The E24 gets her new rear end.

As I’ve mentioned, this E24 was purchased as a chassis. All the running gear had been upgraded with parts from an E28 M5. That included the motor, transmission, differential and brakes. However, none of those powertrain parts were part of the purchase deal; all I owned was the chassis. That’s a real shame, since this car had one of the best choices – a rare 3.91 limited-slip 210mm “large case” Motorsport unit. Out it came, off it went, to another enthusiast.

So…she needs a new rear end. I wasn’t unhappy with the situation – I usually prefer to customize cars with a ratio suited to my driving style, my overdrive gearbox and some slightly oversized wheels. In the short-term, all I had was an oddball large case/medium gear set 3.09 “open” diff from my Euro parts car. That’s not an ideal ratio, it’s designed for reasonable fuel economy and high-speed highway driving.

The E24 chassis has been fitted with a wide variety of differential ratios over the years, and compatible parts can be found in many other BMWs. Unusually, the early E24 cars (’77-’81, E12 chassis) cannot donate a differential. It’s a completely different design, called a ‘side-loader’. It’s incompatible with the later cars (’82-’89, E28 chassis). The newer design is a ‘rear-loaded’ differential. inside two different cases (medium & large) over the late-model production run.

I was also set on a limited-slip locking differential. They’re a real bonus for rear-wheel drive cars. As I was researching diffs for my older E30, I learned that a few of the late-model Z3 six-cyls have a “torsen” locking diff. Since many E30 diffs are easily modified to fit an E24, that’s what I started searching for. Unbelievably, after several months of eBay, CL & forum searches, one turned up virtually right in my backyard. The donor was an unfortunate Z3 2.2, rear-ended and being parted out. I took a chance on the diff – not all of them are LSDs – and they aren’t labeled. Much to my relief it ended up being just the part I was searching for. I needed a few other parts – the Z3 unit requires the proper E24 rear cover version (mine’s the offset type), along with a new gasket, E24 output flanges (straight swap with the E30 parts). I also had to swap over the Z3 speed sensor, since it’s not the same shape as other ones. That can be troublesome, the two small 10mm mounting bolts are very vulnerable to rust. Once I got the bolts out, the sensor plugged right in to the E24 rear cover. It’s also directly compatible with the E24 speedometer & OBC, but the speed sensor connector was changed in later cars. Just a couple quick splices and it was in!

Need more details? Read on!

Pro tip: the torsen (TORque-SENsing) diff is a pretty rare option; it’s not the usual BMW clutch-pack ramp type. These are a more complex design. While still completely mechanical, it uses gears instead of clutches to transfer torque. The upsides are less maintenance and quieter operation. The downsides are that it’s difficult to test and will completely lose traction, on both sides, if there’s zero grip on either driven wheel.

My late-model E24 can accept a wide range of differentials, including some cars you might not expect. The “medium case” diff (typ 188, due to the 188mm ring gear) can be found in all E28 (except oddballs like the M5). It was also fitted to all post-82 E24 (except the M6 and some European trim cars), and many E23 7-series. If you find one of these diffs, you’d just need to swap the rear diff cover to use it in your E24. Happily, we can also choose the wide selection of six-cylinder E30 diffs as well, but they need a bit more work to fit. The lighter E30s all used smaller driveshafts, so you’ll need to do the added task of swapping the output flanges. They pop out with a stout prybar, and are easily exchanged.

FYI, there are a lot of E30 & E36 Compacts running around out there with a small-case (type 168) diff. Those won’t work in our cars without significant modification, and should be avoided. It’s easy to tell the difference by counting the bolts holding the output shaft seal collars. The small case uses four, the medium case uses six, and the large case has eight on each side.

Remember that the E30 chassis didn’t end until the final Z3 rolled off the line in early ’02. It turns out that many six-cyl Z3s have ratios well-suited to our cars.

There are a few things to watch out for when upgrading your differential. If your car was owned by an enthusiast, it’ll typically have an unsuitable ‘final drive’ ratio. Your choice will be dictated by several factors, but the most important is what kind of driving you normally do. If you’re in an urban environment, I’d recommend a ‘shorter’ ratio (higher number). If you do a lot of highway driving, go with something a little ‘taller’ (lower number). Be aware that your gearbox and wheel/tire choices will significantly impact your choice of diff ratio. Supply and demand also makes this task more difficult: some less-useful ratios are very common (like 2.93 & 3.07); a few very useful ones are quite expensive (3.73 & 3.91). Also be aware that more powerful cars generally require taller ratios. That’s not just because of the higher torque output. The taller the diff, the more torque it can sustain, all else being equal. There are some wonderful calculators out there. I like this one.

My typical driving style is relaxed; I do a significant amount of highway driving; my wheel/tire combo is very close to the stock 25″ circumference; I have an overdrive manual gearbox. Given those choices, I felt a 3.09 or 3.27 was too tall. A 3.55 or 3.64 would be a great choice. I’d also have gone for a 3.73. A 3.91 or a 4.11 would have been far too short for my needs. Eventually, I found a great deal on a 3.46 LSD. While that’s slightly too tall for my tastes, I’ll get some additional latitude in choices of wheel/tire combos.

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