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.
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.
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