How the C8 Corvette’s Dual-Clutch Has Changed Since We Drove the Car
The new Corvette has an eight-speed Tremec DCT. We weren’t crazy about it in the pre-production C8 we drove, but engineers tell us the final version will be better.
For the C8 Corvette, Chevrolet abandoned the traditional manual and torque-converter automatic for a new, eight-speed Tremec dual-clutch. And in our Performance Car of the Year testing, the gearbox was the weakest component in the pre-production C8 Stingray we had on hand. It’s part of why the Corvette didn’t win.
In automatic mode, the DCT dolled out nice, snappy shifts, but when using the paddles, it could be clumsy. Too often we found ourselves running into the rev limiter, or having downshifts denied after a paddle pull. But, the C8 we drove wasn’t a finished product. There’s been development work since we drove the car, and that work will continue for the foreseeable future. At a powertrain engineering seminar held by Chevy last week, we asked Glen Hoeflinn, controls program manager for the DCT, what will change from the car we drove.
“Maybe you get some humpy-bumpy shifts here, you get a little bit of that there. That all gets refined out,” Hoeflinn said. “It’s in final refinement, and then it’s in final checks and looking what we’re doing and making sure that it’s behaving exactly [how] we want.”
“That’s what we’ve done since the car that you had. Doing all that refinement and making sure it’s ready to go for everybody across the all the cars.”
A dual-clutch presents unique challenges, no matter what sort of car it’s in. “There’s a lot of pre-selection interaction that goes on in the background,” Hoeflinn said. “It’s the same choreography” between the engine and transmission, he added, but without the “luxury” of a torque converter, there’s a lot more programming work involved.
As you’d expect, the transmission has different automatic shift strategies for the various drive modes, which adapt in real time. The more aggressive, the more spirited you drive, the more aggressive the car’s going to respond,” Hoeflinn said. “As you start to relax, the car’s going to start to relax.”
The DCT uses latitudinal and longitudinal accelerometers, and looks at information like throttle position and steering angle to gauge how the car is being driven, and react accordingly. For example, in Track mode with the transmission set to automatic, the car will downshift aggressively when the driver is braking hard into a corner, and hold upshifts until corner exit.
The C8 has two manual modes. If you pull a paddle while in Drive, you get a temporary manual mode, which automatically times out, or can be exited sooner by holding the upshift paddle. In this mode, the car will automatically upshift at redline. If you press the M button in the center console, you get full manual mode. There’s no time out, and the car won’t upshift at redline.
There are two other neat tricks available for drivers to exploit. First, if you hold the downshift paddle, the DCT will serve up the lowest possible gear. Do that while braking, and the transmission will keep downshifting as engine speed allows. And second, pulling both paddles at the same time is equivalent to pushing in the clutch pedal on a manual car, which allows you to rev the C8’s new V-8 as much as you want.
In the C8, the paddles are directly wired to the transmission control module (TCM) for quicker response times. “In other applications, from the paddle, the wire will go to the body control module and then from the body control module back over to the transmission. You have obvious latency there,” Hoeflinn said.
“It could be 25, 30, 40 milliseconds from the time you pull, to the time that transmission actually got the message. When you wire them directly from the paddle straight to the TCM, we’re getting the message instantaneously.” This doesn’t mean the paddles will give you a downshift that over-revs the engine—the TCM prevents that—it just helps reduce delay.
One of the headline figures of the C8 Corvette is its incredible acceleration. We timed a pre-production Z51 Stingray as hitting 60 mph in 2.8 seconds and running the quarter-mile in 11.2 seconds at 122 mph. With the C8’s Performance Launch mode, the car will actually use the inertia of the engine coming down between revs to propel the car forward. Chevy calls these “Boosted Shifts,” and they’re only used with a Performance Launch. In any other mode, they make the car feel unsettled.
From a mechanical standpoint, this new Tremec transaxle isn’t a radical departure from other DCTs. There are concentric clutches and input shafts for the odd and even gears. The even gears and reverse live near the front of the transmission, while the odds are at the back. A limited slip-differential is integrated within the unit. Base Stingrays get a mechanical diff with a 4.89:1 final drive ratio while Z51-pack cars get an electronic LSD with a 5.17:1 ratio. The overall gear ratio spread of 8.8:1 is the same regardless of differential.
The packaging of the transaxle is such that there’s a common oil sump—filled with 11 liters of Pentosin FFL-4 fluid—for all components. A cooler mounted to the top of the transaxle assembly means there’s no need for additional hydraulic lines, while two filters keep things clean. An externally mounted pressure-side filter requires replacement every 20,000 miles, while the internal suction filter mounted to the sump is a lifetime part.
We asked about why the C8 team didn’t try to do a manual. Hoeflinn and the other engineers present gave us a similar answer to Corvette chief engineer Tadge Juechter, when we interviewed him before the car debuted. They’d need to develop a new manual just for the C8, and considering the stick-shift market is shrinking, it would be an expensive endeavor seemingly without much reward. There are packaging constraints with the Corvette’s central backbone tunnel, too, which would require a hole to accommodate the shifter and gear linkage, hurting structural rigidity. Juechter also said the pedalbox would be cramped with a clutch.
Our first experience with this DCT was less than positive, but this is a gearbox that shows a lot of promise. We look forward to driving the finished product.
Originally written by Chris Perkins; Road&Track