BMW E30 differentials are essentially the gearing module that connects the input driveshaft to each of the rear wheels. Some differential are “open” and some are “limited slip” or “posi-traction”. The primary difference being the ability of both wheels to spin in a slip event. An open differential is oftentimes cheaper because it is not designed to maintain traction of both rear wheels. You can most commonly experiment with this in the snow. One wheel spins and the other does nothing. In limited slip, at least 25% of the force will be distributed to the most heavily loaded wheel, or the wheel that normally would not spin, now gets 25% of the torque.
But there are clutch discs inside of the limited slip mechanism, and they do wear out. So a hard driven 25% diff may only have 5% lock. Which is why you should stay tuned for a more detailed blog about changing LSD clutch packs!
From the outside, you can easily denote a limited slip differential by 2 means: The tag on the differential has an S before the ratio. Example: S2.93, S3.73. You will also find an “S” stamped on the top of the differential. Be careful. The tags can always be removed and added to an open differential as a scam. However, the stamps cannot be added, so this is something to definitely check for.
The “ratio” is that of the number of rotations of the input to the number of rotations of the output. Most BMW E30 Differentials vary from 2.79 and up to 4.45. That means that the input shafts spin 2.79 times for each spin of the output shafts! With the transmission gear and vehicle’s speed being constant, the higher the differential ratio, the higher your engine will rev. A lot of racers experiment with the perfect ratio as to get the most useful horsepower during race day. The good thing is that you are not limited to simply E30 ratios! You can use the E23 7 Series, E24 6 Series, or most commonly, the E28 5 Series.
Just note that there is a bit more work that needs to be done if you are swapping from a different model. You must swap the rear cover and the output shafts, but this is easy. The output shafts remove very easily with a wooden stick, pry them out at the flanges. They should just pop right out!
You have 2 versions: small case and large case. Small case has a ring gear diameter of 168mm and the large case has a diameter of 188mm. The large case differential has been proven to handle upwards of 500ft-lb. See below for a common comparison between small case (left) and large case (right) E30 differentials. You almost never want a small case. The two sizes are 100% interchangeable in the E30 subframe.
Do you already have big power and want to provide additional bracing? Or perhaps want a sportier feel – – Something more engaging for the driving experience?
Well, Garagistic has you covered!
Feel free to browse store.garagistic.com and check out their selection of 80A, 95A, or delrin differential bushings. Installation is easy and can be done with basic hand tools!
If you’re looking for supporting more power, click below for the installation video on the ONE OF A KIND IN THIS INDUSTRY second differential mount. This is a must for any >250bhp upgrade to your E30. Installation is, well….. see for yourself!
To complement that power, strengthening your subframe is key and can also be sourced right here. Garagistic offers a simple, easy to weld kit to strengthen your subframe to where the differential mounts. 4 holes, all can be strengthened. Examples time and time again on the track have shown this area to be particularly susceptible to weakening during hard acceleration. With additional bracketry, this problem is now dissolved.
You can certainly swap your own gearing. But if you do, you must take note on what ratio differential you currently have and what ratio gears you WANT to go to.
The rule for swapping gearsets is that you have 2 ranges. The low gears (3.07 and below) and high gears (anything higher than 3.07). To make this easy, you must have the gearset for the differential that you want to change.
Example: You currently have a 3.25 differential and want to swap to 3.46. You can easily acquire a 3.46 gearset and swap into your 3.25 by exchanging the pinion and the input gears. This is because both are above the 3.07 ratio.
Same goes for the lower gears. Stay within the range.
If you decide to cross the 3.07 threshold with swapping gearsets, you must use 2 extra shims that are necessary to make up for the geometry of the gearset fitment. Without the shims (or removing them), you will certainly have fitment issues and have premature failure, noises, or vibration.
Thanks for reading!