The differential (aka diff) splits the power coming from the propshaft to the two rear wheels.
Two main types of differential exist, Open and Limited Slip. Limited Slip Differentials (LSD) were made using two technologies; Mechanical (LSD-Mech) and Viscous (LSD-Visc). The latter was an open differential that provided 10-100% locking based on its viscous characteristics.
The ratio of a diff, often referred to as final drive, or final drive ratio, is last set of gears that the power goes through before hitting the wheels. If the ratio is 1:1 then one turn of the prop-shaft would equal one turn of the wheels. If we know the wheel size and gear box ratios as well as the final drive ratio we can work out road speed for any given engine speed. A good gear calculator is on the Unixnerd site []
The ratio will affect the performance characteristics of the car. A shorter gearing (larger number) will make the car accelerate faster. By contrast, a longer gearing (smaller number) will accelerate slower for any given gear. The other thing to remember is that shorter gearing will mean higher engine revs for any given speed and longer gearing will mean lower revs for the same speed. Using the above gearing calculator I can demonstrate:
For 3250 RPM, 5th gear, 205/55/R15 wheels:
Turning around the maths would show us that putting a 3.64:1 diff in a 318iS would save around 350 RPM at motorway cruising. But of course this isn't all the story. In making the overall gearing longer I would have had quite an impact on the acceleration of the car. It will be less noticeable in the first and second but as the gears go up the effect is more and more noticeable.
If I were to start with 4.10:1 and move to 4.27 I would experience the opposite effect. Cruising at 3250 RPM would only give me 66 MPH so I'd need to drive at 3400 to get back to 70 MPH. This would have an impact on my MPG and also my top speed - with the shorter (easier for the engine to turn) gearing I would possibly find my top speed reduced -- although in all but extreme cases it would still be over 100 so that's enough to cost me my licence for a while :)
But even this is still not the whole story. If I'm willing to sacrifice some MPG for the extra acceleration of a shorter (higher number) diff - what else will be affected. For spirited country lane or track driving changing the final drive can be advantageous but if you go too short you might render 2nd gear all but useless and too long will leave 3rd gear struggling in the lower RPMs. The trick is to find the right balance which BMW almost always gets pretty close. Generally if the car's power is standard you'll not want to move more than one step in either direction. For example, a 318iS as a daily cruiser would be fine on a 3.91 to ease the cruising revs and help with economy. The same car driven for track days or just for more spirited country lane driving may do better on a 4.27 diff.
To calculate the best diff for any given wheel or speed, [check out this link].
Below is a table of E30 diff ratios, all are interchangeable.
|316 before 9/84||M10||x||small||x|
|316 after 9/84||M10||x||small||x|
|318i 2- & 4-doors||M40||x||x||small||x|
|318i Cabriolet & Touring||M40||x||x||small||x|
|320i before 9/85||M20||x||small||x|
|320i 9/85 to 9/87||M20||x||x||small||x|
|320i after 9/87||M20||x||small||x|
|320i Cabriolet & Touring||M20||x||x||small||x|
|323i before 9/84||M20||x||medium||x|
|323i after 9/84||M20||x||medium||x|
|325i before 9/86||M20||x||x||x||x||medium||x||sport|
|325i after 9/86||M20||x||x||x||x||medium||x||sport|
|325i Cabriolet before 9/86||M20||x||x||medium||x||sport|
|325i Cabriolet after 9/86||M20||x||x||medium||x||sport|
|325, 325e US-models||M20||x||x||medium||x||sport|
|325e before 9/85||M20||x||medium||x||sport|
|325e 9/85 to 12/86||M20||x||medium||x||sport|
|325e after 12/86||M20||x||x||medium||x||sport|
|M3 Evolution I, II and III||S14||x||medium||x|
The E30 was fitted with two sizes of differential. The larger is known as a medium case (Type 188), and was used on 325 models. It has a crown wheel diameter of 188mm.
The smaller case was used on 4 cylinder and 320i cars, and has a crown wheel of 168mm
They differ in length but not width, and therefore are interchangeable without changing drive shafts. However, you will need to adjust the prop length if changing between small and medium case difff. This is an easy adjustment underneath the exhaust heat-shield, but if you are also changing the gearbox due to an engine swap, you may also have to change your prop shaft.
The two types of diff are easily distinguished by the number of bolts holding on the back plate. Small case diffs will have six bolts, while medium case diffs will have 8.
Sometimes the term "large case" diff is used. This is usually to refer to the bigger 210mm diffs fitted to some E34 models, and which are not compatible with E30s without serious modifications.
The E28 also used the small case diff, which can be fitted to the E30, but the rear cover and output flanges must be changed to 325i E30 items. Long ratio diffs from 525e's might be useful for engine swappers and are more easily obtainable than the long ratio E30 units. To tell them apart, count the bolts on the back plate; six bolts means it'll fit an E30, eight bolts means it's too big.
325 E30 diffs can be rebuilt with the crown wheel & pinion and gearset/LSD assembly from medium case E34 diffs.
Learn how to fit a differential.