Transmission

For the transmission, you have many choices to pick from but the most commonly used ones are the Getrag S5D 250G or ZF S5D 320z from e36s. Transmissions have a manufacturer name and different name that BMW refers to. For example, the "ZF S5-31" is called "S5D 310Z" by BMW. The S5D 320Z is an updated version of the 310Z with minor changes including +10Nm rating, black plug covers for shift springs, and supposedly different fluids but they should be interchangable [21]. I believe the 310Z was used before 1996 and 320Z was used after that (Unconfirmed). They also have their own variants like the 320Z-THCH, 320Z-THMW, 320Z-THDY, etc with minor changes. [22] Since they're all practically the same, I'll be referring to them as the ZF 5 speed

Below is a table on decoding transmission names [20]

• Getrag S5D 250G
  • E36: 320i, 323i, 325i, 325is [15]
  • E46: 320i, 320Ci, 323i, 323Ci, 325i, 325Ci, 325ti [16]
  • E34: 520i, 525i (1992-1995) [17]
  • E39: 520i, 523i, 525i [18]
  • E36/7 Z3: 2.0, 2.2i, 2.5, 2.5i [19]
• ZF S5D 310Z
• 1991-1992 (up to 7/92) E34: 525i [24]
• 1994-1995 E34: 530i (this version has a different bell housing that fits the V8 engine. Not compatible with this swap)
• Getrag 260/5
• 1989-1990 E34: 525i (M20) [24]
• E30: 325i Sport [23]
• E28
• Getrag 260
• E30: 323i, 324td, 325e, 325i, 325iX [23]

Transmission Mounting

If you have a transmission that was used with M20s, like the getrag 260, it will sit tilted 15 degrees to the right once bolted up to the M50 [25]. This means you will have to use a custom transmission crossmember to account for the angle like this one from garagistic.

If you used any of the other transmissions, which came factory with the 24v engines, you will need the stock E30 manual transmission crossmember P/N: 23711176574

For the transmission mounts, you can use mounts from any small-6 (non m30) manuals (ZF 5mt, ZF 6mt, G260 5mt, M3 6mt) [26]. In my E30, I have a ZF 320Z from an E36 328/M3 and I used the stock mounts that came with it P/N: 22316799331. You can also choose to go with an aftermarket polyurethane, delrin, or solid aluminum mounts but this will increase vibration. I chose the stock ones for more comfort.

Differential

When picking your differential, there are multiple factors that need to be considered: ratio, case, and open vs lsd. This section will go over each one of these aspects. I have also created a tool to help you pick which kind of gearing you want. Below is a table of all the diff options that came with E30s. Note that this info may or may not be accurate so make sure to search online for your specific model.

Note: The data from this table comes from e30zone.net[30] and may be inaccurate as I've been told all E30 verts have LSD. Again, verify your diff by the tag, part number, or spinning the input/output flanges and comparing the ratio.

Ratio

The differential you pick will affect your torque, mpg, and top speed. The diff ratio (aka final drive ratio) is the ratio of the mating gears, usually expressed in decimal format. Example: A 3.25 diff meaning it takes 3.25 turns on the input to make 1 full rotation on the output. Automatics are usually paired with a higher ratio diff whereas manuals are usually lower ratio. 4 cylinder cars are also usually paired with a higher ratio diff whereas 6 cylinders are usually lower ratio.

• Lower numerical value = lower ratio = taller gearing: less revs, less torque, higher top speed, better mpg
• Higher numerical value = higher ratio = shorter gearing: higher revs, more torque, lower top speed, worse mpg

The diff you pick depends on what your goal is with the car. If you want better highway cruising, choose a lower ratio diff.

Case Size

E30 diffs are either medium or small case. This depends on the size of the ring gear (aka crown wheel) which can be seen in the diagram below. Medium case diffs have a 188mm ring gear and came on all 6 cylinder E30s besides 320i. Small case diffs have a 168mm ring gear and came on 4 cylinder E30s plus the 320i 6 cylinder. In terms of case size, "they differ in length but not width, and therefore are interchangeable without changing drive shafts" [28]. To do this, simply loosen the drive shaft locking nut and adjust the length of it.

Open vs LSD

Open diffs send power to the wheel with least resistance. This won't cause any problems during normal driving but can be an issue if one wheel looses traction. It also means you can't do donuts and drift so open diffs are boring. You'll just end up doing one wheel peels and look goofy. If you don't plan on getting sideways with your car an open diff is fine but an lsd will always be more fun.

LSD stands for limited slip differential. There are multiple different kinds of mechanisms used to achieve limited slip. For E30 diffs, there are mechanical LSDs (uses clutch packs) and viscous (uses viscous coupling, similar to torque converters in automatic transmissions). LSDs came standard on the M3, iX, and sport models (is, es) where iX models had viscous and all others used clutch packs [29]. For other models, an LSD could be optioned. To find out if yours is an LSD, lift up the rear of the car and turn one wheel by hand. If the other side spins the same direction, it is an LSD. If it spins the opposite direction, it is an open diff. Another way to check is by using a vin decoder like this one. Make sure to physically check it though because a previous owner may have swapped the diff in your E30.

Swapping Internals

Gear Ratio Calculator