This is from the BMW press release on the E90:
6-speed Sequential Manual Gearbox
Available on the 330i as of September ’05 production, this is an electrohydraulically shifted, electronically controlled rendition of a 6-speed manual transmission, including an automatic clutch – a wholly different transmission from the STEPTRONIC automatic. There is no clutch pedal; the driver selects the desired mode (N, R, D, S) with a consolemounted selector lever, and can execute manual shifts via that lever or two “paddles” on the steering wheel. SMG fully preserves the performance of a manual transmission, entails little or no penalty in fuel economy, and facilitates both automated and very sporty driving.
Features and characteristics of the SMG driver interface includes the following:
• A Drive mode (D) in which shifting is automated
• A Sequential mode (S) in which shifting is mostly driver-controlled
• A Sport button on the console, which selects the SMG Sport program
• An instrument-panel display of the gear currently engaged.
In D, shifts are automatic, programmed according to how the driver is currently driving; more aggressive driving effects shifts at higher speeds. D is not a substitute for the D of a fully automatic transmission, but rather a convenient operational mode for those times when the driver values ease of driving over maximum performance.
With S selected, the driver essentially controls all shifting by means of the shift lever or paddles –
• Tip lever or paddle(s) rearward = upshift
• Tip lever or paddle(s) forward = downshift
– with the following exceptions:
• If the driver takes the engine up to its rpm limit, SMG will automatically shift to the next higher gear.
• Upon deceleration, as the engine comes down to approximately 1100 rpm, SMG selects the next lower gear. When the vehicle comes to a stop, SMG selects 1st gear automatically; upon moving off, the driver again chooses each upshift.
Engaging the Sport program via a console button influences shifting as follows:
• In S, shifts occur more quickly (sharply).
• In D, shifts occur not only more quickly, but at higher road speeds. This is parallel to the Sport mode of BMW automatic transmissions.
An Acceleration Assistant function (also referred to as Launch Control) is available for very sporty driving: With Sport activated, the driver presses and holds the DSC button (on the dash) for 3 sec. or more, then presses the accelerator to the floor. The engine is automatically revved to the optimum rpm for a wheel-spinning start and the vehicle is “launched.” The Owner’s Manual cautions: “Do not use the Acceleration Assistant too frequently, as this could cause premature wear of the components.”
With all transmissions, a new generation of differential is employed in the 3 Series. Its main distinction is a new type of bearing, called “double angular-contact ball bearing.” These bearings reduce oil temperatures within the differential, with benefits for both function and durability. Power losses due to oil flow are reduced and gear design has been adapted to the new bearings, with the overall results that efficiency is increased (meaning a small increase in fuel economy) and durability is enhanced.