outside so you will get some good grip out of it, but a proper clutch pack will as well with perhaps less than half engine torque

                    of course it could just be personal preference, I myself have never driven a rwd (awd yes but...) with torsen so I'm just asking based on what I've heard


                    • #11
                      The way the torsen works allows more power to be put to the ground, far better than a clutch unit. When I installed one, it made a huge difference in handling. The car can still be made twitchy be other suspension changes, but the torsen will make it more stable. They also last longer, as there is nothing to wear out.
                      Attached Files
                      Chuck Stong
                      300+ Parts and Performance owner
                      2002 ZCOT president and always active member