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BMW Differentials: What Cars Came With Plus Swaps and Upgrades, Part 1

BMW enthusiasts, especially those with older BMW cars, have a wide variety of gearing and drive ratios at their disposable as BMW differentials have a wide variety of ratios and offer broad interchangeability across multiple generations of cars and various platforms.

People talk a lot about the most 'bang for the buck' modifications they can do to improve their car's performance (and by performance they are usually referring to acceleration), and a differential gearset swap or change to a lower (numerically higher) ring and pinion gearset is the single most effective way to increase acceleration. Many call it the 'poor man's supercharger'. It's as bolt-on of a mod as you can get, and has nothing to do with the engine, emissions, or EPA regulations.

BMW used to intentionally undergear its cars to give them slightly better fuel economy so government corporate average fuel economy (CAFE) requirements would be met. The ironic thing about that is the real world trade off that makes you drive harder in order to accelerate faster, thereby offsetting any gains anyway! While this article focuses mostly on E30 BMW 3 series cars, those with E24 6 series, E28 and E34 5 series, and E36 3 series should also pay close attention. I will recommend some general guidelines for each platform and generation at the end that will make choosing a differential easier.

Keep in mind that a (numerically higher) lower ring and pinion/differential/gearset will make your engine turn higher revs, run faster, at any given MPH. This means that when taking highway driving and fuel economy into consideration, there is a point of diminishing returns when going lower (higher numerically). Some BMW's have an overdriven 5th gear that helps mitigate the buzziness of higher revving during highway driving. For example, if your fifth gear is an overdriven 0.81:1 (for example, E30 BMWs) going to a 4.10 diff or gearset means you'll still have a final ratio of only 3.32 in fifth gear (4.10 x .91 =3.321). This translates to just under 75 mph at 3500 rpm in fifth gear using 205/55-15 tires. A 4.45 gearset would raise the revs only to about 3750. So it's not like you'll be getting buzzed out of the cockpit on the highway.

Believe it or not, the top speed of your BMW is usually affected less by a lower gearset (aka higher numerically differential) because not many cars have enough to pin the tach in top gear. Wind resistance, aerodynamics, enigne horsepower and torque output etc conspire to stop you from reaching to attain maximum rpm in an overdriven fifth gear. This is why your 318is won't do 150 mph. Now, of course your car will have a theoretically lower maximum speed, but improving the gear ratios can help you get to a higher real world top speed by giving you more leverage vs wind and drag by optimizing the usable power curve with the gear ratios. Make sense?

If you do have a BMW that was available in both 4 and 6 cylinder guise, remember that differentials will not be interchangeable. Don't believe me? Compare the diff housing, casing, and rear axle carriers of a M42 powered 91 318is and a 91 325ic. The only exception to this is the E30 M3, which has a 'large case' 6 cylinder style differential. This 4.10 LSD is really nice on a 87-91 325i or 325is. In fact, give one of these cars a 4.10 limited slip diff and a chip and you'll find the car accelerates at least as hard as the vaunted E30 M3. Be warned: the 4.10 differential is considered too peaky for an M30-powered 535i, 633csi, or 635csi.

An important thing to bear in mind about swapping is that six- and four-cylinder E30 BMW differentials are different and will not interchange. The housings and rear axle carriers are totally different. Interchangeability only exists among four-cylinder cars and six-cylinder cars separately. The sole exception is the E30 M3, which uses a six-cylinder-type differential, thus providing a source of 4.10 limited slip diffs for the 325i. In addition, differentials from the E28 5 Series BMWs (1982-88) will also interchange with the E30 six-cylinder 3 Series cars, and this becomes key to modifying the early eta engine E30s.

Have an E28 5 series or E24 6 series? Check this out. E28 models came with one of four differentials: 2.93 open in the 528e; 3.15 open in the 524td (diesel); 3.25 in the 533I and 535i/is; and 3.91 in the M5. Limited slip differentials are identified by a white "S" painted on the outside of the housing, usually on top. You can also turn the input shaft and note what happens: If you're looking at an LSD, both flanges will rotate in the same direction. If it's an open diff, either only one will turn or they'll turn in opposite directions. The ring and pinion gearset ratio is identified on a small metal tag on one of the cover bolts. If the tag is gone, you'll have to remove the cover and count the teeth on the ring and pinion gears. The 3.73 325i/is differential is a really nice setup on 82-88 5 series and 1982-88 6 series. Some people give these cars the M5 3.91 (some M6 models had this or the 3.73), but the M5 and M6 cars have the M88/1 aka S38 DOHC engine that revs higher to support this low gearing, whereas the SOHC M30-powered cars are much milder (comparatively speaking, of course).

There are a ton of 533I and 535is models in junkyards, and this means easy picking for owners of E30 cars that have the (lame in stock form) "eta" engine and a 2.79 or 2.93 diff. A swap to the 3.25 differential is super cheap and makes the car really come to life...til you hit that 4500 RPM wall in stock form - but we can cure that easily, just see my recommendations for E30 E/ETA performance setups. Junkyards practically give these away. The 1991-only E30 318is already comes with a 4.10 differential (note; not all 91 318is models have a limited slip diff, check the differential to be sure, this in spite of the fact that it was an IS or sports package car!). Thee rare 4.27 unit from 1991 318i Cabriolets is really only good for short track racing or for a 91 318is with aftermarket camshafts and porting. Also, not many of the 4.27 differentials were limited slip. A lot of people also like a 3.91 differential in the 3.91 IS. Time 0-60 acceleration suffers with this swap, but usable acceleration is improved and it allows you to stay in the same gear longer. Great for mildly modified cars.

Part II of this article will cover BMW Differential Repairing, Maintaining, and Instructions for swapping. Hwo to tell if your differential is worn, how to drain and refill the transmission and differential oil/lubricant etc.

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