Increasing the number of ratios from six to nine improves fuel economy and reduces CO2 emissions. The more closely-spaced ratios result in better acceleration response, while the higher top gear provides added refinement.
Land Rover, who is the lead partner on the ZF 9HP project, says the robust gearbox is suited to its models’ rugged all-terrain ability. The ZF 9HP’s lowest ratio is far lower than the current six-speeder’s, and is specifically designed for off-road use.
Unlike the outgoing gearbox’s sequential operation, the new boy has a ‘skip-shift’ function – which means it can skip a gear or two when downshifting under heavy deceleration or rapid kickdown. It also matches the driver’s mood within seconds, sharpening up for sportiness or settling down for economy as need be.
Curve Mode, longitudinal acceleration and pedal position all control upshift prevention, while Fast-Off mode measures the rate of throttle release, anticipates further requests by the driver for high power, then holds the gear if necessary. If the driver requests a downshift when the vehicle is travelling too fast, the transmission will remember the request and make the shift when the speed drops to an appropriate level.
Despite the extra three gear ratios, the new nine-speeder is only 6.0 mm longer and weighs 7.5 kg less than the outgoing six-speeder. It’s produced at ZF’s Gray Court facility in South Carolina (more details on the ZF 9HP here).
Late last year, ZF boss Stefan Sommer was quoted as saying nine gears was the “natural limit” for transmissions, because any more than that would add weight that he opined could not be offset by gains in fuel efficiency.