All the horsepower in the world is useless until it's transferred to the wheels. Which makes the transmission an unsung hero of the new '14 Stingray, since the only obvious manifestations of its pivotal performance role are either a shifter or a pair of paddles on the steering wheel. Otherwise it's just a mysterious box concealed in the chassis.
All the more reason to dig into the remarkable details of the C7's "trans actions," in eager anticipation of the next installment in our Stingray series. That's when our editors plan to man the wheel and stomp on the loud pedal. We can hardly wait, but for now we'll continue to whet your appetite for what is shaping up to be the best Corvette ever.
To that end, let's take a closer look at the Stingray's new transmission offerings. Looking at Corvette-buyer preferences overall, historically the breakdown has been 70 percent auto, 30 percent manual (on models offering a choice between the two). We'll start with latest developments on the latter.
First off, the seven-speed manual does not merely consist of an extra OD gear grafted onto the existing C6 setup. For a look inside the case and all the upgrades, we spoke with GM's Todd Rooney, Global Assistant Chief Engineer for RWD Manual Transmissions. His background at GM dates back more than a dozen years, through three eras of Corvette trannies, as he's worked on the C5's T56, the C6's TR6060, and now the C7's TR6070. His responsibilities have also included overseeing clutches, clutch-release hydraulics, and manual shifters for RWD drive applications, on both U.S. and international GM models. Additionally, he's worked on automatic transmissions and AWD/4WD systems, as well as other driveline components.
Rooney's first assignment was as development engineer on the C5 manual transmission clutch, so he has an in-depth historical perspective on all the proverbial "gear changes" over the years. Since that initial work, he has been involved in a number of other Corvette-related projects, both in production and prototype arenas. Out of all his varied assignments, he notes that, "I feel that being a part of the Corvette legacy at GM is truly an honor."
Starting with the basic overall unit, Rooney notes that the gears from First to Sixth on the C7's manual are fairly similar to the C6's, except for the aforementioned additional OD, which required extending the case 42 mm to make room for the extra gearset. This change upped the overall weight slightly, by about 3.3 pounds, a gain that was partially offset by the shortened torque tube required.
All of the upgrades for the new transmission (including Active Rev Match, twin-disc clutch, dual-mass flywheel, and Seventh gear, which we'll cover below) are included in both the base and Z51 models. The Z51 option, however, has different ratios since it's intended for track duty, as noted here:
Clearly, the Z51's lower (numerically higher) close ratios in the First through Third gears will make for quicker acceleration, while the higher ones should provide fuel efficiency similar to the base model's—assuming drivers are able to keep a light touch on the throttle. From there, things get more involved, with an all-new output shaft, counter shaft, and internal shift system (forks and rails). The most obvious difference is evident in the relocation of Reverse from the upper left to the lower right, since the new Seventh gear fills in where Reverse is found on the C6 shift pattern.
The change in the shifter layout only hints at the extent of the upgrades. We're told the feel of the clutch is improved, thanks to the incorporation of a new twin-disc unit from LuK. Each of these discs measures 240 mm, compared with 290 mm (total) for the previous single-disc setup. The twin configuration lowers inertia, so there's less effort required, with a cleaner, smoother action, Rooney points out. Pedal effort is a touch lighter, too.
Sounds simple enough, but Rooney admits to sweating the details on the dual-mass flywheel, also supplied by LuK. Designing it presented a quandary, in that a heavier one reduces noise and vibration, but its extra rotational mass inevitably degrades acceleration. Conversely, one with less mass makes for better performance but feels harsher. Balancing these two competing concerns required some fresh thinking and a lot of late nights on the CAD system.
"The C7 market demands a step up in refinement," Rooney notes. "But we didn't want to sacrifice any performance either." So what they came up with is an arrangement called Dual Flywheel Lite. Sounding a bit like the name for a "great tasting, less filling" beer, this relatively new setup has reduced mass at the outer edge. Yet it's still up to handling ZR1 power levels, and likely more, a trait that will be increasingly important as Stingray owners begin enhancing the output of the car's LT1 engine. To minimize noise and vibration, the engine's firing pulses, which are normally transmitted to the driveshaft, have been isolated somewhat with springs, among other measures.
Also contributing to the driveline smoothness is a new Active Rev Matching system. Basically what it does is make the Corvette both easier and more exhilarating to drive by blipping the throttle to match engine speed with each anticipated gear change, resulting in smoother shifts. Although the concept is not completely new (the Nissan 370Z offers a similar feature), the methodology is different on the Stingray, Rooney says.
Breaking it down, this setup stars with a GAP (Gear Absolute Position) sensor. It basically consists of two components, a 3D Hall Effect transducer on the transmission and a magnet on the main shift rail. As the magnet moves, either in rotation and/or back and forth, the 3D Hall Effect transducer identifies its spatial location (hence the 3D reference). That information is relayed to the engine controller, and an algorithm and other computer programming adjust engine speed so it's in sync with the transmission. In other words, its taps the throttle for you.
But don't think it turns the Stingray into a numb, driverless automaton. It's really just a helping hand for entry- to intermediate-level drivers. Feel free to insert names of our editorial staff here. More-seasoned track stars can turn off Active Rev Matching and heel-and-toe shift to their hearts' content. We'll report back on how well we managed this tap dance in our next installment.
From a historical perspective, the Stingray's six-speed, paddle-shift automatic has been in development for at least three years. It's about as different from the '53 Corvette's Powerglide two-speed slushbox as the new 450hp LT1 is from the 150hp Blue Flame Six. In other words, they're not even in the same ballpark.
Comparing the Stingray's automatic with its more immediate predecessor from the C6 is more relevant. For that, we sought some input from two of GM's automatic transmission engineers, Rich Mardeusz and Gabe Gibson. Together they're sort of a tag team, with Mardeusz providing details on the hardware side, and Gibson covering the software algorithms, in particular as they relate to both the AFM (Active Fuel Management) and the various Driver Modes that can be selected with the turn of a knob. We'll start with the hard parts, and then move onto the intricacies of the software that controls them.
By way of background, Mardeusz has worked at General Motors since 1974, starting out in the Cadillac division on the chassis, electrical, and HVAC engineering groups. He then spent several years in the engine- engineering group, overseeing the controls hardware group for various powerplants. From there he moved to the transmission-engineering group, where he has spent the last 10 years. In addition to his current responsibility for the C6 and C7 Corvette automatics, he also oversees the six-speed, rear-wheel-drive automatic transmissions in the Cadillac ATS/CTS, Chevrolet Camaro, and Holden models.
Comparing the transmission layouts of the C6 and C7, Mardeusz notes that the Stingray's automatic has been moved forward in the chassis a few inches. That's been done for a couple of reasons: first, to make room in the rear to accommodate the new suspension structure (see our previous chassis/suspension installment for details); and second, to provide space for the transmission cooler, now located in the rear corner of the driver's side. (This placement has the added benefit of eliminating various piping and fluids.)
While the more-forward mounting provides better balance within the chassis, the new automatic is also 8.15 pounds lighter than the C6's unit. "We consider this a significant difference in weight," Mardeusz says with a note of pride. Trimming off excess poundage is a recurring theme on the new Stingray, as alluded to previously.
But how were GM engineers able to reduce the case size of the automatic, in order to allow this relocation? They went to a smaller torque converter—258mm, as opposed to the previous 300mm unit—which offers a 14 percent reduction in diameter and a seven percent shorter axial length.
According to Mardeusz, other numerical benefits of the smaller converter include a 31 percent reduction in both primary and secondary inertia. We won't delve into Newton's laws on this arcane aspect of physics (due to limits on both space and your author's intellect), but suffice it to say that this change alone resulted in a 0.1-second quicker 0-60 mph time.
Knowing Corvette owners' inordinate interest in enhancing performance, we raised a question about how much power the new automatic can handle. With an engineer's typical emphasis on precision, Mardeusz declined to provide a specific number, citing the need to do a damage analysis of the gearset, along with clutch capacity. Adding to the complexity, he says it would also require knowing the weight of the specific vehicle, along with the wheels and tires. But given GM's powerful computer modeling, used extensively during the three-year development of the transmission, that could be determined in just a couple weeks, if ever needed. Some of that work has actually already been done, as the data on torque and horsepower outputs of the new LT1 indicated that a major redesign of the C6 transmission was not needed, since the gearsets and clutch packs were already up to the task.
Even so, other upgrades were made, such as modifying the isolator clutch to improve the functionality of the Active Fuel Management feature. This unit now moves 2.5 times faster, Mardeusz says, providing a smoother transition to the engine's four-cylinder mode. In addition, a 258mm turbine in the torque converter provides additional driveline isolation to reduce vibration, needed in part due to the change in the length of the torque tube. Otherwise, most of the hardware is fairly similar, except for the size of the bellhousing, which is now slightly smaller since the innards take up less space.
Moving to the software programming, Gabe Gibson started at GM in 2006, working on RWD transmission calibration. He's been on Corvette calibration specifically for the last five years, serving as Lead Clutch Control Calibration Engineer. In these days of electronically controlled transmissions, this responsibility is critical for optimizing performance, even if the technical vocabulary employed might be a challenge to comprehend.
Gibson says that one of the main upgrades involves heavy-throttle upshifts. Keeping those upshifts smooth requires suspending the engine's torque via a fuel cut for a very brief and precise moment—about 100 milliseconds. He says this imperceptible pause in fuel delivery decreases shift time by as much as 50 percent.
Even more promising is how the new auto transmission's software takes full advantage of the lower gears, allowing the driver to run right up to the redline before tapping an upshift. On the previous automatic, the rev limiter would kick in sooner, spoiling all the fun.
Crisper acceleration is enabled by the use of a dual clutch face, in contrast to the single clutch face on the previous 300mm unit. In addition, more cooling is available via the rear ducting on the driver's side; this helps keep the automatic from cooking while the driver burns rubber. Of course, not everybody drives like John Force on a dragstrip. That's where the Drive Mode Selector comes in. In the standard Touring setting, the automatic provides quick, efficient shifts. But turn the dial to Sport, and it'll hold longer in the lower gears. This software setting is identified by the acronym PAL (Performance Algorithm Lifting), for the "lift foot" style of shifting.
Switching to Track mode engages PAS (Performance Algorithm Shifting). This programming factors in several variables—such as throttle rate and position, as well as brake pressure—to stay in the lowest possible gear for optimum performance on a road course. Putting it simply, PAL is for straight-line acceleration, and PAS is for the twisties.
Another shift-control enhancement involves the "Torque-Phase Fill," in which the automatic "is asking for a positive torque request from the engine," Gibson says in engineer-speak. To break that down into simpler terms, we asked if this is roughly akin to the throttle blip provided by the manual's Active Rev Matching feature. "It's the same but different," he offered. Hmmm. Put another way, in a conventional automatic there's a slight loss in torque output between shifts, but on the new C7, the automatic carries torque continuously so there's no drop-off or disturbance to the vehicle.
How does this Torque Phase Fill function work with the fuel cut mentioned above? After all, at first glance they might seem like mutually exclusive features, since one eliminates torque while the other adds it. Gibson explains that these two different torque requests occur during different phases of the upshift. The Torque Phase Fill happens both before and after the ratio change, while the fuel cut kicks in for just an instant during it, since it's the fastest way to get to the next gear. He adds that it happens so quickly, right in between the Torque Phase Fill events, that the driver experiences ratio changes with minimal acceleration disturbances during shift events.
Looking briefly at other driver modes, in Eco, the software mapping optimizes the AFM torque operation for better fuel efficiency, while in Weather mode, it provides softer shifts to minimize tire spin on wet or snowy pavement. Also enhancing performance in slippery conditions are StabiliTrak, Traction Control, and Performance Traction Management. All three (abbreviated as Stabil, TC, PTM) are technically under the same system because they're all managed by the same unit: the Electronic Brake Controller, which also controls ABS.
The eLSD (electronic Limited Slip Differential) described in our previous article is a completely separate system under a different controller—the Integrated Chassis Control Module, which also controls Exhaust Value (exhaust note/sound), among other things. Although they are different systems, they all communicate and work together all the time. For instance, when you enter PTM, it configures the rest of the vehicle (eLSD and TC included) according to your situation, including track type and conditions, as well as the amount of traction available.
In sum, while the Stingray's new transmissions play a pivotal role in putting the power to the ground, there are several other sophisticated systems that help tailor the car's performance to particular applications. We can hardly wait to try out all of them.