Rumor: 8-Speed AT to replace current 6-Speed AT transmission?

Rumors | November 30th, 2009 by 16
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The January issue of Car and Driver suggests that the 8-speed ZF transmission will highly likely going to replace the current 6 speed automatic transmission …

The January issue of Car and Driver suggests that the 8-speed ZF transmission will highly likely going to replace the current 6 speed automatic transmission across the BMW line-up.

While we don’t have an official confirmation on this, we reported in the past that BMW is considering replacing the six speed auto with the more power and fuel efficient eight speed ZF automatic transmission. The change will take over the course of the next two-three years and by 2012, the entire line-up will feature the new ZF transmission.

Car and Driver goes ahead and gives a great review to the new 8-speed gearbox, confirming what we knew already: almost near perfection shifting, always in the right gear and higher fuel efficiency.

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The ZF 8-speed automatic made its debut on the BMW 5 Series Gran Turismo and 760Li, and will also appear on the Bentley Mulsanne and Rolls Royce Ghost models. It was named recently the 2010 Automotive News PACE Award Finalist.

For a while, the ZF 6-speed was regarded as one of the best automatic transmissions featured in a production car, but the new ZF 8-speed takes it a bit further. It is not based on the 6-speed ‘box with two extra gears, but rather an all-new design, similar in size to a 6-speed automatic, with fewer parts.

The modular design allows for integration into hybrid vehicles, simply by replacing the torque converter with an electric motor, resulting in a fuel economy improvement of approximately 25 percent when compared to the ZF 6-speed, and still weighing the same – 200 lbs. It also has an optional wet multiplate clutch pack like the new E63 AMG.

According to the company, their goal was to develop a new transmission that improved fuel economy by at least 6 percent over the current-generation of 6-speed automatic, while also improving the power-to-weight ratio, shift speed and quality, while increasing torque load. It can handle up to 738 lbs-ft of torque.

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All the new 2011 BMW 5 Series models will feature the ZF 8-speed automatic transmission and all the details can be seen in the excerpt below taken from the official press release:

All new BMW 5 Series models will feature an 8-speed automatic transmission, which brings smooth shifting, sportiness and efficiency to new levels of perfection. Compared to the exemplary 6-speed automatic offered in past BMW models, the 8-speed’s “taller” cruising gears make a significant contribution to reduced fuel consumption and emissions, as well as quietness. And its wider spread of ratios from lowest to highest gear enhances acceleration and response across the range of driving speeds.

Technically speaking, this new automatic’s attributes have been achieved via an innovative concept that provides the two additional ratios (7 and 8) with no increase in bulk or weight. In an arrangement not heretofore employed in an 8-speed automatic, 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. The modest increase in the number of mechanical elements allows the new transmission to achieve unusually high efficiency: the so-called “gearing efficiency” is higher than 98% in all eight gears. In 6th gear, it’s highest of all because that is direct drive, with no gearing reduction at all. Combine this with reduced friction and the ability to keep the torque converter “locked up” more of the total driving time, and the transmission becomes very much a part of the new 5 Series’ overall efficiency concept.

Two additional gears mean threefold progress: quicker shifts, greater smoothness, enhanced efficiency.

The wider spread of ratios allows the engine to run at lower speeds, mainly in the “tallest” gear, 8th. Yet even with this wider spread, the steps between adjacent gears are reduced; in turn this means a stronger, smoother flow of power during acceleration.

And for the same reason, faster shifts are a further benefit because only one clutch pack is disengaged to shift up or down by one or two gears. On the other hand, downshifting more than two gears is accomplished as a direct shift. For example, a downshift from 8th to 2nd gear is made with only one clutch pack disengagement, and thus occurs without stepping down through the intermediate gears. Thus at one instant the driver can be enjoying the quiet, fuel-efficient low rpm of 8th gear, and in the next instant getting maximum acceleration.

It is interesting and at the same time, pleasing to see that BMW decided to go down this route with the new 5 Series since Mercedes-Benz offers the four-cylinder E-Class with the lower-end 5-speed transmission.

As far as the next generation M5, there are several reports out there mentioning a similar 8-speed automatic replacing the current M-DCT, but we’ll keep this at a rumor level for now.

As we enter 2010 and new models are being unveiled, we will learn more about BMW’s future plans.