Deeper Tech Dive

TwinPower Turbocharged 3.0-litre I6 (M55)
In the spring of 2010, the BMW 535i Gran Turismo will debut the automaker’s next-generation turbocharged inline six-cylinder engine. Designated internally as the M55, it will eventually replace the current twin-turbo I6 (known as the M54) in all BMW applications (but over time, not all at once).

One major development is the move to a new turbo-charging design, called TwinPower, where two turbochargers are fitted into one housing. (This design is more generically referred to as “twin scroll.”) This new single turbocharger unit (with two scrolls) not only pushes more air than the twin-turbo set-up on M54, but also helps the engine reach its peak torque lower in the rev range.

The other major advance over the M54, is the adoption of the newest and fastest version of Valtronic, BMW’s fully variable valvetrain system. (M54 is fitted with the VANOS system, which only continually adjusts valve lift).

The engine also features BMW’s latest direct injection technology, capable of producing between 40 and 200 bar of fuel pressure.

As such, it becomes BMW’s first engine to combine turbocharging, a fully adjustable valvetrain, and direct injection. Net result: a very busy and complex combustion chamber and cylinder head area, and a powerplant with similar power outputs to BMW’s naturally-aspirated 4.0-litre V8, but with 29 percent better fuel efficiency. (Official fuel ratings TBA).

Preliminary information from BMW lists peak horsepower at 300, between 5,800 and 6,250 rpm, and peak torque at 300 lb-ft, between 1,200 and 5,000 rpm. The 535i GT is said to accelerate from zero to 100 km/h in 6.5 seconds.


New 8-Speed Automatic
The 550i GT and the 760i debut ZF’s new 8-speed automatic transmission. A new compact design, means it is the same size and weight of the 6-speed automatic it replaces.

Four planetary gearsets and five clutch packs are controlled in a way that no more than two of the five clutch packs are freewheeling at any given time. With less moving parts, the transmission doesn’t lose much in energy transfer; the so-called “gearing efficiency” is said to be higher than 98 percent in all eight gears. Efficiency is even better in the so-called 6th gear, which is actually not a gear, but when the transmission is in “direct drive.”

Efficiency is furthered by a torque converter that is “locked up” during more driving modes, and the wider spread of ratios, which allows the engine to run at lower speeds, mainly in that tall 8th gear.

But this wider spread also means steps between gears are smaller, resulting in a stronger, and smoother flow of power during acceleration.

BMW notes faster shifts are possible, because only one clutch pack is disengaged to shift up or down by one or two gears. It’s equally fast when you want to go down more than two gears, because that enacts a “direct shift.” For example, a downshift from 8th to 2nd happens with only one clutch pack disengagement, and with no stepping down through the intermediate gears.