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Review- 4.10 to 3.73 differential, '91 318 (M42)

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    Review- 4.10 to 3.73 differential, '91 318 (M42)

    While researching the swap I wished there were more detailed thoughts shared regarding each ratio. I hope my thoughts and observations will be helpful to others considering this! No, I didn't take pictures. Pretty sure you all know what a differential looks like. :p

    My 1991 318i came with a small case open 4.10 and was leaking a lot of fluid. While chasing a drive line vibration I noticed the diff bushing was pretty shot, so the time was perfect to do the swap. Even better, my friend had a medium case 3.73 LSD just chilling in his garage!

    Before I share my thoughts, let me point out my driving preferences and style. Yours are likely different. I drive slow and shift around 2500-3000 most of the time. With driving on the track for a few years and being 28, I don’t care to drive aggressively in most situations. I certainly enjoy pushing it through twisties and see on-ramps as “fun zones”. But overall, pretty laid back. Plus this is the wrong car to own if you care much about power related performance.

    Physical details-
    The medium case is indeed heavier. 20 lbs more is the number I have heard and it sounds about right. There is absolutely nothing that needs to be changed when going from a small case to a medium case. It is about 1.5” longer, if I remember correctly, otherwise the same. With my urethane engine mounts the engine is mounted at least 1” further back than stock and I still had no issues (although it makes getting the driveline in/out a touch harder). For those that haven’t looked at them side-by-side the diff covers are different and cannot be interchanged.

    Driving thoughts-
    I wish I had documented gear/rpm/speed relations better before. If you search around another forum has a full documentation of how rpm/speed relationships change with different differential gearing. In any case, at 73 mph in 5th gear the engine is at 3000 rpm’s. I think it was around 3400-3500 with the 4.10 before.

    Biggest difference is it feels like a “normal” car now. I tend to cruise in lower rpm’s and was always in 5th gear when cruising around town (35-45 mph speeds). Now I stay in 4th and I prefer that. It’s arbitrary what gear you end up cruising in, but with 5th gear feeling about right in town that means freeway speeds feel far too high. Also, 1st gear is actually useable now! I hated 1st gear before.

    The feel has certainly changed and after a week of driving it still feels strange when I first get into my car. I don’t feel that the car is slower. Best description I can think of is it feels like going from a lightweight flywheel back to stock. Still accelerates fine, but the snappiness is gone. The 4.10 makes it feel torquey on the freeway, not so much now. My guess is that any snap lost in acceleration is nullified by not shifting as often on the way to 60. Biggest impact this has is how beneficial a limited slip is. Quite frankly, limited slip is just not needed with the 3.73 gearing for a daily driver. I primarily wanted limited slip since I will likely be back in WI this year and it makes a world of difference when the roads get bad.

    Overall thoughts-
    If you daily drive your M42 powered 318 and spend much time on the freeway, do it. If you are primarily in town, I’d stick with the 4.10 and enjoy the extra snap. This gearing would be killer with a very low boost turbo set up! Or, not considering money, it would be awesome with a slightly closer ratio 6-speed – that would be the best of both worlds in my book.
    sigpic

    1991 318i Sedan Project

    #2
    I just recently went from a 3.64 to a S3.73, I'm happy with the swap and pretty much feels the same.

    I imagine that the 4.10 feels good on a track, but I can't stand crusing at high RPMs. Even 3.5k is almost too much for me. I guess that's what happens when our cars are from the 80s and expected 55mph speed limits? ;)


    btw-- if you happen to see a friend of mine driving her pink/red volvo S70 in Tigard, give her a honk! haha
    '84 318i M10B18 147- Safari Beige
    NA: 93whp/90ftlbs, MS2E w/ LC, 2-Step
    Turbo: ?whp/?ftlbs, MS3x flex @ 17psi

    Comment


      #3
      Quick update:

      First full tank of gas I saw an improvement of 3-4 mpg. Nothing was different with driving style or city/highway ratios. If anything I drove it harder at times since I went on a a few back road drives and wound out 1st, 2nd, and halfway through 3rd to get a feel for acceleration.

      Further thoughts on acceleration qualities of the taller gearing - it's just fine! Still would say the "snappiness" is gone, but it really pulls just fine.
      sigpic

      1991 318i Sedan Project

      Comment


        #4
        Originally posted by s_ribs View Post
        First full tank of gas I saw an improvement of 3-4 mpg.
        Man, that's pretty sweet. I always averaged 33-34 with the 4.10 in my old 318is. I miss that car. M42's are great..

        Comment


          #5
          I don't get these cars in regard to mpg. Some get 30+, others (like myself) only get low to mid 20's. Last tank I average 24.7 mpg.
          sigpic

          1991 318i Sedan Project

          Comment


            #6
            s_ribs: thanks for this thread. I'm running my original LS 4.10 and have been thinking about a taller ratio. According to your info, I think I'll keep what I have.

            Comment


              #7
              Originally posted by s_ribs View Post
              ...others (like myself) only get low to mid 20's. Last tank I average 24.7 mpg.
              That's a bummer.. My driving with that car was 75% highway. Even splitting city and highway driving evenly I never did less than 30 mpg.

              Comment


                #8
                Originally posted by Ryann View Post
                s_ribs: thanks for this thread. I'm running my original LS 4.10 and have been thinking about a taller ratio. According to your info, I think I'll keep what I have.
                Fantastic, glad it was helpful! This is exactly what I was hoping to find before I did it, glad it's fulfilling it's purpose.

                Originally posted by rThor432 View Post
                That's a bummer.. My driving with that car was 75% highway. Even splitting city and highway driving evenly I never did less than 30 mpg.
                Not to get further off topic, but some research points to intake vacuum hose leaks, coolant temp sensor, and O2 sensor having a significant impact on M42's mpg. Well worth the cost for these maint. items if mpg jumps up from 24 to 30+ like many get.
                sigpic

                1991 318i Sedan Project

                Comment


                  #9
                  When comparing MPG, remember that some folks are going off the economy guage, some people take the miles travelled divided by gallons used ( the most accurate IMHO). Another consideration is the terrain, where some HWY miles you encounter may have mountains or significant grade changes, versus flat midwest plains types of driving environments.

                  As the golden rule says.... YMMV.

                  synopsis: one drivers CITY driving might consist of the STEEP and HILLY streets of San Francisco, or the FLAT lands of another metropolis.

                  btw, I <3 my s4.10 m42 e30.



                  Originally posted by s_ribs View Post
                  Fantastic, glad it was helpful! This is exactly what I was hoping to find before I did it, glad it's fulfilling it's purpose.



                  Not to get further off topic, but some research points to intake vacuum hose leaks, coolant temp sensor, and O2 sensor having a significant impact on M42's mpg. Well worth the cost for these maint. items if mpg jumps up from 24 to 30+ like many get.

                  Comment


                    #10
                    ^^For what it's worth, my numbers are calculated based on miles travelled divided by the number of gallons used.

                    Comment


                      #11
                      primarily, the main difference in gas mileage is your right foot.

                      If you're serious about trying to get the most mileage out of your car, lightening it is a good start. light weight oil everywhere, more plug gap without detonation, freeing up induction/exhaust(to a certain extent exhaust for non-turbos) and moving to the top of a mountain and never coming back also helps.


                      If you're thinking that a lower geared diff is going to hurt your gas mileage because you'll be accelerating more, consider the actual amount of time that you spend accelerating on a daily basis, in terms of percentage. Somehow, I doubt it's more than 30% of your commute, even if you live in a big city.
                      '84 318i M10B18 147- Safari Beige
                      NA: 93whp/90ftlbs, MS2E w/ LC, 2-Step
                      Turbo: ?whp/?ftlbs, MS3x flex @ 17psi

                      Comment

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