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  • Gearing

    Getting opinions on what works best as far as gearing. I am wondering if it is better to run one gear (1st only) or top out in 2nd with first for the really slow stuff and when I blow a corner
    I have put a stock LS1 in my BMW E36 and running an old school 4 speed. Yes it's way over powered I know but it's a "what the hell project"! I am getting ready to rebuild a transmission (2 options) for it and starting to consider gearing options. Also have 2 diff ratios to choose from.
    Below are the options - max speed is at 6000 rpm with 205/65/15 soft compound rally tires.

    A) 2.43 first gear - 3.23 diff = 58 mph / 20 mph = 2000 rpm
    B) 2.43 first gear - 3.91 diff = 48 mph / 20 mph = 2500 rpm
    C) 3.42 first gear = 41 mph / 2.28 second gear - 3.23 diff = 62 mph / 20 mph = 2000 rpm
    D) 3.42 first gear = 34 mph / 2.28 second gear - 3.91 diff = 51 mph / 20 mph = 2500 rpm

    Personally I'm considering option C as it will keep me in the meat of the rpm range 2500-5000 for most of the course but A would do that too. Just not sure if gearing for 1 gear is smart.

    Thanks!
    Last edited by OffRoadE36; 09-27-2018, 12:54 PM.

  • #2
    This really depends on what your local courses and venues are like. Locally to me and for the National Championship, A would probably work very well for you. C isn't bad either, though I'd be curious where 2nd @ 25mph puts you on your torque curve. Basically you want to shift no more than once a run, so if a 20-25mph corner puts you into a torque no man's land, I'd eliminate that option. B and D are non-starters in my opinion unless your local region has tiny venues.

    Since you are designing this around 205's, C has the added benefit that you can further tweak it by stepping down to 195 or even 185 tires to get you exactly where you want to be with speeds. Unless you're running hard packed clay venues, the loss of width likely won't hurt you much, if any.

    In regards to gearing for first only, at Nationals this past year, the first course allowed Andy Thomas to run his PF Celica in first gear, whereas Bob Seelig and I were having to use second in our cars. Andy was able to put 2 seconds on us every run! Rain somewhat screwed up the comparison on the latter courses, but we essentially ran comparable times on the third course.

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    • #3
      2nd gear should be good for about 70-75mph, assuming you have enough power that traction is kind of an issue. You will always be spinning the tires to some extent with a two wheel drive car so you need to account for that. Geared such, my car's actual top speed in 2nd was closer to 50-55mph as measured by radar gun. (4.78 rear, 2.2 second gear, 62cm tires, 10000rpm max)

      Ideally, you'd never need 1st gear. If you have "enough" power to make 2nd gear work for you, then 1st will be completely useless anyway. In theory mine was good for a hair over 40, in practice I tended to shift to 2nd after walking it out a car length or two past the start line.

      I keep getting goaded to do a V8 swap, and my best estimation is to gear it so FIRST is good for 70-75mph. It will still probably have traction problems, especially with all of that engine inertia keeping you from being able to effectively pedal the car. (Goal would be an engine that makes power in the 3000-6500 range with an 8000rpm redline)
      Last edited by Pete; 09-25-2018, 08:14 PM.

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      • #4
        I personally would use options B. this should put you in the meat of the torque curve coming out of corners, you will rarely need to shift and it can be adjust slightly with tire changes if really needed. It would also be really good to see the dyno curves on the car to see the torque and HP curves. It is fairly common for max torque to be around 4000K but I have also seen some pretty flat curves that have close to max torque at 2500 rpm. I would look at where you will be on the curves when you are at 20 mph. Slower corners will be in that range. IMHO - I think you want to be in the meat of the power band coming out of corner so the car will accelerate. additionally putting your peak RPM near the mid 40s means you should have some work done by the motor to slow the car down coming into corners which is also beneficial. My current setup is most like D which I think is not the optimum setup.

        Mark
        Mark Macoubrie
        RXB
        Kansas City Region
        2005 STI 41 PA

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        • #5
          I have edited the original post to show rpm at 20 mph. I also looked up the torque curve a stock LS1 and it's pretty flat from 2500-5500 @300 ft lbs. And yes peaking about 4000.

          If I don't want the rpm's to drop below 2500 it looks like D is optimal. Mark, what makes it not optimal in your opinion/situation? I'm guessing your torque has dropped off too much.

          My thought is I want to be geared a little high to (help me) minimize wheel spin. Also to have rpm's left to use shorter tires when desired, I'm definitely not getting anything taller in there.

          I know I'm not going to get this right out of the box, just hoping to not be too far off with the options I have. I could probably swap the 3.23 for either a 3.15 or 3.36 but they would be minimal changes. Transmission and or gearing changes there are more than I want to do at the moment.

          Also I will running in the Middle Georgia area but haven't been yet, just moved to the area. I was running in the Central and Finger Lakes, NY area and would go to the Nationals in the DC area every year. All those courses were pretty wide and fast which suited the car. The Central NY steward would make the courses tight in the morning session and open them up in the afternoon. Needless to say I did better in the afternoon.

          Richard

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          • #6
            For what it's worth, with the gearing I described, I was seeing no lower than 3500ish in the middle of slow corners, which translates to about 20-25mph assuming perfect traction. Apply the throttle and the revs would pick right up to the 5000-6000rpm range. At this point the engine was making roughly 170-180hp.

            Tire spin makes gearing calculation kind of a bugger. It's also why I like to see a redline a few thousand RPM higher than peak power, with a power curve that tapes off instead of falling off a cliff, otherwise you never actually get to use peak power. If you're spinning the engine higher than peak power, when the grip comes back then it will pull the engine down and you'll accelerate that much harder. But until then, you're making less power, which makes it easier for grip to come back.
            Last edited by Pete; 09-27-2018, 07:42 PM.

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            • #7
              Originally posted by OffRoadE36 View Post
              If I don't want the rpm's to drop below 2500 it looks like D is optimal. Mark, what makes it not optimal in your opinion/situation? I'm guessing your torque has dropped off too much.

              Richard
              For you setup D may also be fairly decent. For me in a 2005 STI I have top speed of 35 in first and 51 in second. Car starts making boost at about 2800 so in slow corners I either have to downshift and then upshift quickly (losing time) or come out of the corner slow (losing time). Because your car in naturally aspirated and make significant torque at 2500 RPM you may be find with option D. As David said you only want to shift once if possible or not at all. Pete also make a good point about wheel spin so you are never hitting max speed. there is typically going to be some wheel spin.

              Biggest competitor to the STI I have seen is an EVO IX. I race an EVO IX in my region on a regular basis. I have noticed they have much better acceleration off slow corners. The STI can hang with the EVO when it is sloppy and slick or when it if more wide open and corner speeds stay above 25 mph. If there is traction and corner speeds drop below 25 the STI starts to fall behind.

              I do not drive RWD cars in competition. Pete has better experience than I do in that realm.

              Mark
              Mark Macoubrie
              RXB
              Kansas City Region
              2005 STI 41 PA

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              • #8
                Thanks for the advice, I think just walking through the options with others has helped too.

                I'm going to set up for option D to start and if that gearing is too short I can easily switch to option C. This may be a little tall but I can shorten it a bit with tires.

                Now to get it completed and out on the track....

                Richard

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                • #9
                  That is the fun thing about Mod. You can do whatever you want. I've never dealt that deeply with the E36 but I remember that the E30 rear diff is easy enough to change that you could probably do it on lunch break, so you might even be able to do back to back testing on the same day.

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