Differentials
2002 differentials came in two different styles, the long neck and the short neck. The long neck differential was used on pre-'69 cars, and is often considered to be useful as a boat anchor. There is a considerable amount of extra weight, as well as less strength, than the short neck differentials. Also, the long neck differential was used with the older style half-shafts, with U-joints and sliding joints, which required maintenance and were more trouble prone than the later sealed CV joints. Nevertheless, the long neck differential is still used in instances where originality is important, or because rules dictate it (such as vintage racing). Switching between the two styles requires a driveshaft swap, as well as switching the axle carrier (suspension crossmember).
Here are the commonly available short neck final drive ratios:
3.36:1 2002 Turbo
3.45:1 early euro tii, E21 320/6,323i
3.64:1 most 2002s, all 2-liter 320i's, 1.8L 320iA's
3.90:1 1976 49-state 2002, 1.8L 5-speed 320i's
4.11:1 1600-2, 1602
Most short neck differentials are readily interchangeable with each other. The things to watch out for when swapping a differential are:
Output flange bolt hole size/style: Early 2002s have 10 unthreaded holes in the output flange. Later 2002s and pre-'79 320is have 6 threaded holes. The 10 hole flange allows the use of either the CV jointed half-shafts, or the early sliding U-joint shafts. The 6 hole flange only allows the use of CV jointed shafts. Half-shaft bolt lengths are also different between these two styles of flanges. 2002s with CV joint half-shafts can use differentials with either style flange. '79-up 320is have a 6 hole output flange, but the threaded holes are 10mm in diameter, vs. 8mm for the earlier versions. This was accompanied by different CV joints. A discussion on dealing with the different sized bolt holes is contained below, in the Limited Slip Differential section.
Rear cover: 2002s use two interchangeable styles of covers. One is finned, the other is not. The 320i uses a different rear cover, but the 2002 cover will fit on a 320i differential, and vice versa.
Here are the commonly available long neck final drive ratios:
3.64:1 most 2002's
4.11:1 most 1600's
Keep in mind that you may need to swap the input flange to match your driveshaft when swapping long neck differentials. There were a couple of different types of input flanges available.
An important note on input flanges and short neck differentials: Do NOT remove the nut on the input flange on a short neck differential unless you intend to rebuild the differential. The nut requires a large amount of torque to hold it in place. There is a crush collar inside the differential that can only be used once, that prevents the torque on the nut from being transferred onto the bearings. Tightening the nut without replacing the collar first results in a large amount of force being applied to the bearings in the differential, which will cause premature failure.
What about limited slip differentials?
Original 2002 limited slip differentials are extremely difficult to find. It was an optional extra that was rarely ordered in the US. 2002 limited slips were mostly 40% lockup, but a 75% lockup was also available. A 40% unit can be turned into a 75% unit, or vice versa, by adding or removing clutch plates in the limited slip unit. This percentage rating refers to the percentage of lock up present in the differential. An open (or standard, nonlimited slip differential) has 0% lockup. The higher the lockup percentage, the more aggressive the limited slip action. The percentage number denotes the percentage of torque applied to the slower turning wheel from the faster turning wheel. In a straight line, both drive wheels turn at the same speed, so no limited slip action is occurring. In a turn, or when one tire is spinning more than the other (such as on snow or ice), with a limited slip differential, 25, 40, or 75 percent of the torque applied to the faster wheel is applied to the slower wheel, effectively 'limiting slip'. A higher lockup percentage will cause increased rear tire wear on the inside tire during cornering -the tire itself will have to slip slightly to counteract the limited slip's desire to have both tires turning at the same speed. It will also increase oversteer in wet or slippery conditions, but it will also increase understeer in tight corners under dry conditions. This is simply due to the fact that with a limited slip, the drive wheels tend to want to turn at the same speed, making the car tend to want to go in a straight line. When it is slippery, however, both drive tires will tend to lose traction at the same time, increasing oversteer. The advantages are less inside wheelspin when accelerating out of a tight corner. This also translates into more horsepower to the pavement and faster autocross times -provided that the suspension is tuned for the limited slip. The ability to accelerate out of corners without excess wheel spin can be a great advantage.
A common question is what percentage is appropriate for the street? The issues are tire wear, traction requirements, and understeer/oversteer characteristics. For dual street/track use, a 40% unit is probably the best compromise. Serious autocross and track use would benefit from a 75% unit. For plain street use, a 25% unit is unobtrusive, and helps traction/handling mildly.
The most common limited slip differential installed in a 2002 is usually a unit out of a 320i. Limited slips were much more prevalent in 320i's because they came standard with the 'S' package. These units are mostly 25% units, and are not as easily upgraded to a higher locking value.
The most popular 2002 differential retrofit seems to be the 3.90:1 limited slip unit out of an '80-'83 320i with the 'S' package. The lower gearing helps acceleration, and the limited slip helps traction. A good addition for autocrossing or racing. The main problems with this retrofit is that the 320i differential is 13mm narrower between the output flanges, the bolt holes in the output flange are larger, and the rear covers are different. The rear cover can be swapped from a 2002 differential, but the width and bolt problem require more involvement. There are several options available to overcome this problem.
The easiest way to overcome this problem is to purchase modified output flanges from any of several aftermarket sources. These flanges have been modified with smaller bolt holes, and are thicker to make up for the differential width difference. The flanges simply pop out of the differential, and snap back in. You will probably have to return the old flanges as a core, however.
Another method that can be used is to mix and match half-shaft components. This requires the half-shafts from a '79-up 320i. Basically, the 320i half-shaft is used, but the outboard side CV joint is removed, and replaced with the old 2002 CV joint. This way, the inner joint matches the differential, the shaft is longer, and the outer joint matches the 2002 stub axle.
Limited Slip Spotter's Guide
This section has been added so that when you're trudging through the salvage yard looking for goodies for your 2002, you can immediately determine what kind of differential is in the car. The original factory 2002 limited slip units are easy to spot. On the driver's side of the external case, there will be some numbers that are STAMPED into the side: 40 11S 40
The first two numbers, the 40 and the 11, indicate the final drive ratio of the unit. For this particular example, 40/11 = 3.636, identifying this as a 3.64:1 unit. The other markings, "S 40" indicate that this is in fact a limited slip, factory set with 40% lockup. Please note that differentials like these are a rare find. They are also GREAT, because they bolt right in.
The best way to determine if a differential is limited slip is to empirically test it. With the transmission in neutral and both tires off of the ground, turning one tire should cause the other tire to turn in the same direction if the differential is a limited slip. With the transmission in gear, the tire will become very difficult to turn, whereas an open differential will cause the other tire to turn in the opposite direction.
One final word on differentials for racing and performance use. In "Grassroots Motorsports" magazine's ITB 2002 project, they covered locking up a standard 2002 differential for track use by welding up the spider gears. "Locked" implies that there is NO differential action. Each wheel gets equal amounts of driving torque all the time, like a live axle go-kart. Please note: this is for track use only. You do no want to run a locked differential on the street! Furthermore, one has to be a little careful when attempting this procedure, and it is not recommended by us. The increased stress on the gears can cause premature and possibly spectacular failure.