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4.10 Differential and RPMs

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    4.10 Differential and RPMs

    Hi,

    Recently converted my 86 325e to the 2.7i with 885 head, 1.3 motronic, etc. My differential was a 2.93 and very drive-able on the freeway with 70mph being around 3500 RPM (if i remember correctly, been awhile since I drove it).

    As of yesterday, I swapped in a 4.10 LSD from an 89 325i vert auto. While it is much faster off the line, 70mph is now at 5000 RPM. This pretty much makes it useless to drive on the freeway without a fear of running it that hard constantly.

    My questions: does this high of an RPM sound correct for my set up? It seems pretty high. Could my tach possibly have been wrong this whole time? I know my speedometer is off by around 4mph on the low end and about 8mph on the higher end, according to GPS speedo apps. I am running stock bottlecap wheels aswell.
    Last edited by Untouched; 05-09-2016, 12:04 PM.

    #2
    Use this:
    http://www.bokchoys.com/differential...calculator.htm

    With your ETA gearing info from here:
    http://www.strictlyeta.net/technical...ications2.html
    Ratios:(1/2/3/4/5/R): 3.83 / 2.20 / 1.40 / 1.00 / 0.81 / 3.71

    I put in the required info with the stock 195/65r14 tires and got 3259 @70 mph. My best guess is that you still have the 325e coding plug in the cluster and the wrong RPMs are showing. I would verify that you have the correct coding plug and that you are getting a accurate RPM reading.
    sigpic
    1987 325e Lachssilber w/ MarkD chip and late bumpers, Zender EVO spoiler, s3.25 LSD

    Originally posted by nando
    I don't think there's really strong evidence that ZDDP harms cats.

    Comment


      #3
      Originally posted by itsonlygeorge View Post
      Use this:
      http://www.bokchoys.com/differential...calculator.htm

      With your ETA gearing info from here:
      http://www.strictlyeta.net/technical...ications2.html
      Ratios:(1/2/3/4/5/R): 3.83 / 2.20 / 1.40 / 1.00 / 0.81 / 3.71

      I put in the required info with the stock 195/65r14 tires and got 3259 @70 mph. My best guess is that you still have the 325e coding plug in the cluster and the wrong RPMs are showing. I would verify that you have the correct coding plug and that you are getting a accurate RPM reading.
      The problem is that I have switched it to a 7k tach a while ago. I do not remember if I switched coding plugs around or not or what this tach is even from at this point. Since I am running off the 89's electronics (motronic 1.3), is it fair to say I can get a tach from an 89 and it will work?

      EDIT: I have just switched the tach from the same car I got the differential from ('89). The tach reads the same as before. So... I do not think this is the problem. Any other suggestions?
      Last edited by Untouched; 05-09-2016, 11:10 PM.

      Comment


        #4
        FIXED. It was the coding plug! For some reason I thought that the coding plug resided within tach and just depended on what year/engine it had based on tach alone. Swapped out the white coding plug on the bottom of the cluster and all is good. Revs are reading normal and I fear not to drive on the freeway. Thanks!

        Comment


          #5
          Originally posted by Untouched View Post
          FIXED. It was the coding plug! For some reason I thought that the coding plug resided within tach and just depended on what year/engine it had based on tach alone. Swapped out the white coding plug on the bottom of the cluster and all is good. Revs are reading normal and I fear not to drive on the freeway. Thanks!
          Sweet! The devil's in the details.

          Just out of curiosity, how do you like how the car drives with the 2.7i? I am told that it is great off the line and has good torque through out the rpm range, but that the engine falls flat in 4th and 5th gears.
          sigpic
          1987 325e Lachssilber w/ MarkD chip and late bumpers, Zender EVO spoiler, s3.25 LSD

          Originally posted by nando
          I don't think there's really strong evidence that ZDDP harms cats.

          Comment

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