Proven in Volume Production Specialized for Motorsports
With its 8P45R, ZF presents a motorsports product that transfers major advantages of an eight-speed automatic transmission for volume production passenger cars to racing applications. Modifications make the transmission system even faster, lighter and more robust on the racetrack.
There were good reasons behind the idea of adapting the ZF volume production eight-speed automatic transmission for a new application in motorsports. Most importantly, unlike a conventional manual-shift transmission with clutch system, an electronically controlled planetary gearset allows power shifting without torque interruption. That boosts the dynamics and performance of race cars. Spur gear drives apply the gear ratio change to just one pair of gear wheels, whereas planetary gearsets come with the advantage of multiple use of planetary gearsets and shift elements to facilitate an extremely compact design. As a result, a planetary gearset can be lighter than other transmission designs with the same power output.
Furthermore, the electronic control unit increases drivability as well as safety for drivers, who often include beginners and amateurs, especially in the envisaged mass sports classes. An automatic transmission system also prevents engine-damaging gear steps, in other words over-revving the internal combustion engine. It is also a big plus that the driver can keep their hands on the wheel at all times, because a planetary gearset together with a corresponding shift system allows fast gear changes using steering wheel paddles. Also significant is the high reliability that comes with a transmission already proven millions of times in volume production vehicles.
From the idea to the track
In 2014, initially an exclusively software-modified 8HP45 transmission completed its premiere season in motorsports. It was used in the BMW M235i Racing Cup for private teams as well as in BMW’s own young drivers program.
The volume production basic transmission, designed for up to 450 Nm, a maximum engine speed of 7,200 rpm and a racing service life of 30,000 kilometers, proved to be powerful and robust enough to withstand all longitudinal and transverse dynamic requirements that arise during racing. Installed in a total of 16 vehicles, this transmission had already survived the rigors of motor racing – including a total of 22,000 km on the North Loop of the Nürburgring – without any transmission failure. This was one of the reasons why, as early as 2014, ZF decided to systematically further develop its automatic transmission technology for the new application field of motorsports, building on the company’s solid experience from volume production.
The objective was to create a proven, also mechanically modified, racing transmission, the 8P45R. It was initially built as a prototype. The development targets detailed in the requirement specification were lighter weight, faster shifting and a modified transmission-ratio spread with the transmission optimized for racing. “Ten months after project launch and just nine months under development (including detailed engineering, component procurement, assembly and application), the first 8P45R prototype was successfully used in the VLN Endurance Championship on the Nürburgring in October 2015,” is how Marc Seeberger, “Advanced Engineering Driveline” project manager at ZF Headquarters in Friedrichshafen, looks back full of pride on the successful development of this motorsports automatic transmission.
Development of transmission and application
One of the most important modifications to the racing transmission compared to the volume production model is that the 8P45R transmission does not require a hydraulic torque converter or the associated lock-up clutch. This innovation has reduced the system weight compared to the volume production version by around 12% to some 60 kg, while also significantly reducing mass inertia.
The shift element designed as a multidisk brake takes care of setting off – combined with adapted control of the other transmission clutches. This involves an internal startup clutch which operates in an oil bath and can therefore withstand high thermal stresses. This clutch now engages in a noticeably harder way so that the engine speed drops only minimally during gear changing. Even without mechanical modifications compared to the volume production solution, the internal startup clutch meets challenging, explicitly racing-specific starting criteria such as flying starts after a safety car phase or pit stops including maneuvering. These beneficial characteristics are also closely related with software measures.
Overall gear ratio spread optimized for racing
Once the car has moved off the starting grid, the 8P45R transmission designed for a higher input torque of 500 Nm shifts gears under load just like the volume production transmission. However, the gear steps have been completely realigned to the engine’s optimum power range. Due to their close-to-volume-production, comparatively long gear ratio, the top two gears (seventh and eighth) in the software-optimized racing transmission previously remained largely unused. Theoretically, the top speed in eighth gear would have been 425 km/h.
The 8P45R transmission therefore has smaller gear steps. These are the result of a much narrower spread of gear ratios overall, i.e. from 7.1 to 4.2. To achieve this, three of the total four planetary gearsets were re-engineered. The rear-axle drive was also adapted and the overall gear ratio decreased accordingly. However, this general development does not apply equally to every gear. The first gear covers an even greater speed range than before – now up to 80 km/h instead of the previous 60 km/h. However, the other seven forward gears on the new transmission are closer together in order to achieve better acceleration and more powerful traction, which limits the top speed (achievable in top gear) in the current application to a realistic 320 km/h.
“The much shorter shifting times in all gears lead to a further significant performance boost,” says Peter Leipold, Driveline Development project manager at ZF Race Engineering in Schweinfurt. “This is true for changing to both higher and lower gear ratios. It means the 8P45R racing transmission beats even very sporty volume production transmissions and simultaneously comes close to the level of sequential racing transmissions,” adds Leipold.
Due to the much shorter shifting times, lower stress factors and sequential shift system specified in the rules, the load on the shift elements has dropped massively. That ensures the required service life and stability despite the use of volume production components. Added to this are software measures which also accelerate shifting and provide a higher performance potential.
The unconventional project which aimed for the first time to adapt an advanced, proven volume production automatic transmission to a pure racing application at minimum outlay has been a resounding success. After just nine months under development, the 8P45R racing transmission meets all customer requirements as well as ZF’s own targets. “Given the huge interest in the new racing transmission, it could also soon find its way into other race vehicles,” predicts Norbert Odendahl, CEO of ZF Race Engineering GmbH, with more than a touch of pride.