GM Says Corvette’s New 8-Speed Gearbox Rivals DCT

MILFORD, MI – General Motors says its traditional 8-speed automatic transmission for the Chevrolet Corvette Stingray and coming Corvette Z06 rivals the vaunted dual-clutch 7-speed automatic gearbox in the Porsche 911, giving the automaker’s halo sports car quick shifts and improved fuel economy with bowtie value.

The rear-wheel-drive Hydra-Matic 8L90 8-speed automatic bows on the Corvette for ’15, joining the Performance Data Recorder and Valet Mode as new technologies for the coming model year. The latest-generation Z06, promising 650 hp and 650 lb.-ft. (881 Nm) of torque on top of other mechanical and exterior upgrades over the base Stingray, arrives in first-quarter 2015.

But despite the lightning-fast promise of a DCT, which German automakers favor for many performance-model products, Corvette Chief Engineer Tadge Juechter says GM will not go that direction on its all-American supercar.

There are a number of reasons, he says, starting with the fact no DCT presently exists on the market capable of handling the power output of the Z06, which will share the new 8-speed automatic with the 455-hp Stingray.

That means GM would have to design and build its own DCT to superhero specifications, and the result would be an expensive niche transmission sure to drive up costs on the value-oriented Corvette.

“One of the reasons we sell the Corvette at a reasonable price is we leverage General Motors’ (production) volume, whether it is the engine or the transmission,” Juechter tells WardsAuto during a media preview here for the 8-speed and PDR system.

GM uses a modified version of the Corvette’s 6.2L small-block LT1 V-8, redesigned for ’14, in its large pickups and SUVs. Trucks with that L86 engine will add the 8-speed in ’15 as well, moving annual production numbers for the gearbox to upwards of 700,000 units. GM historically sells fewer than 20,000 Corvettes annually.

Driveline expert ZF supplies the Porsche 911 DCT, known as Porsche Doppelkupplungsgetriebe, or PDK. Getrag lent its expertise to Ford for its small-car DCT, and Volkswagen sought out BorgWarner for the unit it uses in its namesake and Audi products.

8L90 Compact, Lightweight

Juechter also says the Corvette demands certain packaging requirements and the GM 8-speed marries up better to the automaker’s cylinder-deactivation technology, a key fuel-economy feature on THE small-block engines that shuts down half the cylinders during low-load operating conditions.

“There’s a very constrained package where the transmission has to live,” he adds. “It is a unique spec.”

The 8L90 transmission matches the Corvette’s outgoing 6L80 6-speed unit at 24 ins. (598 mm) long but drops 9 lbs. (4 kg), using advanced lightweight materials such as magnesium and aluminum on a number of parts. Coupled with a unique design, which in an industry-first includes an off-axis oil pump, the work resulted in 24 new patents.

Working in tandem with cylinder deactivation, a feature GM calls Active Fuel Management, the 8-speed provides a 3.5% bump in Corvette fuel economy to as much as 29 mpg (8.1 L/100 km) highway, according to EPA estimates. It’s rated at 16 mpg city (14.7 L/100 km).

GM needs a transmission with a torque converter to wring out all the benefit of AFM, another reason to avoid a DCT, Juechter says. “That lets us be more aggressive in AFM, and that’s very important because AFM is the single biggest card we can play in terms of fuel economy.”

Juechter expects the new 8-speed to push the mix of automatic transmissions slightly beyond the Corvette’s historical two-thirds of customer orders during the early months before settling back down to normal levels. “It’ll be the hot new thing.”

Also new for the ’15 Corvette is the PDR system. Developed in conjunction with motorsports-engineering house Cosworth, PDR records track-day laps, as well as what Juechter calls “epic drives.”

Owners can immediately play back videos of their laps or drives, which include key data overlays, on the Corvette’s 8-in. (20-cm) color touchscreen. The drive and data also can be downloaded from a removable SD card and shared.

“It was born out of social media and how people like to share their experiences,” Juechter says, noting development of the PDR began before work commenced on the fifth-generation Corvette that bowed last year.

On the Track

Mated to the Corvette’s LT1 V-8, the 8-speed and PDR perform well during brief testing on the Milford Road Course at GM’s proving ground outside Detroit. Transmission shifts occur swiftly and with authority under hard throttle. In a nifty bit of engineering, software flattens shifts during hard lateral acceleration to retain overall stability.

There’s no need to switch over to the paddle shifters, either. Even GM’s best drivers forgo manual mode, as software controls are five times faster, Juechter says. Consider them art on the wall.

GM says the 8-speed makes the Corvette 0.1 seconds faster in both 0-60 mph (97 km) tests and over the quarter-mile (0.4 km) to better than 3.7 seconds and 11.9 seconds, respectively, compared with the outgoing 6-speed.

In a brief stint on the roadway, the Corvette returns 23 mpg (10.2 L/100 km). However, the test never reaches highway cruising speeds, where GM says the eighth gear dials the small-block V-8 down to a fuel-sipping 1,600 rpm. Here's a video of a WardsAuto test drive with PDR:

The PDR recorded two laps on GM’s Milford course, providing both a high-definition video and audio of the symphonic Corvette exhaust note, with data overlays such as speed, rpm, G-forces, steering angle, GPS position and track times.

By the time the ’15 Corvette arrives at U.S. dealers next year, a website with a Cosworth Toolbox of software technology will be available for owners to download additional telemetry similar to the sophisticated data used by the Corvette racing team.

A Valet Mode feature on PDR allows owners to lock interior storage, disable the infotainment system and record video, audio and vehicle data when it is active and out of the owner’s hands.

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