Developing a high-performance sedan from a rear-drive model is an "expected" approach, and so a "Vsport" twin-turbocharged V6 derivative certainly fits the 2014 Cadillac CTS, joining the V8 model. However, there also was a marketing need for a Vsport version of the front-drive XTS luxury sedan. And although the two cars use basically the same version of the twin-turbo 3.6-L V6, the XTS required specific engineering from the grille back to the rear axle. The CTS and XTS turbo engines share only the front cover and cam drive with the naturally aspirated version.
The twin-turbo system (http://www.sae.org/mags/aei/11941) was designed to also be packaged for the 90º mounting orientation of the transverse front-drive V6. And the air intake box at the top of the engine, which also holds the intercooler and throttle body, incorporates a bolt pattern that accommodates a 180º turn. That permits routing the coolant plumbing for the coolant-to-air intercooler from the intake air box, so it exits from the flywheel side of the engine for connections to the front cooling module.
With a Mitsubishi Heavy Industries turbo machine nestled alongside each cylinder bank, and the throttle body within the intake air box, the boosted and direct-injected V6 has short but smooth airflow paths into the cylinders. It is approximately a 32-in (820-mm) passage up to and through the intercooler, into the throttle body, and through the intake ports into the cylinders.
That intake box also contains one of the two temperature-humidity sensors on the car. The latest of these combination sensors has improved accuracy and lower cost and permits an enhanced level of systems control. Humidity acts as a fuel dilutant, so with a humidity input the computer can compensate by changing the valve timing, and this additionally drives a change in ignition timing.
The complete group of intake system sensors provides key inputs to control systems for the vacuum-solenoid operated wastegates of the turbos and the engine's variable valve timing. They are the temperatures pre- and post-turbine (to calculate the pressure ratio) and intake air in the air box (pre- and post-intercooler). Also included are the mass airflow sensors (MAF) for each bank, which also incorporate temperature sensing, as well as throttle position (to calculate torque request) and manifold absolute pressure (MAP) sensors.
With the MAP, the system can provide altitude compensation to increase turbo boost beyond the specified 12 psi (83 kPa).
The second temperature-humidity sensor is in the passenger compartment, in the rearview mirror holder, up against the windshield, where it uses the signals to control the automatic HVAC system, including defrost. Other GM cars also have this HVAC control, including the Chevrolet Impala and Cadillac CTS and ATS.
Muscular driveline to handle turbo torque
Completing the drivetrain for the XTS mounting was a specific challenge. There was no transverse equivalent to the high-torque-capacity 8-speed used with the new rear-drive CTS Vsport or even a transmission equivalent to the 6L90 6-speed coupled to the CTS V8.
Instead, GM's highest-capacity front-drive transaxle available, the 6T75, was beefed up internally for increased torque and higher duty cycle. It now carries a 6T80 designation and features a stiffer case to better hold gear alignment. The shift cable mounting was redesigned and the torque converter was upgraded, and there is a new converter housing mounting interface specific for the twin-turbo engine.
Bill Peterson, lead development engineer for the XTS Vsport, said with the internal modifications the 6T80 can handle the direct-injection twin-turbo V6, SAE rated at 410 hp/369 lb·ft (306 kW/500 N·m) throughput. This compares with the twin-turbo used in the CTS Vsport that is SAE rated at 420 hp/430 lb·ft (313 kW/583 N·m).
By comparison, the XTS's base 3.6-L V6 produces 305 hp (337 kW) and 265 lb·ft (355 N·m) and is paired with the 6T70.
The XTS Vsport has an all-wheel-drive system, an Audi-like configuration that is designed to mitigate the torque steer associated with a front-drive. It is used with an all-mechanical Haldex AWD coupling for front-rear torque distribution into an electronically controlled limited-slip rear differential.
There is no defined torque split in this system. It is physically capable of transferring up to 1000 N·m (736 lb·ft) of propshaft torque to the rear, which is double the engine's maximum torque output. On a normal launch (all wheels on dry pavement), the front-rear percentage split is about 50-50. Under dynamic conditions, the transfer is based on inputs from vehicle sensors to maximize driving control.
The XTS Vsport's engine does require more cooling, but it didn't take a major front-end change. The radiator is a carryover, and cooling system capacity is the same (9.4 L). But the grille was reshaped and the fan wattage was raised, so there is about a 10% increase in airflow, Peterson said.
The turbo engine cradle was modestly reworked for packaging, but there were no structural upgrades necessary, as it already had the capability for the additional 60 kg (132 lb) greater weight of the twin-turbo version. The powertrain has the same torque-axis mounting layout as the non-turbo. The east-west axis mounts are the load-bearing ones; the front-rear axis mounts counter torque reaction, and their rate for the turbo was dropped about 10%, Peterson said, "to give us more ride isolation and the control we wanted of tip-in and tip-out."
The car's Magnetic Ride (magnetorheological damping) suspension was recalibrated for increased ride sportiness, he added. Front MacPherson struts on all XTS are the "Hi-Per" design previously introduced on Buick LaCrosse and Regal GS. They are a dual-path design that separates the spring and damper loads going into the body, which not only improves ride quality but also provides some additional moderation of torque steer. The XTS Vsport rides on 20-in Bridgestone all-season tires.
The braking system on all XTS features two-piece four-piston Brembo aluminum calipers in front, two-piston TRW calipers in the rear, with TRW vented rotors on all four corners.
Although the front-drive model has rack-mounted electric power steering that is a new addition for the 2014 model year, the conventional hydraulic system was retained for the AWD turbo, Peterson said. At this early stage it enabled Cadillac engineering, with more experience with the hydraulic system, to get the desired Vsport handling characteristics. The 15.3:1 steering ratio remains the same.