TECHNICALITIES

BY DON SHERMAN

Shifty business

From the engineer’s perspective, the Corvette is a Rubik’s cube of interlocking, interacting systems. A small change here can inflict major consequences there. Corralling an additional 36 horses under the hood for 2008 sent transmission engineers scurrying to make sure their chunks of the cube could take the gaff.

Design release engineer Dave Howe, who’s responsible for the Corvette’s six-speed manual transaxle, explains, “Our objective was to provide the necessary torque capacity without major disturbances to the basic design. But while we were at it, we seized the opportunity to install improvements every customer will appreciate.”

Changes for 2008 include wider gears for increased torque capacity, higher-capacity synchronizers, finer splines and machined teeth for the dog clutches, and new single-piece (replacing two-piece) shift forks. For increased stiffness at the front of the transmission, the oil-circulating pump has been moved inside the case and its flow rate has been increased by 15 percent. Various ribs, reinforcements and flange thicknesses are increased, and the countershaft is a new, more robust single-piece (versus two-piece) design. Differential ring and pinion gears are now shot-peened twice for enhanced durability, and the span between their support bearings has been increased to the Z06 dimension.

When the chips settled, Howe and his platoon of colleagues had altered over 90 percent of the Corvette’s six-speed manual transmission’s components. That prompted a new TR-6060 identification label to supersede the previous T-56 nameplate. Validation testing proved that all the attention was warranted. The new gearbox is not only tough enough to stand up to the LS3 V8’s ferocity, it’s also slicker-shifting with a 10-percent reduction in motion and effort.

Across the hall at GM Powertrain, the engineers responsible for the Corvette’s six-speed automatic requested equal time to upgrade their game. Control integration systems manager Jim Springer explains, “Corvette owners have urged us to tighten up the response between a command at the steering wheel paddle and an up- or downshift. That’s an area we’re always striving to improve, so we used the 2008 model year to implement strategies we’ve developed.”

During downshifts, a new “quick shift” technique automatically raises engine rpm in the brief interval between the release of one gear and the engagement of the next. It took an intense calibration effort and cooperation from Corvette engine experts to refine how it works, but the end result is a 40-percent quicker response.

“Thanks to the scope and sophistication of our electronic controls, this improvement was accomplished in software with no hardware changes necessary,” says Springer.

“To quicken upshifts, we implemented a strategy called torque management,” he says. A momentary reduction in the amount of engine output shortens the torque phase of each paddle-commanded upshift. The net result is a 25-percent faster response. Since minimal torque management is used when the transmission handles upshifts automatically, the sport mode should still be used if the driver’s priority is flat-out acceleration.

David Kimble cutaway illustration

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