Drivetrain

It's great that your engine produces all that lovely power, but getting it to the Wheels is a serious matter. To make the bridge between them, you need to understand the Drivetrain.

Overview

The gearbox, propshaft and differential make up the drivetrain. To get power to the gearbox, the clutch presses itself up hard against the flywheel of the engine.

Gearbox

The E30 was available as both a Manual and an Automatic. In total, seven different gearboxes were fitted to the E30; Four manuals, all produced by Getrag, and three automatics by ZF. The overall performance of a gearbox is measured by its ratio, found by dividing the ratio of 1st gear by the ratio of 5th.

Gearbox ratios are also dependent on the ratio of your diff. For example, a high ratio diff (4.27) combined with a high-ratio first gear (3.72) will make first gear useless. In contrast, a low-ratio diff (3.25) with a low-ratio gearbox (2.48 in first, for automatics) would be an appalling combination for city driving, with the acceleration of a glacier.

The gearboxes on all cars are bolted to the back of the engine via a bell housing, and are held to the car by a crossmember. While it is possible to remove the gearbox from the car in situ, it is considered much easier to disconnect the Engine and remove both units in one.

Operating the gearbox is done via the gearstick in the cabin, which operated the gears via the selector.

Manual

A range of gearboxes were used in the E30.

Manual Gearboxes

	Four-Speed	Five-Speed
	316 318i	316 316i 318i 320i 324d	318iS	323i Sport	323i 324td 325e 325i 325iX	325i Sport
1st	3.76	3.72	3.72	3.76	3.83	3.35
2nd	2.04	2.02	2.02	2.33	2.20	2.03
3rd	1.32	1.32	1.61	1.32	1.40	1.36
4th	1.00	1.00	1.00	1.23	1.00	1.00
5th		0.81	0.80	1.00	0.81	0.81
Reverse	4.10	3.45	3.71	4.10	3.46	3.20
	Getrag 242	Getrag 240 or 242/5	Getrag 240	Getrag 245	Getrag 260	Getrag 260/5

The M42 engine mates up to the same gearbox used in M40 engines, but is not campatible with earlier M10 gearboxes due to the angle of installation, which will cause problems with the shifter and rear gearbox mount. In a similar way, although the M40 and M42 boxes are interchangeable, the input/output shafts are different and so is the propshaft. This means that replacing a 318iS gearbox with the more mundane 318i unit will necessitate a change in prop.

Notice the similar ratios between the Getrag 240 and the Getrag 260/5. This means that for those replacing their M20B20 engine with the more powerful M20B25, Sports performance can be achieving by keeping the original gearbox.

However, don't let parts sellers convince you that the boxes are identical. Getrag gearboxes can be identified by looking for the serial number stamped around the edge of the bell housing. This number should also contain letters, which will identify the following boxes:

• Getrag 240 - MN or BMV
• Getrag 242 - LF, MD or BMU
• Getrag 242/5 - AY, AW or LG
• Getrag 245 - BJ
• Getrag 260 - NW, ANB or TNW
• Getrag 260/5 - TNN or HM

Automatic

Automatic Gearboxes

	Three-Speed	Four-Speed
	316 318i	320i 323i	325i
1st	2.48	2.73	2.48
2nd	1.48	1.56	1.48
3rd	1.00	1.00	1.00
4th		0.73	0.73
Reverse	2.09	2.09	2.09
	ZF 3 HP 22	ZF 4 HP 22

Propshaft

The propshaft is one of the least technical components in the drivetrain, being a simple metal pole that connects the back of the gearbox to the input flange of the differential. To cushion the blow of the engine and gearbox moving (during clunky gear changes and heavy braking), there is a rubber doughnut known as the guibo bolted to the gearbox end of the propshaft. Holding the prop of the body of the car is a bearing, held in a cradle, which allows the prop to spin freely.

Props come in two parts, connected with a swivel joint. These two parts are balanced together at the factory, and it is ESSENTIAL that they go back together in exactly the same way. Before splitting a propshaft, use a white paint or correction fluid to mark the two halves at their joint, so that you can reassemble the prop in exactly the same way. Failing to do this will result in an imbalanced prop which will shake itself apart at high revs.

If you're really lucky, you'll find white dots already in place from the factory, but don't count on it. An imbalanced prop is both useless and worthless.

Even though all E30s are a standard length, gearboxes and diffs are not, and depending on the car a different length prop will be fitted:

Propshaft centre joint and bearing, with collar nut highlighted

	Manual	Automatic
316 318i
318iS
320i	1526mm	1337mm
323i	1468mm	1337mm
324td 325i	1483mm	1327mm

However, it is possible to adjust the length of each propshaft by a certain amount (up to 10mm) by loosening the collar nut (pictured) and pulling the two halves of the prop apart. Make sure you mark the two halves first in case you separate the prop completely.

Guibo

Between the gearbox and prop sits the Guibo. This rubber cushion allows a certain degree of flex in the drivetrain, and without it the rattle of the prop would be insufferable.

Over time you'll notice that changing gear produces a noticeable clunking noise from under the car; this is caused by the guibo failing and flexing too much.

Bearing

The centre bearing is essential for holding the prop to the bottom of the car, and is mounted inside a carrier frame. Depending on the model, the bearing fits around either the front or rear half of the propshaft.

When making adjustments to the prop, it is essential to tension the centre bearing; this means applying pressure to it before it is bolted in place, otherwise it will simply fall apart.

If the centre bearing is on the front section of the prop (as in 325i, 318i and), then push the bearing about 3mm towards the gearbox and then tighten the bolts up. If the bearing is on the rear section (as in E30 M3), the bearing is tensioned toward the diff.

Differential

Main article: Differentials

One of the most curious components in the drivetrain, the Differential is a massive lump of metal bolted to the rear subframe. It receives power from the propshaft and splits it to both wheels.

Most people are scared of the diff, seeing it as a magic box of grinding gears and wheels, but it isn't that complicated. While it may take a bit of explaining to know how it works, all you need to know is that it allows each wheel to spin at different speeds. This is really useful when it comes to corners, where the inside wheel needs to spin slower than the outer wheel, since it's travelling a shorter path. If it helps, think of Olympic sprinters on a track; the ones on the inner lane don't run so far as the runners in the outer lane, which is why they start further back on the straight.

The differentials fitted to E30s come in two types; Open and Limited Slip, with the latter being highly desired as a "performance" unit. They also come in two sizes; Small and Medium case, with the bigger being fitted to the 325i.

To tell whether you have an Open or Limited Slip diff, get both output flanges in your hands (one each is best!) and try and turn them in opposite directions, If you can then it's an open diff. If they feel like they're joined together and you can't turn them in opposite directions then it's a LSD.

A variety of ratios were available on the E30, from as high as 4.45 to as low as 2.95. Generally speaking, the ratio is a multiplier, where the number is how many times the propshaft has to turn to make the wheels go round once. Lower numbers mean good cruising speed and fuel economy, while high numbers mean great acceleration but noisy motorway driving. The ratio you use completely depends on the body style and engine. Heavier cars like Cabriolets and Tourings will have slightly higher ratios to help them accelerate, while economy and power models like the 325e and M3 will have low ratios to make the best use of the engine.

The differential is held in place using four bolts, screwed in from the top, as well as one bolt through the rear bush, allowing the diff to move with the rest of the Suspension.

Output Flanges

Output flange and drive shaft

The output flanges are effectively sockets which run from the inside of the differential to the outside, and provide a flat face to bolt the driveshafts to.

All E30 output flanges are the same. However, when fitting a differential from another BMW such as the E28, the output flanges from that diff will need to be replaced with E30 flanges to allow the drive shafts to be attached.

Drive Shafts

To get the power from the diff to the rear Wheels, two drive shafts are used. These bolt to the output flanges at one end using six Allen-head bolts, and slot into the wheel hub at the other, secured by a 22mm nut.

At each end of the drive shaft are CV or constant-velocity joints, which allow the driveshaft to turn at an angle to both the wheel and the diff. Although the shaft as a whole is a sturdy unit, these joints should not be pushed to their limits. Therefore, when removing the differential it is important to hang or support the driveshafts somehow. Allowing them to drop to the floor will damage the CV joints.

Common Upgrades

A Z3 short shift conversion will help you change gears quicker.