Ratios Galore: A Deep Look at ZF’s 9-speed Automatic

When is enough, enough? When it comes to transmissions, the available ratios from any trans maker are on a steady climb into two-digit territory. Take ZF’s newest for example: the 9HP. It’s a nine-speed transaxle for transverse applications sporting a 9.8 ratio spread (that’s good) and it promises 10-percent better fuel economy when compared to a six-speed slushbox. In a market that goes full frenzy by one-percent here and two-percent there, 10 can’t be ignored.

The trouble with engineering a transmission for a transverse application is packaging. The gearbox, along with the engine, has to fit between the shock towers. According to ZF, which announced it will initially supply the 9HP to the Jeep Cherokee and the Land Rover Range Rover Evoque with more customers to come, the maximum width for such a transmission is about 14.6 inches. It’s no surprise, then, that the 9HP is 14.4 inches wide. With four planetary gear sets and six shifting elements (brakes and clutches) that’s a very dense 14.4 inches.

Of those four planetary gearsets, two of them are nested. In this nested pair, the annulus, or ring gear, of the smaller planetary doubles as the sun gearbox  of the larger set. This arrangement trims some width from the four-gearset tranny.

Simply adding ratios to a transmission might not make it a tool for greater efficiency, however. More ratios require more shifting elements and these add weight, complexity, and drag to the transmission. This is why ZF implemented two dog clutches in the 9HP. The beauty of a dog clutch is there’s little-to-no parasitic loss when they aren’t engaged, whereas a conventional friction clutch zaps some efficiency, and they’re relatively compact.

Parallel shaft gearbox

The tricky part of the canine clutch is how, or rather when, to engage them. Synchromesh helps dog clutches within a manual transmission from grinding away and easing gear changes. ZF relies on computers to anticipate the precise moment both halves of each dog clutch are spinning the exact same speed, and engagement happens without the slightest crunch, as proven by our short sample in an Evoque test mule.

• Comparison Test: 2013 BMW X3 xDrive28i vs. 2013 Audi Q5 2.0T, 2013 Land Rover Range Rover Evoque
• First Drive: 2014 Jeep Grand Cherokee V-6 / V-8
• Instrumented Test: 2014 Jeep Grand Cherokee SRT

Internal efficiencies aren’t the only source for the claimed fuel-economy improvement, either; the nine ratios play a part, too. Internal-combustion engines are most efficient in a relatively small rpm range and the biggest ratio step is just 1.65, between first and second. Having many small ratio steps in hand allows any engine coupled to a 9HP to operate in that ideal rpm window more of the time.

Oh yeah, one of the best parts of the 9HP: it’s 100 percent American made. Every