General Motors faced tough challenge when searching for the proper transmission to pair with its all-new 625 horsepower supercharged LT4 engine in the 2015 Corvette Z06. The General wanted to match the lightning-fast shifts offered by Porsche’s PDK transmission, but no dual gearbox existed that met both of GM’s requirements — compact size and the ability to handle the LT4’s 635 lb.-ft. of torque.
The answer? Build one that could. The 8L90 eight-speed automatic transmission found in the new Z06 was developed entirely within GM. It’s a traditional automatic gearbox in its design, but GM says it shifts not only faster than any dual-clutch, but smoother as well.
Lead Corvette engineer Tadge Juechter highlighted the stringent packaging requirements the team had to meet for the 8L90.
“It’s engineered by GM, internally, and many of the parameters were focused on this car,” Juechter told Motor Authority. “So it’s almost a custom transmission.”
Juechter explains that the transmission, which offers 5 percent better fuel economy than the equivalent six-speed automatic, was designed to work seamlessly with the Corvette’s cylinder deactivation system.
“We wanted a transmission that provided everyday drivability and compatibility with cylinder deactivation,” he said. “When you run in four-cylinder mode, a torque converter is really your friend.”
Juechter confirmed the transmission will find its way into other applications, even though it’s design was influenced solely by the Z06 project.
A key piece of the design in the 8L90 is what GM is calling a “smart pump”. Under heavy acceleration, the transmission uses two inlet discharge ports, providing more flow to the clutch packs. When cruising, the system automatically calms down a bit and resorts to a single inlet port, reducing drag.
What’s more, the 8L90 is designed to be compatible with the standard Corvette Stingray’s torque tube, suggesting that it could eventually find its way into the standard car as a rolling update or mid-cycle refresh. The gearbox is the same physical size as the existing six-speed automatic, but thanks to a lightweight aluminium and magnesium construction, its eight pounds lighter. Despite its light weight, the 8L90 can handle up to 738 lb.-ft. of torque.
The transmission features a tall overdrive ratio for seventh and eighth gears (0.86 and 0.65, respectively), while the sixth gear is a direct drive. The new setup also allows GM to lower the final drive to 2.41 from 2.73, though the taller base axle is still available as an option. GM is still honing in shift patterns, shift points, and the map for cylinder deactivation, so we’ll have to wait a bit for official fuel economy figures for the Z06.
But between the efficient eight speed transmission, lightweight construction and cylinder deactivation system, the Z06 is shaping up to be one of the fastest and most fuel-efficient supercars on the market.