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Gearing: CR Trans + diff or just 4.45 diff

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  • Gearing: CR Trans + diff or just 4.45 diff

    The standard US tranny gearing is pretty far apart. I've been thinking about going with a CR gearbox. I know in europe CR M3's had 3.25 gearing but has anyone tried running the CR box with the US standard 4.11 diff or is it just too too short? I've seen most people change the diff gearing but 5th gear in the CR box is just like driving around in 4th (both 1:1) which isn't bad with the 4.11.

    Changing the tranny and diff gets expensive so on the other end, how about just changing the final drive to 4.45? I realize the tranny gearing will still be far apart but perhaps the shorter final drive can be a good enough substitute. Let the discussion begin...


  • #2
    3.25 is decent with the CR box.
    3.73 is a good compromise with the CR box.
    4.10 is a waste as you'll end up "losing" time shifting all the time.

    4.45 is a good diff to run if you're just looking for some more "get-up and go" type accel. Going to be a bit painful on the highway crusing at 80mph though I think.

    Thats all I got there....
    Chris L.

    Spray paint and tire shine doesn't equate to a "restoration!"


    • #3


      • #4
        Interesting that you guys would bring this up right now. I guess the US shift pattern was chosen to favor the 1-2 shift over the 5-4 shift, but, why did the US spec cars get the OD tranny and 4.10 diff, and the Euro cars got the 1:1 5th gear with 3.25 diff, when, as Han's chart shows, they work out about equal?

        Did the Wizards of Munich think we Yanks drove in 4th more than 5th? Anybody know why they did this? The US box gives a "shorter" 1st gear and less drop between 1st and 2nd. Does the US car have a better 0-60 time?

        If I am not mistaken, the euro setup has the engineering advantages of:

        1. a larger and thus stronger pinion in the rearset
        2. less friction loss in 5th gear since it's 1:1

        According to the chart, there is no real difference in overall final drive ratio between the US and Euro setups in 3rd, 4th, and 5th. If you put a dogbox in a US car with the 4.10, the math says that the dogbox 5th will be exactly the same MPH vs RPM that you had with the original US trans. in 4th gear. What you would gain is a 1st gear with 15.2:1 reduction instead of the stock US 13.8. I personally find the stock 1st gear "short" enough and I think the Euro setup would suit me better personally, but I dont' do much stop and go driving. So putting in a dogbox and keeping the 4.10 would be about the same as putting in a 4.45 rearend, in 3rd, 4th and 5th anyway, the dogbox has the engineering advantages above but costs more, and it gets you the dogleg 1st, which you may or may not like better than the US shift pattern. Of course going to the 4.45 rearend would be cheaper and easier, and would allow putting in a more aggressive positive traction clutch pack if you want to do that.

        What I want to know is has anybody put a 3.73 or the next step up into a US car? I could do with a few LESS revs at 70 to 80 cruise, or at least I think I would like that better...

        Now living in LA one has a lot of opportunities to accelerate to about 80, and frequently need to get there as fast as you can, and you wouldn't hold 80 for a couple of hours at a time. So LA has always been the home of 4.10 and 4.11 rearsets on American iron that revs to about 5000, 6000, so maybe either the dogbox or 4.45 would be what you want...Out here in Eastern Washington, you can go faster than 80 and can cruise 80 for 4 or 5 hours if you want to.

        To me anyway, this is a really interesting thread.


        • #5
          I have a 4.45 with a 2.3 and I really like it. It makes a very noticeable difference on acceleration. The gearing is still a bit long but the acceleration is way better.

          I also should admit that this is not a cheaper route than installing a CR box. My gearset alone was $750 and then I had to pay for my mechanic to rebuild the diff. I probably have $1500 in the 4.45 alone, albeti, with a completely rebuilt diff. The 4.45s are somewhat hard to come by.

          FWIW I do not find the 4.45 to be a problem at 80mph. My car runs at about 4200-4300 rpm compared to about 4000 rpm with the 4.10. My car is mainly a DE car and I do not have to rely on it for a daily driver however.


          • #6
            I have the CR box with 4.10's. Acceleration is quick, but the freeway is aweful over 70mph. I plan on upgrading to a 3.73 or something close to that (Z3 LSD unit maybe?). Either way, the CR tranny was worth every penny if you ask me. It made the car much more fun to drive on the twisties.


            • #7
              Originally posted by wookie1976
              I have the CR box with 4.10's. Acceleration is quick, but the freeway is aweful over 70mph. I plan on upgrading to a 3.73 or something close to that (Z3 LSD unit maybe?). Either way, the CR tranny was worth every penny if you ask me. It made the car much more fun to drive on the twisties.
              What rpms do you see at 65-80 MPH?

              88 M3 - LACHSSILBER/M TECH
              89 M3 - ALPINEWEISS II/SCHWARZ
              85 323I S52 - ALPINEWEISS/SCHWARZ


              • #8
                those kind of gearing charts which just plot
                a gear ratio are common but unfortunately
                they dont tell you much.

                you need to look at a rpm, speed, drop between gears,
                and how wide in terms of rpm each gear is. looked
                at with respect to your engine characteristic.
                then you have to decide if you want pure acceleration,
                or closer gears to make easier downshifts (more suitable
                for curves/tracks). for track use you can use taller
                first and second gears to have them closer to 3/4/5.
                acceleration from a stop light will be a bit slower,
                but on the track or backroads, the taller gearbox is
                the quicker one because you can downshift easier.
                on tight serpentines e.g., the first gear of the CR is
                still usable, whereas the US 1st gear is much too short.
                on the US box you would keep it in 2nd gear, on the CR
                you would be in first. its obvious which one will power
                out faster from the corner.

                in the case of the CR, 1/2/3 are pretty decently spaced.
                4/5 is also ok. but the 3/4 shift is not that great.

                I ran the CR with a 4.10, the acceleration is quite
                good to say the least. but it is not practical for the street.
                you will just end up using the next higher gear compared
                to your previous setup. top gear running is of course high
                rpm even at moderate cruising speeds of 100 mph.
                the net result is that you basically give up a (lower)
                gear for street use.

                The 3.25 is too long. A 3.46 is OK but still a bit long.
                I can run easily over 165 mph with a 3.46 diff. But,
                you can run a little lower rpm for 5th gear cruising.

                3.64 or 3.73 is pretty much in the middle. good performance
                but also higher rpm (about 500 or so) in the respective

                I would use 3.46 on a street only car and 3.64/3.73
                on a car that gets used on track.

                Regardless of which diff you use, you cant get rid
                of the 3/4 shift gap that is present with the CR box.



                • #9
                  Originally posted by Reelizmpro
                  What rpms do you see at 65-80 MPH?
                  65 is about 4k, 4500 maybe at 75-80, but I would have to go drive it to be sure.


                  • #10
                    Here's something I put together last year after doing a DE on this track. It's not a comparison between trannys, but, between diff ratios. A similar chart could be made for a tranny comparison. After you have built the comparison charts you can see what the differences in RPM and Speed are for your intended use.

                    My car is a euro with the original 3.25 diff, but, it has the US G265. I put this chart together to compare the 3.25 and 4.10 mated to the US G265 on this particular track.

                    One problem the 3.25 has on this track is turn 2. Going into turn 2 I downshift from 4th to 3rd. But, I can't carry enough speed into the turn (about 40 mph) and because of this my engine speed is just under 3k. I have no torque at this RPM and don't get any drive into and out of the corner. With the 4.10 I would be around 3.5k and just getting into the power band of my car.

                    I've got a DE at this track in about three weeks and should have my 4.10 installed by then. It will be interesting to see if my calculations are correct.

                    Damon in STL

                    Resized the image and added this one.

                    Last edited by peak; 03-20-2005, 02:42 AM.
                    Damon in STL
                    '99 e39 540 Sport 6spd (DD), '88 e30 M3 - GTS2 #72 - Motorcraft Ignition, Volvo Injectors, Thrush Turbo Muffler, Open Source ECU, Aerospace Connectors, Lowes' Polycarbonate, Alumacore Front Splitter and Rear Diffuser, Racer's Tape.


                    • #11
                      so are the cr and the us boxes the only ones people ever put in our car?
                      and ofter all this what would be the best track set up?