Analysis: ZF 9HP 9 Speed Transverse Automatic Transmission

Featured Posts, Interesting | January 26th, 2011 by 6
nine speed transmission 630x500 Analysis: ZF 9HP 9 Speed Transverse Automatic Transmission

What the heck! One more speed and all of us old Schwinn riders would have felt right at home, see what Jalopnik had to say. Nine …

What the heck! One more speed and all of us old Schwinn riders would have felt right at home, see what Jalopnik had to say. Nine forward speeds appears, on the surface, to be overkill. ZF has been rather coy with the details of the new gearbox, but some bits of information seem to be becoming available, if not from ZF directly.

Currently ZF manufactures a very good 8 speed automatic, the 8HP, for longitudinal mount applications. This is the gearbox that’s showing up in the ‘F’ code BMWs. It utilizes three ‘overdrive’ gears, with fifth having a ‘1:1′ ratio. Six speed gearboxes often are set to use fifth and sixth as overdrive gears, fourth being 1:1. Having that additional overdrive gear available in the 8HP helps with fuel economy and having one additional gear at the bottom can help when dealing with a power to weight ratio that could use a little extra torque multiplication.
nine speed transmission Analysis: ZF 9HP 9 Speed Transverse Automatic Transmission

What ZF is doing with the 9HP for transverse mount applications (which accounts for 80% of current car production) is leapfrogging the six speed competition with three extra speeds. And ZF isn’t saying how they’ve managed to do that. The first thought is that it will be limited to pretty small engines, that don’t have a lot of torque. Say, four, three, and probably even two cylinder units. Since the first user of the 9HP is supposedly Chrysler, that would make some sense. Chrysler could use all the extra fuel economy they can find what with the prominence of ‘Hemi’ in their sales pitches. And Fiat has a two cylinder ‘multiair’ engine that could plug into a small Chrysler.

However, the 9HP’s torque limit is said to be 295 lb-ft, per an article by K. C. Colwell in Carlovers Magazine. That’s plenty of capacity to handle a good turbo four. That’s just a bit more torque than Nissan’s CVT has to deal with in the V6 Maxima. And ZF makes a CVT also, so there has to be a good reason to build a nine speed automatic rather than utilize the existing CVT, right? Well CVT’s haven’t been accepted by US consumers and the 9HP may also better fuel economy than a CVT to boot.

As long as we’re talking about Chryslers, remember the Dodge Colt ‘Twin-Stick’, effectively an eight speed manual from the 1970s? Maybe that’s what we’re seeing with the 9HP in automatic form. The ability to mix a number of different engines, with a number of different final-drive ratios and effectively deliver the needed torque for acceleration, and the desired fuel economy while cruising. And given the upcoming 36 MPG fuel economy CAFE requirement, the 9HP will be a direct benefit to meeting it.

One other tidbit that leads to my belief that two and three cylinder (and small displacement four cylinders) may be in the offing is the inclusion of a, “shock absorber systems in the torque converter, which make a rapid lock-up of the converter clutch possible”, per the ZF press release. This sounds like the LuK torque converter lock-up clutch dampers available to allow the torque converter to lock at lower RPM (and with fewer cylinders) where vibration would normally be unacceptable to owners. And that too has a fuel efficiency advantage over a conventional arrangement.

Basically it boils down to the need for greater fuel economy. Optimize a range of engines and final-drive ratios around a flexible gearbox so that the best possible fuel economy can be recovered. And with BMW’s purported front wheel drive ‘B’ class coming, this gearbox may be a good fit.

But let’s end on this note, how to shift a Ranger nine-speed manual gearbox:

BMWBLOG

NEWSLETTER