An automatic transmission in a Ferrari must give the driver the feel of a true Ferrari without compromise. So when in the early 1990s, Ferrari identified a requirement for an automatic transmission option for its 456 GT high performance, luxury touring car, it approached Ricardo for a potential solution. The new design was required to make use of an existing donor transmission, as the high investment and long lead times required for a completely new automatic (coupled with the relatively low volumes expected) would not have been viable.
The result was the creation of a transmission that could give a minimum of four speeds with torque converter lock-up, ratios arranged to give maximum vehicle performance, an engine power of 442PS at 6200rpm, engine torque 540Nm at 4500rpm, a maximum engine speed 7200rpm, and automatic shifts at speeds up to 7000rpm. In addition, the controls had to be capable of high energy shift management, and the electronics had to to interface with Bosch Motronic engine management ECUs. What is more, the unit had to be compliant with US OBDII (on-board diagnostics) regulations and compatible with a Ferrari diagnostic tool.
The 456 GT is configured with a front engine, torque tube and a rear, manual transaxle. The six speed manual transmission is long and slim and packages under the rear seats with an integral differential mounted at the rear of the transmission. The automatic transmission also had to be rear mounted and packaged into the existing space using the same differential centre-line. Further package space was made available behind the rear axle which was facilitated by the small luggage space requirement.
Design concept studies by Ricardo showed that there was not enough space available to allow the core automatic transmission components (planetary gear sets, clutches and pistons) to be placed in the same position as the core of the slim manual transmission. This constraint forced the development of a concept design with the automatic transmission core placed behind the axle centre line. This concept required power flow in and out of the transmission at the front end, which necessitated the selection of a donor transmission with hollow shafts to allow the fitment of another shaft to take power out.
This restricted the list of possible donor transmissions. The core of the GM 4T80-E provided the closest match to the configuration requirements. This transmission is typically used with the Northstar V8 in the Cadillac Seville. Although the configuration suited the concept required, several key components needed uprating. The gearset was input torque limited to 637Nm, which had to be increased to allow for the higher torque Ferrari engine and the effects of torque multiplication at torque converter stall conditions.
Friction elements (wet clutches and band brakes) had insufficient energy capacity due to the Ferrari application having higher engine speed, shift speeds, torque and inertia. Other design challenges included ensuring that the donor parts would retain their reliability in the new application. This included developing an independent differential oil flow circuit between the torque converter and the transmission core.
Torque converter packaging had to be remote from the transmission core for the new Ferrari automatic, and the 4T80-E offered a separate torque converter which could be readily adapted. The front end of the new transmission was designed to hold the torque converter and mount with the torque tube. The new front casing rigidly mounts to the torque tube with a splined connection to the torque shaft. Input shaft bearings were arranged like a wheel hub for stiffness and sealed input shaft bearings were selected. To connect the input shaft to the torque converter a new flex plate was designed. The final design allowed the bell housing to package under the rear seats as intended.
Due to the overall layout of the transmission, the final drive required a ring gear and pinion to be designed with rotation in the reverse direction to that normally used. To allow the drive to pass from the torque converter to the transmission core, a hollow pinion gear was designed. A ZF plate type limited slip differential (LSD) which was in current use in the Ferrari F40 was included in the package. The final drive requires a separate oil type from the automatic transmission fluid (ATF) that is used by the torque converter and the transmission core. Hence the final drive needed its own lubrication system with passageways to take the ATF to and from the torque converter designed into the drive housing.
Parts of the transmission core were modified to take the higher power, torque, speeds and shift energies. The modified core along with carry over valve bodies, pumps, filters and park brake mechanism were all packaged in a new single axis maincase.
Transmission control has a major role to play in ensuring the success of any automatic transmission. For this project, there were three key challenges for the control engineers. Firstly, the transient event control at high speed, secondly, the shift management calibration and torque modulation to ensure good shift quality, good performance and ensuring that it feels like a Ferrari.
When used with the Northstar engine, the 4T80-E uses a combined engine and transmission control unit. This unit was neither available nor appropriate for the Ferrari application. GM’s transmission control strategy and software were also not available to Ricardo or Ferrari. Therefore, it was necessary for Ricardo to devise a new TCU with new control algorithms and software.
The new TCU was designed to control all transmission functions, to communicate via CAN with the two engine management controllers and the ABS controllers, and to be compatible with OBDII scan tools, Ferrari end of line (EOL) testers and dealer diagnostic equipment.
To improve driveability, a number of features were included in the software. Speed of throttle apply is used by the new software to modify the shift behaviour to give improved driving response. Sport function is automatically engaged when D3 is selected: no mode switches are used as these were considered to detract from the driveability.
Sign-off of the new transmission followed an extensive program of rig and vehicle based testing. This included some specially devised durability tests to identify any problems associated with the extended operating envelope required of a Ferrari transmission.
Ricardo Tel: 01926 319347