GM 8-Speed In Corvette Z06 Shifts Faster Than Porsche PDK; Here's How

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With the new 2015 Chevrolet Corvette Z06 that was introduced this past week at the Detroit Auto Show, GM raises the threat level to the bragging rights of virtually every other supercar in the world.

And while performance numbers are still months away, there were plenty of interesting details released with GM’s Z06 announcement—like an all-new ‘LT4’ engine, as well as a new, GM-designed eight-speed automatic transmission designed for exactly this application.

That new ‘8L90’ transmission, designed completely within GM, by a dedicated team, is a traditional hydraulic automatic—solenoids, valves, planetary gearsets and all—but GM says that it provides the super-quick shifts of dual-clutch boxes, yet with more smoothness.

The eight-speed automatic is actually quicker-shifting than the PDK gearbox in the Porsche 911, GM officials say; and that’s a transmission that engineers benchmarked for the project.

“Almost a custom transmission”

Corvette chief engineer Tadge Juechter pointed to some tough packaging challenges. There wasn't any existing dual-clutch gearbox that could take the torque of the engine and would fit in the space permitted.

“It's engineered by GM, internally, and many of the parameters were focused on this car,” said Tadge Juechter, at a pre-show event. “So it's almost a custom transmission.”

“We wanted a transmission that provided everyday drivability and compatibility with cylinder deactivation,” Juechter explained, while they also wanted the speed of a DCT. “When you run in four-cylinder mode, a torque converter is really your friend” in everyday cruising conditions, he said.

The transmission will be used in other applications, Juechter confirmed, but many of the attributes of it were driven by the parameters demanded of the Z06 project.

Also of note is that the transmission offers up to a five-percent fuel economy gain over a six-speed automatic used in the same application

Two-port ‘smart pump’ design a key to being lean and quick

2015 Chevrolet Corvette Z06 - GM 8L90 8-Speed Automatic Transmission

2015 Chevrolet Corvette Z06 - GM 8L90 8-Speed Automatic Transmission

That’s due to a host of engineering changes and analysis of the transmission from scratch, looking at the most efficient path for torque flow. But when we pressed the engineer overseeing the project for the single most important design attribute, he pointed to the ‘smart pump’ setup, in which GM incorporated a binary operation with two inlet discharge ports.

To put it as simply as possible, this clever hydraulic pump system works a bit like the cylinder deactivation the Corvette already has. Under acceleration and aggressive driving, the transmission uses both ports and provides more flow to the clutch packs, to speed up shifts. And then at cruising speed it relaxes to a single port, which reduces drag.

One other key to help minimize drag and mechanical/friction losses for this transmission, which has five clutches and four planetary gearsets, is that only two clutches in any single gear are open at one time. That plays a role in speeding up downshifts, too.

The unit, overall, is the same physical size as the existing six-speed, with the casing and input shaft area designed to be compatible with the C7 Corvette’s ‘torque tube’ that runs down the center of the car and is essentially part of its structure. And thanks to aluminum and magnesium construction it’s eight pounds lighter than the outgoing six-speed. Yet it was designed to allow up to 1000 Nm (738 pound-feet) of input torque—which could suggest, perhaps, that GM could eventually sell them to niche ultraluxury makers with torquey turbocharged V-8s and V-12s.

Tall overdrive ratios? Check.

The new transmission offers deep overdrive ratios for seventh and eighth gears (0.86 and 0.65), while sixth gear is a direct drive. The new setup also allowed them to drop the base axle down to 2.41, from 2.73 (the latter will still be available as an option).

GM is still making decisions about shift pattern, shift points, and the the map for cylinder deactivation, officials said, so any announcement on fuel efficiency will be months away. And its operation in the Z06 would of course be tailored by the Corvette’s mode selector.

The transmission, as it was engineered, would to be able to go into any of GM's rear-wheel drive applications. We take that as a strong hint that this transmission may be headed into the C7 as a rare mid-cycle improvement.

While we’re curious to see where this transmission lands next, we’re eagerly awaiting performance and fuel economy numbers for the Z06. By offering a choice between a seven-speed manual or eight-speed automatic, the 2015 Chevrolet Corvette Z06 stands out among 600-hp-plus vehicles. Based on the suggestions of those involved in the project, we’re also expecting the Z06 to be one of the most fuel-efficient supercars on the planet.

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