Drivetrain

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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.

Drivetrain comprising gearbox, selector, propshaft, differential, clutch, flywheel and pedals
Drivetrain comprising gearbox, selector, propshaft, differential, clutch, flywheel and pedals

Contents


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. The clutch then drives the gearbox input shaft to make the main gearbox shaft turn. Through a series of cogs, the gearbox sends power to its output shaft, which bolts to the differential via the propshaft. From the differential, power is sent to each rear wheel through the drive shafts.

Clutch

Main article: Clutch

The clutch on E30s is a hydraulic unit, whereby two fluid cylinders transfer pressure from the pedal to the clutch mechanism.

When the pedal is pressed, the pedal arm pushes in the master cylinder. The master will then push fluid through its pipe to the slave cylinder, which operates the clutch release lever.

The clutch itself is a typical three-piece mechanism of clutch disc, pressure plate and release bearing. It is a service component, and therefore has a fixed lifetime - however, distances of up to 130,000 miles on an original clutch are not unheard of. If replacing the clutch, all three components should be replaced at the same time.

Failure to maintain the clutch will result in the unit slipping, meaning the clutch disc is not engaging with the flywheel. This can be for a number of reasons; a worn disc, a failed bearing, or oil on the disc face. Any or all of these necessitate clutch replacement.

To test the clutch for slipping:

  • Put the handbrake on full
  • Select 2nd or 3nd gear
  • Give the engine plenty of revs
  • Now gently release the clutch pedal, giving more throttle as the clutch bites.
  • If your handbrake works well then you should be able to stall the engine. If you can get the clutch all the way up with the engine still revving then the clutch requires maintenance.

Because of its location, the only practical way to replace the clutch is to remove the entire gearbox from the vehicle. For those that have access to a lift, the car can be raised enough to remove the gearbox without disturbing the rest of the drivetrain, but for those working on their own driveway, it is often better to remove the engine and gearbox together in order to fit new clutch components. Learn more about replacing the clutch.

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 compatible with earlier M10 gearboxes due to the bolt pattern; you cannot physically fit an M10 gearbox to an M40/M42 engine. 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 and BMV (M10 carbureted models), BMO, TBCU and BCX (M40 cars and some 318iS)
  • Getrag 240 - BCO, BCP and BCT - 318iS and E34 518i (pre 1992 overdrive). These boxes are the longer units with the shorter prop.
  • Getrag 240 - BMS, MB and BMT - 320i models.
  • Getrag 242 - LF (316 M10 only), MD and BMU (318i M10)
  • 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

Selector

Gear stick and shifter rod
Gear stick and shifter rod

The selector rod is the system that allows you to manually control which gear is selected. The gearstick itself pivots around a ball, and at the other end of this pivot is a long shaft that runs into the gearbox and moves the internal gear selectors. While these rods are a relatively simple concept, the various joints and clips can deteriorate over time, leading to sloppy and clunky gear changes.

The selector rod is specific to each gearbox, having different lengths depending on which gearbox is used. Therefore a 318i will have a longer rod than 325i because the 318i gearbox is shorter. However, all 4-cyl models (316, 318i and 318iS) share the same selector rod.

If your gearstick is wobbling all over the place, and doesn't feel how it should, learn how to replace the gear linkage.

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
Propshaft centre joint and bearing, with collar nut highlighted
Manual Automatic
316 318i (M10) 1560mm? 1480mm
316i 318i (M40) 1480mm
318iS 1570mm
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
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 32mm 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 Problems

Clutch Issues

Clutch problems manifest themselves in all sorts of ways, from incredibly low biting points to reluctant gear changes and kangarooing. Because most people think the clutch works through black magic, they always fear the worst.

Surprisingly, a lot of these issues are simple fixes that can be done yourself with simple tools. For more info, check out our list of clutch problems.

Oil Leaks

It's not uncommon to see a fluid leak when you look under the car, especially at the back of the gearbox. A lot of people immediately jump to the conclusion that the rear oil seal for the gearbox has failed, which it may have, but the problem is far more likely to be the small selector shaft seal, which is directly above it.

Access to this seal is the main problem. Removing the rear crossmember and letting the back of the box drop a little helps.

Remove the linkage, slide the spring clip around the joint on the end of the selector shaft out of the way, push the pin out and remove the joint. Hook the seal out with a small screwdriver, and replace.

Sloppy Gearstick

Sloppy, loose, wobbly, slushy; however you call it, it's when the gearstick isn't as tight and responsive as it should be. This is caused by worn joints and bushes in the selector. Learn more about replacing the gear linkage.

Common Upgrades

Auto to Manual

While automatic 'boxes give a smooth and easy ride, many people feel that it detracts somewhat from the driving experience. If that's your case, learn more about converting your car from Auto to Manual.

Short Shifter

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