Commercial success in the auto industry invariably rests on forward thinking, carefully calculated bets on new technology, and shrewd up-front investment. Engineers at high-performance transmission designer and manufacturer Xtrac applied those criteria as they settled on the development of a generic gearbox for future supercars. At the outset of the program they decided to buck the general trend of longitudinal configuration, focusing instead on transverse installation.
It has proved to be the right recipe. The latest 7-speed version of that transmission (available initially with six speeds but now with seven) is installed in the new Pagani Huayra. Initial design specification of the gearbox, designated 1007, included capability to handle torque inputs of more than 1000 N·m (737 lb·ft). The mid-engined Huayra (named after an ancient South American god of the wind) is powered by a 6.0-L V12 with a peak torque rating of 1100 N·m (811 lb·ft).
It was a fortunate coincidence that, as Horacio Pagani was mulling over a replacement for his remarkable Zonda model, Xtrac’s design team was considering the development of a transverse transmission to meet future supercar needs.
Xtrac’s thinking was that although supercars historically have a longitudinal engine mated to a rear-mounted longitudinal transmission, the layout results in a considerable mass behind the rear axle. The engineers regarded this as a constraint on optimum vehicle performance and handling, on the space available for a rear crash structure, and even on vehicle aesthetics.
So they set about designing an efficiently packaged gearbox with a very high torque capability for transverse applications. Its primary and secondary shafts are stacked directly above each other for space efficiency. The drive is transmitted by an engine-mounted push-type clutch and input shaft with a twin-cable gear change actuation system.
“The transmission was configured to give the shortest possible rear overhang, with the input drive turned through 90º by a spiral bevel set of gears,” explained Adrian Moore, Xtrac’s Technical Director.
Moore told AEI that helical gears were specified for the main cluster due to their refinement, durability, and efficiency. They are fully form-ground with a nominal 30º helix angle, complemented by a multicone synchronizer-based gearshift mechanism.
For the original 6-speed transmission, the H-pattern gearchange, single rail selector and cable mountings were attached to the top and toward the rear of the gearbox with a left- or right-hand shift cable linkage to allow for a variety of vehicle applications.
"The initial 6-speed and subsequent 7-speed gearbox were further developed into a fully integrated, paddle-operated automated manual transmission (AMT) which allows gears to be selected in any sequence,” he said.
Moore added that the plate differential used is a ramp-type Salisbury design offering limited-slip capability, ideally suited to a high-end, high-performance road car because it offers both superior handling and reduced harshness. The differential was calibrated on Xtrac’s quasi-transient differential test rig.
The 1007 gearbox's lubrication system is an integrated recirculating type using a single-stage eccentric rotor pump with no oil tank, taking advantage of the minimal number of components and low weight pump assembly. The system also incorporates a thermostatically controlled pressure-regulating valve for rapid warm-up. Oil cooler inlet and outlet ports are positioned on the side of the casing to avoid any disturbance of a vehicle’s underbody airflow.
For the earlier 6-speed gearbox, a separate bell housing was required to package the transmission into different vehicles, Moore explained, while the aluminum casing and optimized design of internal components ensured a relatively lightweight 97-kg (214-lb) gearbox. Four location points allowed the transmission to be mounted to a rear subframe. But for the new Huayra, the bell housing was designed to be fully integrated with the transmission to further optimize packaging efficiency.
In selecting a suitable mule for the Huayra development, Pagani engineers elected to use the Zonda R that has an Xtrac race transmission. This is also a transverse configuration with a semi-automatic sequential gearchange but with straight-cut gears. With car and transmission both designed for the track rather than the road, the R shared only 10% of the road-homologated Zonda F's components; this helped provide an “independent” test bed chassis for key components of the Huayra. It also provided a major link with Xtrac.
The two went together very effectively. A 6.0-L V12 Mercedes-Benz AMG-built engine sourced from the race version of the Mercedes CLK-GTR powers the Zonda R. Its output is 552 kW (740 hp) and maximum torque is 710 N·m (524 lb·ft).
The Huayra also uses a bi-turbo 6.0-L V12 Mercedes-AMG engine that produces slightly less power (515 kW/690 hp) although significantly more torque of 1100 N·m. It gives the car a top speed of more than 370 km/h (230 mph).
Said Moore: “A challenge for a mid-engine sports car is matching a longitudinal engine with a transverse gearbox. We achieve the transfer of power and torque through 90º by means of the bevel gears. The advantage is a gearbox shorter in relation to the length of the vehicle. This can be extremely useful for positioning the mass of the powertrain exactly where it’s needed to enhance handling and to adjust the overall balance of the car for the optimum road holding.”
The involvement of Xtrac engineers throughout the Huayra program and Pagani’s experience with an Xtrac race transmission in the Zonda R were major factors in the 1007 gearbox’s successful integration in the road car.
Xtrac is using its Klingelnberg gear-grinding machine to manufacture the bevel gears for the transmission. It enables production of components that have a longer life and much improved NVH qualities when compared with those using the traditional soft cut and lap process, according to Moore.