M Variable Differential Lock. Together with the German division of GKN Viscodrive, BMW M engineers developed a special mechanical limited-slip differential for the M3s. This M Variable Differential Lock specifically addresses low- and split-traction situations in a way that reinforces sporty handling while contributing to M3s' remarkable slippery-road abilities.

Any time a speed difference develops between the two rear (driven) wheels, a shear pump, driven solely by this difference, develops pressure in the silicon viscous fluid in which the lock operates. In turn, this pressure is directed to a multi-disc clutch that transfers driving torque to the wheel with the better road grip ("select high"). The greater the speed difference between the two wheels, the more aggressively the clutch engages. As soon as the difference between the two wheels' speeds begins to diminish, the clutch starts to ease off.

This mechanism achieves its sophisticated action by entirely natural means. There is no external pump, no external source of lubrication or operating fluid. The very motion to be controlled - differences in speed between left and right wheels - generates its locking action. Viscous fluid is so-called because it develops internal force (via an increase in viscosity) whenever it is sheared; this is why the relatively small difference between one wheel speed and the other can generate the necessary action.

The new M5 also employs the M Variable Differential Lock.

Dynamic Stability Control. This electronic traction and stability system, standard on all current BMWs, complements the M Variable Differential Lock.

DSC optimizes traction by electronic means, sensing wheel-speed differences and reducing engine torque and/or applying individual rear-wheel brakes. The crucial difference to the M3 driver between the M Variable Differential Lock and the DSC traction function is that the former in no way impedes power delivery, and is hence suitable for performance driving.

Yet in fact, even DSC's traction function in M3s is calibrated to M-specific parameters. In cooperation with Continental Teves, BMW M engineers developed a logic that, in combination with the fast-reacting engine, performance-oriented gearing and M Variable Differential Lock, achieves the desired traction optimization in an M-compatible way…in other words, without undue interference with the differential lock's ability to get power to the road.

The DSC stability-enhancing function is essentially unrelated to traction. Sensing differences in wheel speed in a critical cornering or avoidance maneuver, DSC detects any deviation from the normal cornering path (abnormal understeer or oversteer) and gently applies individual wheel brakes to help the driver keep the vehicle on the intended path.

Sequential Manual Gearbox (SMG): special way to drive a performance automobile
Given the M3's performance nature, BMW M doesn't offer an automatic transmission as such; no matter how good - and BMW's 5- and 6-speed automatics are among the best - an automatic transmission incurs some performance loss relative to a well handled manual gearbox. On the other hand, given today's capabilities in electronics and hydraulics, it does make great sense to enhance the M3's 6-speed manual transmission with some automated operation. This is accomplished with the optional Sequential Manual Gearbox (SMG).

In conceptual terms, the SMG system consists of:

• The same 6-speed manual transmission as is standard in M3 models.
• An electrohydraulic mechanism for the gearshifting and clutch actuation.
• Electronic controls that regulate the electrohydraulic mechanism.
• The driver interface, which includes a shift lever on the console and shift "paddles" on the steering wheel.

There is no clutch pedal. On the console is a short, sporty shift lever with R (Reverse), N (Neutral) and S/D (Sequential/Automated) positions, plus "-" and "+" directions. The shift pattern appears on the shift knob and in an instrument-cluster display. Behind the shift lever is a program selector, with which the driver may select:

• In the Sequential mode, 6 programs ranging from "softest and slowest" shifts to "firmest and quickest" shifts; i.e. from most leisurely to sportiest.
• In the Automated mode, 5 programs of similar gradation.

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