We first heard rumors that General Motors was developing an all-new eight-speed automatic transmission in-house years ago, and at the North American International Auto Show in January, we learned that it would debut on the 2015 Corvette Z06. The transmission, called 8L90, is unlike any others before it, designed specifically to handle all 635 lb-ft of torque on offer from the range-topping ‘Vette’s supercharged LT4 V8 and to work in conjunction with its unique cylinder deactivation system. Now, thanks to a recent article in Engine Technology International, we know about a little more details about the unit’s technicalities.
“The transmission is made up of four separate gearsets and five clutches,” Bill Goodrich, assistant chief engineer for the 8L90 told ETI. “A key feature is that in every gear state we have three of the five clutches applied; with only two open, you minimize the drag and spin losses you get from an open clutch pack.”
Goodrich also explained the extensive weight saving measures taken by the engineering team when developing the 8L90. They were able to keep mass down by carefully choosing the materials for the transmission. The gearset carrier is made of aluminum, while both the the clutch piston housing and the cover plate for the control system are made from magnesium. The housing for the clutch is made from aluminium, but a steel insert is used on the high-stress area where the shafts are connected.
“Where we have low-stress areas on parts, we’ve cast, stamped or machined holes in them to remove unnecessary mass, while maintaining strength in the critical areas,” said Goodrich. “We were able to achieve a product which fits in the same envelope as the current 6-speed auto but that is 3.5 kg (7.7 lbs) lighter.”
The 8L90 also has a max torque rating of 738 lb-ft of twisting force.
The other main focus of the engineers when developing the 8L90 was improving upon shift times. As reported previously, the team benchmarked the Porsche 911’s dual-cutch PDK transmission during development. GM claims to have matched, if not beaten those shift times through a combination of hardware and software design.
Currently, the 8L90 eight-speed has only been confirmed for use in the 2015 Corvette Z06, although lead Corvette engineer Tadge Juechter has not ruled out the possibility of it eventually finding its way into other vehicles.