So you're interested in 24 valve swapping your E30. I've put together this guide using many different sources as well as my personal experience to ensure all the info you'll need is in one place. Many other write ups exist out there but there is always some conflicting information so I will share what I've confirmed to work in my build as well as what others have said works.
24V refers to the 24 valves in the head, 12 intake and 12 exhaust. M20 engines were 12 valve and all 6 cylinder engines after that were 24 valve. For these swaps, the main engines commonly used are the M5X and S5X engines (M50, M52, M54, S50, S52, S54). This guide will go over swapping in stock motors but there is also the option to customize the engine too. For example, an M50 3.0 liter stroker using an M50 2.5 block with an M54 rotating assembly (crank, rods, pistons)[1].
For the engine arms, you can use any E36 6 cylinder engine arms P/N: 11811138249 for passenger side and 11811141137 for driver side. All of the 24v engines will have the same 6 holes used to mount the engine arms. Only 4 holes are used per engine arm but which 4 of the 6 holes to use depends on the chassis the engine is going in. To fit in the E30, you will use the rear 4 holes. Here's a video that shows this.
For the engine mounts, use the rubber oem if you want a more comfortable ride. You will need E28 mounts. Some say you need 535i mounts and others say M5 but either should fit fine. E28 motor mounts are different on the driver and passenger side. The driver side one is shaped like a hexagon and the passenger side is cylindrical. Some sources say you need to use two driver side ones and others say passenger side works. I was able to use two passenger side ones P/N: 11811128932. Usually the passenger side cylindrical mounts are cheaper. The mounting points on the subframe have 2 holes on each side. Normally, the stock M20 motor mounts have the stud on the inner holes. When you install the E28 mounts, align the studs with the outer holes instead.
You also have the option to go with an aftermarket mount, polyurethane, delrin, or aluminum which are all stiffer than the stock rubber mounts. This would be preferred if you plan on driving more aggressively like for track cars.
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]
Transmission Name | Definition | Options |
---|---|---|
S | Transmission Type | S=Manual, A=Auto |
5 | Number of forward gears | |
D | Type of top gear | D=Direct, S=Overdrive |
310 | Max input torque (Nm) | |
Z | Manufacturer | Z=ZF, G=Getrag, R=GM |
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.
Model | Engine | Ratios | Case Size | Type | |||
---|---|---|---|---|---|---|---|
316 before 9/1984 | M10 | 3.64 | small | open | |||
316 after 9/1984 | M10 | 3.91 | small | open | |||
316i | M10 | 4.10 | 3.91 | small | open | ||
316i | M40 | 4.45 | 4.27 | small | open | ||
318i | M10 | 4.10 | 3.91 | 3.64 | small | open | |
318i 2 & 4 door | M40 | 4.45 | 4.27 | small | open | ||
318iC & iT | M40 | 4.45 | 4.27 | small | open | ||
318is | M42 | 4.10 | small | open | |||
320i before 9/85 | M20 | 3.46 | small | open | |||
320i 9/85 - 9/87 | M20 | 3.91 | 3.64 | small | open |
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.
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.
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 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.
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]Since the transmission is tilted, this means the shifter will be tilted at an angle too. To account for this, you will need a custom shift selector rod too like this one from garagistic.