Five Modern High Gear-Count RWD Automatic Transmissions: Similarities and Differences

First, I want to talk about some qualities they share. Then, I will go over each of them briefly. Lastly, I will highlight some differences as well as my opinions in the form of rankings.

In terms of their commonalities, I would note three points: One, they are all capable of smooth shifts. Whether a gearbox is smooth in operation is primarily dependent on its hydraulic control, including both the programming of the TCU and the design of the valve block. TCU programming is specific to each vehicle-engine combination. Hence, a transmission may be smooth in one car but not so in another. This is particularly obvious with ZF transmissions, which usually performs the best with BMW. BMW has been the launch operator of ZF automatic transmissions since the 1980s with the 4HP. It is likely that BMW has always had inputs to how these transmissions are designed.

Two, all these transmissions are capable of jumping gears. This is not some fantastic feature, as some marketers would have you believe; instead, all gearshifts involve the opening and the closing of one clutch pack each, i.e. two shift elements only. It is not possible to operate four shift elements in a single gearshift.

Three, shift time in automatic transmissions is unimportant, because, unlike manual transmissions or DCTs, power is continually transmitted to the wheels even during gearshifts.

Now, let’s go over the five most common high gear-count RWD automatic transmissions in alphabetic order.

First up is Aisin’s 8-speed. This is the world’s first 8-speed automatic transmission for passenger cars. It uses just two planetary gearsets, including one Ravigneaux gearset. In terms of shift elements, it uses 2 brakes and 4 clutches. Two of these are closed, while 4 are open, in each gear. There is also a one-way-clutch that prevents the rear planetary assembly from turning clockwise that is useful for the 1st gear.

This transmission powers many Lexus RWD vehicles, including the RC F, GS F, and IS 500 F Sport. At one point, it also powered the Cayenne, Q7, and Touareg, among others.

The Ford-GM 10-speed is designed by Ford and manufactured by Ford and GM separately. It uses 4 planetary gearsets, 2 brakes, and 4 clutches. Four of the shift elements are closed in each gear, with 2 left open. There is also a one-way-clutch for the 1st gear.

This transmission powers all modern Ford and GM RWD-platform vehicles. It is also made in high duty variants for heavy-duty pickups.

Hyundai’s 8-speed is the company’s own design. It is a big jump from the 5-speed, which was the only other RWD automatic transmission Hyundai has made. It uses 3 gearsets, including one Ravigneaux gearset. In terms of shift elements, it has 2 brakes and 4 clutches. Two of these are closed for each gear, leaving 4 open. There is also a one-way-clutch for the middle planetary gearset that is useful for the 1st gear.

This transmission powers all modern Hyundai, Kia, and Genesis RWD-platform vehicles.

Mercedes’s 9G-Tronic is also a proprietary design. It uses 4 gearsets, 3 brakes, and 3 clutches, one brake fewer than its 7-speed predecessor. Three shift elements are closed for each gear, leaving 3 open.

This transmission powers all Mercedes RWD-platform vehicles. It holds the record of the largest gear spread in passenger cars, up to 9.15 times.

ZF’s 8HP is a popular design. GM’s 8-speed transmission is also based on the ZF 8HP, with some modifications. This transmission uses 4 gearsets, 2 brakes, and 3 clutches. Three shift elements are closed for each gear, leaving 2 open. It has the fewest number of shift elements among high gear-count transmissions.

One peculiar design aspect that allows ZF to use so few shift elements is the use of large drums that link front and rear gearsets. There are three of them. Similar designs have been used in ZF’s previous gearboxes. The thin-walled drums can be fragile, and stacking them over each other add drag.

Finally, I’d like to rank these five transmissions in three aspects, innovativeness, reliability, and efficiency.

In terms of innovativeness, I think the Aisin deserves the top spot, because it uses just two gearsets (or three, depending on how your count). Its simplicity contributes to its reliability and efficiency. Ford comes second, because it uses an innovative triple clutch housing that greatly simplifies the transmission. It gets 10 gears from what ZF could only 8 but is still mechanically simpler. There is only one drum that connects the 1st and the last gearsets. The ZF 8HP ranks third in innovativeness, because it gets it done with just 5 shift elements, even though its gear train is the most complex. Hyundai ranks last, because it uses one more gearset than the Aisin but still gets only 8 gears half a decade later.

In terms of reliability, I should point out that all these modern automatic transmissions are reliable. Still, Aisin is without doubt the best. Historically, Mercedes is also great; it seems that the 9G-Tronic hasn’t broken this pattern. From what I can gather, the Hyundai is good, too. ZF historically has one of the worst reliability records; however, the 8HP seems to be a good design. Lastly, Ford and GM are having issues with their 10-speed that only time will tell whether they can fix.

Lastly, I want to give my guesses on the efficiency of these five designs. Naturally, this is purely an opinion. Ford takes the top spot, because it has a relatively simple design, leaves only 2 shift elements open in each gear, and has the 2nd largest gear spread. Mercedes ranks second, because it also has a relatively simple design and has the largest gear spread. ZF ranks third, because it leaves only 2 shift elements open in each gear, compared to 4 in the case of the Aisin and the Hyundai. However, its three large drums that may add significant drag, so I am not so certain. Hyundai ranks last, because it is more complex than the Aisin.