Cadillac engineers a 'bi-modal' 200-mph CTS-V

  • 29-Dec-2014 06:16 EST
2016 Cad CTSV.jpg

Count the heat exchangers and heat extractors! Front end of the new Alpha-based 2016 CTS-V is a case study in multiple engineering areas cooperating to execute a high-performance product. The grill mesh profile is optimized for efficient airflow. Radar was left out to improve the front-end cooling package.

To hear Tony Roma explain it, there’s an art in developing a big-power performance sedan that’s capable of shredding lap records at the local road course, then picking up the kids from school and grabbing groceries on the way home.

“That’s what we call a ‘bi-modal’ car”—a track-day hero that’s also a great daily driver, explained Roma, the Chief Engineer of Cadillac’s all-new 2016 CTS-V. Shifted from the previous Sigma platform to a stretched-and-widened—by about 40 mm (1.6 in)—version of General Motors’ lightweight Alpha rear-wheel-drive (RWD) architecture that debuted on the ATS, the third-generation CTS earns its “V” nameplate by packing a supercharged 6.2-L LT4 V8 under the bulge of its carbon-fiber hood. The CTS-V will be publicly shown for the first time next month at the 2015 North American International Auto Show in Detroit.

Essentially the same unit that powers the latest Z06 Corvette, the direct-injected LT4 boasts cylinder deactivation for frugal steady-state running in four-cylinder mode in addition to SAE-certified power and torque ratings of 640 hp and 630 lb·ft (477 kW and 854 N·m, respectively)—significant increases from the previous V-engine’s ratings and requiring a steel (rather than aluminum) driveshaft.

Thanks to an enlightened mix of structural and trim materials as proven on the pioneering ATS, each horsepower in the CTS-V carries 6.5 lb (2.9 kg)—a power-to-weight ratio that handily beats BMW’s M5 (7.8 lb/hp) and helps the 4150-lb (1882-kg) car accelerate from a standing start to 60 mph in a claimed 3.7 s. According to Roma, the car also achieves a 200-mph (322-km/h) top speed, as validated by GM engineers at the Transportation Research Center (TRC) in Ohio.

The LT4 is paired with GM’s recently introduced 8L90E planetary eight-speed automatic that is internally similar to the Z06’s transaxle but naturally uses a case and tailshaft designed for front-engine applications. For more on the 8L90 eight-speed, visit http://articles.sae.org/12782/. For recently released SAE J1349 Certified Power Engine Data for GM’s LT4 V8, see http://standards.sae.org/cpgm1_16ltrcts/.

Optimizing cooling, aero, vehicle dynamics

Viewing the CTS-V from head-on, the hard work done by Roma’s development team to provide adequate engine cooling while optimizing vehicle aerodynamics and handling is in-your-face evident. The engine coolant radiator sits behind the grill racecar style, on an angle rather than upright, and large oil coolers contribute to the aggressive front-end styling.

The front fenders (steel, like the rest of the body except for the hood) are 20 mm (0.8 in) wider at their outer edges compared with the standard CTS and are vented to extract hot air from the engine compartment and brakes.

The carbon-fiber hood itself features a large vent integrated into the upper surface that helps mitigate lift at speed by channeling air pulled through the radiator out and over the roof rather than allowing trapped air to exit under the car, Roma explained.

“Our colleagues in Styling didn’t want fender vents this large; that was a source of months of debate,” Roma revealed. “We used two different CFD (computational fluid dynamics) platforms, one of them Exa Corp.'s PowerFLOW, to give our designers the utmost fidelity across the car, before we built our first scale model for the tunnel.”

With the heat-exchanger array dominating the front end, total grill area is about 50% greater than on the non-V CTS, Roma noted.

“We were unable to adopt active grill shutters, and we also had to do without ACC (adaptive cruise control) because a front-mounted radar would hinder airflow,” he told Automotive Engineering. “This was a major lesson in balancing engine cooling, feeding the blower [a 1.7-L Eaton TVS unit], and engineering in the downforce we wanted while minimizing front-end lift.”

GM’s stylists, aerodynamicists, and engineers worked closely to develop the CTS-V’s chin splitter and decklid spoiler, both of which are extended by about 25 mm (1 in) in an available carbon-fiber package that delivers downforce at high speeds. The aero team helped “tune” the geometry of the mesh grill’s individual lattices to optimize airflow. The package includes extended bodyside sills and ‘Gurney-flap’ air deflectors ahead of the front wheel arches; the flaps originally were devised by racing legend Dan Gurney to improve boundary-layer airflow across Formula and Indy car rear spoilers.

“Our CFD analysis got us very close to the final body shape before the physical test phase,” Roma said. He added that while the blown small-block OHV V8 offers many packaging advantages compared with the much taller and larger four-cam V8s used by the Cadillac’s German competitors, its need for a bulged hood posed a challenge when it came to designing the front end of the car to meet European pedestrian-protection requirements.

“It actually hurt us for ‘ped-pro,’ and we even looked at using a more compact centrifugal supercharger, rather than the TVS type, just to save a few millimeters of space,” but finally opted for the proven TVS unit, he said. And as an interesting attention to detail, all of the carbon-fiber panels are “book matched” to present a neatly aligned weave panel to panel.

Shear panel stiffens chassis

As interesting as the CTS-V’s exterior styling is, the car’s real attractiveness to engineers is underneath. Designing in the nearly 1-g lateral grip that befits a BMW-beater specification required special meats: 265/35R19 front and big 295/30R19 rear Michelin Pilot Super Sport tires mounted on new 9.5 in wide front and 10 in wide rear forged aluminum-alloy wheels that are 45% stiffer than previous CTS-V wheels.

The tires feature Michelin's new Twaron Belt technology that uses tri-compound technology—the rubber is hardest on the outer rib for long wear, and durometer is increasingly optimized in the center and inner ribs for turn-in, wet-surface grip, and durability/long life.

For the 114.6-in (2910-mm) wheelbase sedan, “tire size and performance drove development of our stiffer structure—up about 25% over the previous car,” Roma said. Actions related to improving overall body rigidity included a large aluminum shear panel underneath the front of the chassis. The relatively massive panel (which was mirror-polished on the car displayed during the media preview) connects the front cradle and K-member and is unique to the V-series CTS. The shear panel is an example of GM’s increasing use of its proprietary aluminum spot welding technology. On non-V models, a steel crossbar is used for stiffening in the forward-chassis area.

Other V-series upgrades include a stronger rocker bulkhead; upgraded rear cradle-to-rocker braces; upper tiebar-to-bumper braces, and braces added across the front strut towers and connecting the towers to the plenum.

CTS-V suspension (front MacPherson struts, five-link independent rear) benefits by 11% higher front and rear spring rates and fatter anti-roll bars—increased from 25 to 27 mm (0.98 to 1.06 in) in front, and 23 to 26 mm (0.91 to 1.02 in) rear. BWI Group’s Gen-3 Magneride (dual-coil magnetorheological dampers) and unique-to-CTS suspension bushings are tuned to give “flat, predictable cornering with excellent ride quality,” according to Roma. He added that the rear toe-control links use metal-sphere "cross-axis" bushings that offer virtually no deflection.

Steering is by a ZF Servotronic II variable-ratio EPS. “The ZF systems are expensive, but we feel they’re the best,” Corvette Chief Engineer Tadge Juechter told the author during last year’s C7 introduction. On the CTS-V, the ZF steering gear provides 14% greater system stiffness, which gives a more precise feeling at the wheel, Roma noted. The car’s foundation brakesets are by Brembo and claimed to be the largest steel disc brakes supplied by that company; the rotors measure 15.4 in (391 mm) in front and 14.4 in (366 mm) at the rear. They’re fitted with six-piston front and four-piston rear calipers.

Interior equipment includes optional Recaro seats and a Cosworth-developed Performance Data Recorder like that offered in the Z06 Corvette; see http://articles.sae.org/12735/.

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