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.

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“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.

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