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4:27 rear dif ratio

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  • 4:27 rear dif ratio

    Anyone running a 4:27 ratio in their rear dif? It appears to be a good trade off for both track and street duty. The 4:45 seems a bit high reving for highway cruising.

    Any thoughts?

    Thanks
    1988 Diamond Black / track
    2006 4 Runner LTD V8 / DD
    2003 Tahoe Z71 trail / tugger / 225,000 miles (3 trannys, same original engine)
    1992 325ic S52, OBDI, M3 suspension (Finished!) Fast and Fun for the Mom
    2011 Superfly (Nirvana!)

    sigpic

  • #2
    I run a 4.27 in my E21 320i with M10 motor. Transmission ratios are very close to US E30 M3. Stock ratio in an E21 is 3.91, so this was a very nice improvement. The 4.45 is very short unless you have around 9000rpms to reach an acceptable top speed of about 150mph. The 4.45 makes 1st gear useless and you can start in second everywhere.
    Photos

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    • #3
      Anyone interested in creating some dialog here? Chris? Mick? Lee? Anyone?

      A local expert has a 4:45 that I can purchase. But again, I am concerned about highway cruising.

      Would the 4:27 be a better fit for track and street driving?

      THANKS!
      1988 Diamond Black / track
      2006 4 Runner LTD V8 / DD
      2003 Tahoe Z71 trail / tugger / 225,000 miles (3 trannys, same original engine)
      1992 325ic S52, OBDI, M3 suspension (Finished!) Fast and Fun for the Mom
      2011 Superfly (Nirvana!)

      sigpic

      Comment


      • #4
        I bought my 4.27 from local 2002 guys that run VIR. They run 4.45s. The rpms for crusing on the interstate at 80 are annoying and eat gas. For a purpose built car I'd say go with the 4.45 for the punch it gives, but i'd have top end built to go the extra revs. For a street car no way in my opinion, I'd say with the stock 4.10.
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        • #5
          It all depends what you do with your car.

          Hill climb and rally M3s had up to 5:1 diffs.
          If you race, the final diff ratio will depend on the track. To take advantage of all your gears and get the best acceleration, you should reach top gear and redline at the end of the fastest section of the track. This is a general rule that doesn't take in account how good the car feels in certain corners in a given gear.

          Final ratios are the mechanical way to change the engine's torque. But what are you going to sacrifice? Better accelerations will induce higher engine revs, noise and fuel consumption. How much are you willing to trade? Do you prefer performance over comfort. Personally, I do not race with the M3. I only do HPDEs and daily driving. I am quite happy with the 3.75 LSD from the e30 325, especially on highways.

          BTW Most 4.27 and 4.45 I know come fro small case diffs such as the ones from the e30 318 cabrio auto and e36 318 cabrio auto. The gears are smaller than what the medium case M3 diff needs. They are not as strong.

          To finish my comment. If I were to compete with the M, I would install a higher diff ratio along with the OD tranny, to achieve best acceleration out of corners.


          [email protected]

          1969 2002 racecar
          1989 M3 racecar
          e39 Touring

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          • #6
            I keep telling myself that I'm building it as a dual purpose car. I've even gone out of my way to maintain the A/C. I will never race it. However, with the rear cage, harnesses and all it's really a two seater. I was hoping to maintain a nice mix of improved acceleraton at the track and OK highway cruising by using the 4:27. Maybe the stock 4:10 will be OK after all.

            Thanks all!
            1988 Diamond Black / track
            2006 4 Runner LTD V8 / DD
            2003 Tahoe Z71 trail / tugger / 225,000 miles (3 trannys, same original engine)
            1992 325ic S52, OBDI, M3 suspension (Finished!) Fast and Fun for the Mom
            2011 Superfly (Nirvana!)

            sigpic

            Comment


            • #7
              I have 427 rebuilt by don fields 1st gear is useless but hway criusing isnt too bad but i love drop the a gear when i step on that gas peddle awesome mid range push !!!!!

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              • #8
                If it's a dual purpose car I definitely wouldn't do it. If you're making good power usually 1st gear is worthless, this will make it more so. My engine is screaming at 80mph and I wouldn't want it any higher. I would get the CR tranny and do some calculations and find a rear end that will give you a slightly taller 5th gear and still keep the other ratios similar. (I did it awhile ago but forget what I got).
                "It is needless to say that self-propelling vehicles, like other machines, will never do as much for one who does not understand them as for one who does."

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