ZF 9HP 9 Speed Transverse Automatic Transmission Analysis

13 June 2011 - 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.

ZF Friedrichshafen’s 9HP nine-speed automatic transmission, which was first announced earlier this year, now has more specific details about it, though there’s still no mention of when it is expected to debut in a production vehicle. The unit, which doesn’t replace the 8HP eight-speeder and is meant for passenger cars with front-transverse drive, will be available in two variant forms, covering a torque range between 280 and 480 Nm.

The transmission, which is stop-start capable (with only one friction shift element that needs to be closed for restarting), features four individual gearsets and six shifting elements. A torque converter is used in the 9HP as the standard starting element, with a multi-level torsion damper system minimising hydraulic losses and quick bridging of the torque converter lock-up clutch facilitated at low speeds. In hybrid-based applications, the torque converter is replaced by an electric motor.

ZF says the unit, with its extremely high total spread of 9.84, offers savings of up to 16% compared to today’s standard 6-speed automatic transmissions in front-transverse installations. In ninth gear, only 1,900 rpm is needed to cruise at 120 kph instead of 2,600 in a 6-speed transmission.