Differential

The differential (aka diff) splits the power coming from the propshaft to the two rear wheels.

Small & medium case diffs

Type

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.

Small & medium case diffs

Ratio

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 [[1]]

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:

4.10:1 (318iS) final drive == 69.4 MPH 3.64:1 (325i) final drive == 78.2 MPH

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.

Model(s)	Engine	Ratio												Case size	Type
		4.45	4.27	4.10	3.91	3.73	3.64	3.46	3.25	3.23	3.15	2.93	2.79		Open	LSD-Mech	LSD-Visc
316 before 9/84	M10						x							small	x
316 after 9/84	M10				x									small	x
316i	M10			x	x									small	x
316i	M40	x	x											small	x
318i	M10			x	x		x							small	x
318i 2- & 4-doors	M40	x	x											small	x
318i Cabriolet & Touring	M40	x	x											small	x
318iS	M42			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
324d								x						small	x
324td									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
325i Touring	M20				x									medium	x
325iX	M20				x	x	x							medium			x
325iX Touring	M20			x	x									medium			x
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 US-models	S14			x										medium		x
M3	S14								x		x			medium		x
M3 Cabriolet	S14								x					medium		x
M3 Evolution I, II and III	S14			x										medium		x

Small & medium case diffs

Small & medium case diffs

Case Size

There were two different case sizes used in E30s, a larger known as medium case, used on 325 models (Type 177?) and the smaller, Small case, used on 4 cylinder and 320i cars.

They differ in length but not width so they are interchangeable without changing drive shafts but you will need to adjust the prop length which is an easy adjustment underneath the exhaust heat-shield.

Cross-breeding

The smaller type of diff fitted to the E28 can also be fitter 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 more easily obtainable than the long ratio E30 units.

325 E30 diffs can be rebuilt with the crown wheel & pinion and gearset/LSD assembly from smaller case E34 diffs.

Fitting

Learn how to fit a differential.