Shifting 500 lb (227 kg) of engine mass rearward by 7.5 feet (2.3 m) and moving 300 lb (136 kg) of transmission components aft by almost three feet transformed the 2020 Chevrolet Corvette Stingray into a budding supercar. General Motors’ motive behind adopting a mid-engine layout for the eighth-generation (C8) Corvette was to improve acceleration, braking and handling via substantially increased rear-tire loading.
While numerous test-drive reviews and ‘Of the Year’ trophies mark that mission accomplished, there are lessons behind the hoopla: how GM engineers solved problems they encountered reinventing America’s sports car. “Porsche was our primary benchmark, especially in reference to their PDK dual-clutch automatic transmission’s overall dynamics,” revealed executive chief engineer Tadge Juechter, in an interview with SAE’s Automotive Engineering. He said GM also purchased a Ferrari 458 for testing and tear-down analysis.
The first running Stingray prototypes exhibited a high-frequency whirring noise emanating from accessory drives for the alternator, AC compressor and water pump located at the front of the engine. This caused concern, as the noise was being generated only a foot from the occupants’ ears. Insulation placed on the firewall wasn’t as effective in muffling the noise as the engineers had hoped, Juechter noted. So, they analyzed Ferrari’s solution: an unusually thick rear window. “For C8, we increased the thickness of that piece of acoustic glass to 8.6 mm (0.34 in), which is nearly three times as thick as the 3.2-mm (0.13-in) tempered glass shrouding the engine bay,” he explained.
Jordan Lee, global chief engineer for GM’s small-block V8, added: “It’s a balancing act – hearing the satisfying sounds, such as the engine’s throaty intake and exhaust rumble, over distractions such as belt whir, injector tick and valvetrain clicks.” He noted that while most competitors hard-mount their fuel injectors between the engine’s cylinder heads and fuel rails, GM has been using an isolated arrangement in the small block since 2014.
“For C8, GM’s NVH engineers invested extra effort in developing effective insulation materials and assuring that the seal around the laminated acoustic glass at the rear of the cockpit is sufficiently robust to hush the belt whir,” Lee said. Further dialing-in of build processes is aimed at eliminating audible fuel-injector noise in the production cars.
A related issue is that the driver’s view rearward is through both the near-vertical glass panel in the rear cabin bulkhead and the long, nearly horizontal hatch over the engine compartment. In addition to providing rear visibility, the bottom edge of the glass hatch and surrounding surfaces must vent heat and moisture from the engine bay. What Juechter calls C8’s “chimney” passes large volumes of hot, wet vapor during rainy driving and when the car is parked following a drive. A fan helps vent that heat from the engine bay. In addition, all the electrical connectors subjected to road splash have weathertight seals.
When the hatch glass is soiled, the driver’s view to the rear is diminished. To address this concern, Corvettes with up-level 2LT and 3LT interior trim are equipped with a two-way center mirror. Mode one is a conventional view through the two glass panels. The second choice is an electronic display provided by a high-definition camera mounted to the trailing edge of the roof.
Assuming the lens is clean, the camera provides a broad rear view, unobstructed by the wide roof pillars and soiled hatch glass. As is not uncommon in other rearview camera-mirror applications, reviewers have reported that it takes a second or so for their eyes to focus when the camera view is in use. All Corvettes have a conventional backup camera as standard equipment.
Stymied by the FEAD
Given the C8’s aggressive cornering capability, powertrain engineers knew that it was essential to improve the 6.2-L LT2 V8’s lubrication system. Jordan explained that with the previous-generation Corvette, “dialing in the lube system was like threading a needle. We lost several C7 engines when the oil pickup in the tank was starved during high-g maneuvers.” Another issue was oil blown out the optional dry sump’s vent system.
To solve these problems, the C8 program opted for two additional scavenge pumps to assure that track performance would be uncompromised. “Thanks to the new engine-mounted dry-sump oil reservoir that’s now standard equipment, we encountered virtually no [oil-starvation] drama during development,” Lee reported. “We were astounded how well it works and throughout C8’s comprehensive test program we experienced only one engine failure.”
One challenge that has thus far stymied engineers is a straightforward procedure for replacing the front-of-engine accessory drive (FEAD) belts. Doing so requires dropping the entire engine-transaxle assembly from the car. Corvette owners can only hope that will be a rare occurrence necessary only every 100,000 miles or so.
While shuffling powertrain component locations, C8 engineers also switched from semi-elliptic composite suspension springs (pioneered on Corvette’ rear axle in 1981) to steel coils. “We would have preferred to keep the composite springs because they are quite efficient from a mass standpoint,” Juechter explained. “Unfortunately, with our low-mounted engine and transaxle, there’s no room for the tall cross-car path that a rear composite spring requires. In a rear view of the chassis, the arc of the spring would occupy the exact same space as spinning transmission gears.”
Once C8’s rear suspension design changed to coil springs, the engineers had to follow suit in front. Juechter said this was because of the significant difference between composite- and coil-spring force-versus-deflection characteristics. “To match the ride and roll rates at both ends of the car—something we deem absolutely essential—we switched to a steel coil spring at each corner,” he explained.
Rear-tire mass loading
The other fundamental C8 design change is a new aluminum spaceframe consisting of six elaborately ribbed die castings—which the engineering team dubbed “the Bedford Six” because they’re manufactured at GM’s Bedford, Indiana, facility – plus 14 conventional castings, extrusions, stampings and hydroformed parts joined together with a variety of fasteners and structural adhesive.
Flow-drill screws used at dozens of locations pierce an attaching hole and form threads to secure frame components. As in C7, suspension control arms and knuckles are stiff aluminum forgings and castings. Thanks to the more robust spaceframe, C8’s torsional stiffness is 7% greater than C7 with its roof panel in place and 12% stiffer in the open configuration, according to GM engineers.
In its C8 review, Car and Driver reported an increase in the Corvette’s center-of-gravity height. Confirming that, Juechter shed light on the situation: “While C8’s engine and transaxle are both mounted as low as possible in the chassis, there are several parts that are higher than before,” he said. “The rear coil springs are not only heavier than the previous composites, they reside above the tops of the tires and are anchored at their upper ends by substantial pockets cast into our new spaceframe.”
In addition, C7’s low-mounted torque tube is gone and C8’s exhaust system sweeps upward as it flows rearward, raising the height of the catalytic converters, Juechter noted. The net result: the C8’s center-of-gravity height is 470 mm (18.5 in), 15-mm (0.59-in) higher than C7’s.
An oft-cited reason for choosing mid-engine over alternative powertrain layouts is to minimize the car’s polar moment of inertia for optimum agility. Asked to compare C8 and its predecessor, vehicle performance manager Alex MacDonald calls the polar-moment-of-inertia figures of both C7 and C8 “pretty comparable.” That said, two substantial components now reside some distance from the center of gravity.
“C8 has two large radiators full of coolant situated in the nose of the car,” MacDonald noted. “And our fairly heavy transaxle lives at the opposite end of the car, well behind the center of mass. So, positioning the engine near the middle doesn’t automatically yield a low polar moment of inertia. In the end, the added traction achieved with 60 percent of the Corvette’s mass loading its rear tires is a greater influence on overall performance.”
Another non-trivial challenge facing the C8 team was tightening EPA and global regulations for brake-dust emissions. Studies have shown that the dust created by vehicle brake-pad abrasion is the source of approximately 20% of total PM2.5 (fine particulate) traffic pollution. “We had to reinvent our brakes,” Juechter explained, “because our previous pads’ 25-percent copper content is no longer permissible. To avoid abrasion of the new pad material, we can no longer use the drilled and slotted rotor venting that flushes water from the friction surfaces during wet driving. While I personally prefer the look of slotted rotors, they’re gone in C8.”
Balancing handling and ride quality
An unsung C8 achievement is a base curb weight increased by only 70 lb (32 kg) over its predecessor despite added features: roomier passenger accommodations, the new Tremec TR-9080 8-speed dual-clutch automated-manual transmission with one more gear than C7’s manual transmission, dry-sump lubrication, increased cooling capacity and larger rear wheels and tires. Even with the more stringent 2020 EPA test procedures, highway fuel efficiency increases by 2 mpg (to 27 mpg).
To hold the line on weight, Juechter’s team supplemented its sharp-pencil engineering with a few carbon-fiber composite components. One is an industry-first curved and hollow rear bumper beam. The supplier, Shape Corp., uses pultrusion technology developed by Germany’s Thomas Technik and Innovation to draw carbon fiber material, wet with urethane-acrylate resin, through a die. The resulting beam, which is 4.9-lb (2.2-kg) lighter than an aluminum extrusion, bolts to the Corvette’s spaceframe extensions.
Another ultralight part is the center tunnel close-out panel, supplied by the Molded Fiber Glass company. It is made using MFG’s PRiME liquid composite molding process. Two carbon fiber and three fiberglass sheets wet with vinyl ester resin are molded under pressure, yielding a stiff 10.4- x 49.5- x 0.16-in (26.5 x 125.7 x 4.0-mm) panel weighing only 4.9 lb. It attaches to the bottom of the aluminum spaceframe with 30 fasteners.
The new Corvette’s passenger compartment floor panels are SMC (sheet molding compound) moldings topped with stamped aluminum panels at the rear to support the car’s bucket seats. Front and rear luggage compartment bins made by MFG use ultralight SMC with a specific gravity below 1.0 – indicating that each part would float in water. A die-cast magnesium cross-car beam bolts between the A-pillars to rigidly support the dash panel. For the first time, the Corvette’s bottom surface is flat and smooth to minimize aerodynamic drag.
One enduring C7 owner gripe is front-tire chatter during cold-weather, full-steering-lock maneuvering. According to Juechter, considerable effort was invested in remedying that shortcoming. “In the past, we pushed our front-engine performance limits with near-racing tire compounds and steering geometry favoring handling,” he said. “Unfortunately, below 40-degrees F, this results in stick-slip tire chatter that owners notice. To improve this in C8, we revised the steering geometry to improve the Ackermann correction and walked back a bit from our previous aggressive tire compounds because they’re less essential with mid-engine to achieve our performance goals.”
The net result, Juechter noted, “is nowhere near as much stick-slip chatter as we had in C7 even with the tighter turn circle provided in C8’s equipped with our MR (magnetorheological) dampers.” Now in their fourth design generation, the highly effective MR dampers supplied by BWI cost $1,895 over the $5,000 Z51 Performance Package.
The Corvette development team collaborated five years with Michelin to develop new run-flat tires for C8. The base rubber is a Pilot Sport ALS (all season) while Pilot Sport 4S (summer only) radials are included with the Z51 option. This is the first application of all-season tires on the Corvette, which should encourage owners to use this supercar more months of the year in northern climes.
Juechter is on record not wanting C8 to be a handful at the cornering limit – a preference dating to his youth under the wing of a fighter-pilot father who enjoyed owning and driving Porsche 911s. “We definitely didn’t want our first mid-engine effort to earn a reputation as a car that’s tricky at the limit,” he asserted. “There are numerous variables that must be addressed to achieve benign, totally controllable handling. By that, I mean near-neutral characteristics on the track with a bit of understeer during normal street driving.”
A key variable is lateral compliance, which begins at the tire sidewall and continues through the wheel’s construction, wheel bearings, suspension knuckles, rubber bushings and control arms all the way to the suspension anchor points on the spaceframe. “We want the tire’s stick-slip characteristics to be very progressive with gradual changes in the coefficient of friction,” Juechter explained. Suspension geometry was tailored to provide a small amount of steering into the turn as the car rolls. The lateral compliance built into the suspension bushings also provides a few minutes of steer angle under high lateral loading, he said.
Engine-mounting strategies were also deliberated, with the team deciding to “softly” mount the LT-2 V8 for vibration isolation. “But you definitely don’t want a two-stage step function when the engine reaches the limit of its roll,” Juechter explained. “So, the compliance and damping built into the engine mounts are also important.” Finally, spring, anti-roll bar and damper calibrations were selected to work with all the other variables to give the driver the perception of a totally integrated driving experience.
In the plan view, the 2020 Corvette Stingray has a fighter-jet look with an aggressively curved nose tip and front corners drawn back tight – seemingly a triumph of styling over engineering. The car also is 2.2 in (5.6 cm) wider and 5.3 in (13.5 cm) longer than its C7 predecessor. The increase in length is attributable to ergonomic and storage improvements. Fore-aft seat travel was increased by an inch (25.4-mm) and the seat-backrest recline angle doubled, to 18 degrees, to better accommodate tall occupants. The C8’s extra length also provides storage space in the rear trunk for the removable roof, a feature considered essential in Corvettes.
The increase in overall width is due to wider rear tires and wheels. “It’s not practical to tuck them in closer to the centerline if you want outstanding ride and handling balance,” Juechter said. “We prefer long suspension travel with lower spring rates than are common in competitors. Shortening the halfshafts isn’t practical because that increases universal-joint angularity, greatly reducing the life of the rubber boots surrounding the U-joints.”
Also contributing to the C8’s added width is the substantial duct integrated into each rear quarter panel. These openings are responsible for ingesting an enormous volume of air for engine induction and engine-bay cooling (cars equipped with the Z51 performance package have a third radiator on the passenger side to assure that they’re fully track capable at ambient temperatures of 100-degrees F).
Juechter acknowledges that the wider fenders and rear tires are responsible for a frontal area that’s slightly greater in C8 (2.075 m2/22.3 ft2) than in C7 (2.023 m2/21.78 ft2). Because aerodynamic downforce is notably greater with the Z51 performance package, a C8 so configured has an 0.322 drag coefficient versus C7’s 0.313 Cd.
The bottom line is a 2020 Corvette Stingray chassis and body engineered to fully exploit the performance strides delivered by the SAE-certified 495-hp LT2 V8 and the Tremec DCT. Considering the new Corvette’s $59,995 base price and the sold-out 2020 production run, Juechter’s team has seemingly wrought the supercar value of the century.Continue reading »