GM 8-Speed 8L90 M5U Hydra-Matic Automatic Transmission

Designed and built by GM, the 8L90 transmission speed is expected to contribute up to 5-percent greater efficiency, when compared to the previous 6L90 six-speed automatic. Smaller steps between gears, compared to the Hydra-Matic 6L90 six-speed automatic, keep the engine within the sweet spot of the rpm band, making the most of its horsepower and torque to optimize performance and efficiency.

For performance driving in the Corvette, the transmission offers full manual control via steering wheel paddles, and unique algorithms to deliver shift performance that rivals the dual-clutch/semi-automatic transmissions found in many supercars – but with the smoothness and refinement that comes with a conventional automatic fitted with a torque converter.

The 8L90 has a torque capacity of 738 pound-feet (1000 Nm).

Compact And Lightweight

Featuring four gearsets and five clutches, creative packaging enables the 8L90 to fit the same space as the previous 6L80/6L90 family of six-speed automatics. Extensive use of aluminum and even magnesium also make it more than eight pounds (4 kg) lighter than the six-speed (in Corvette applications). Along with design features that reduce friction, the 8L90 is expected to contribute up to 5-percent greater efficiency, when compared with a six-speed automatic.

Five clutches are used, three at a time, leaving only two open clutches in each gear state. Optimized clutch sizing and off axis binary pump contribute to outstanding spinloss attributes.

All applications feature adaptive shift controls. Several features such as grade braking, Performance Algorithm Shifting, and Driver Shift Control are available on some of the applications. Performance Algorithm Shifting (PAS) monitors how assertively the driver is using engine output and other vehicle sensors to determine at what engine speed to upshift of downshift. Driver Shift Control allows the driver to shift the transmission like a clutchless manual gearbox. Electronic safeguards prevent over-revving if the wrong gear position is selected.

The 8L90 uses VFS control components to monitor clutch pressures with optimum accuracy. The result are faster and more precise shifts.

The 8L90’s 32-bit transmission control module (TCM) monitors transmission performance and compensates for normal wear in components such as clutch plates, so performance is consistent for the life of the transmission. The control module also “tests” the components to optimize their interaction on an on-going basis.

The 8L90’s controller analyzes and executes commands,the processor microcontroller executes instruction at a rate of 120 Million Instructions per second, and wide-open-throttle upshifts are executed up to eight-hundredths of a second quicker than those of the dual-clutch transmission offered in the Porsche 911.

The torque converter is 258 mm in diameter and features a lock-up clutch, although clutch and converter specifications vary depending on the application. The 8L90 uses electronic controlled capacity clutch (ECCC) technology, which employs a small, regulated amount of slip to dampen out engine pulses for a smoother running drivetrain, especially during shifting.

Low Maintenance

The 8L90 uses Dex 6 HP Transmission fluid, which was developed to have a more consistent viscosity profile as well as improved lubrication characteristics.