High-performance Otto-to-Atkinson V8 edition of Lexus GS introduced at NAIAS

  • 20-Jan-2015 01:24 EST
aei-Lexus GS-F.jpg

The GS-F gets a more aggressive front appearance to promote its sports sedan image. Note orange caliper projecting through front wheel. (Click arrow at top right to view additional images.)

Lexus continued on its path to a high-tech performance car image from its long-held soft-luxury reputation, with the introduction at the North American International Auto Show (NAIAS) of the newest sports sedan in its F line, the 2016 GS-F.

The F designation is Lexus' mark of race-tinged performance, and the new edition in the GS series comes on the heels of the RC-F sport coupe, with which it shares an advanced V8 engine and torque-vectoring rear differential. Toyota is serious enough about the performance image to tell the world of motorsports that it will be going full-throttle into the FIA's GT3 racing, officially named "Group 3: Cup Grand Touring Cars."

The GS-F is, like other models aspiring to compete in the German-dominated sports sedan market, a product of suspension development at the Nurburgring, but also on Japan's Fuji Speedway. It rides on wide 19-in forged aluminum wheels with 15-in (381-mm) front rotors and attention-getting Brembo six-piston bright orange calipers. The custom-tuned suspension includes underhood hand-turn adjusters for the shock absorbers.

And because it's a Lexus, the five-passenger sedan production model also will have all the safety features of the more sedate models, including Pre-Collision with radar-guided automatic braking and pedestrian detection. But it's the engine and drivetrain technology (which actually debuted in the RC-F immediately before NAIAS) that draws the engineering attention.

Otto to Atkinson cycling

The 5.0-L V8 is rated at 467 hp (348 kW), a hearty 51 hp (38 kW) more than the previous generation. Peak torque is 389 lb·ft (527 N·m), and the engine carries a 7300-rpm redline. It's bolted to a quick-shifting version of the eight-speed Lexus automatic. Much of the performance increase is from cylinder head porting and valvetrain modifications.

Although this performance boost might be expected to impact fuel economy, Lexus is saying it won't. This is thanks to a major change in engine calibration plus use of a new intake valvetrain electric cam phaser that permit the engine to switch from Otto cycle to Atkinson cycle operation when engine load is low. The engine itself is the latest version of the V8 that was developed for the original Lexus performance car, the IS-F.

The LS460 has had an electric intake phaser for some time. But the new one is a further development for the seamless transitions between Otto cycle and Atkinson cycle (late intake valve closing to simulate a longer power stroke, for improved efficiency). That will enable the GS-F to post EPA window sticker numbers comparable to the standard GS V8, Lexus said.

The direct-injection fuel system is a comparatively high-pressure design, with peak pressure at 5000 psi (345 bar).

Torque-vectoring sans ABS

The torque-vectoring rear differential (TVD) is a step beyond the type that can add power to a wheel to help power a car coming out of a turn. In other systems, the torque-vectoring control also uses the anti-lock brakes/traction control actuators (ABS/TC) to brake the inside wheel (reducing torque) to guide the car when it is entering a turn.

The Lexus differential has a multi-disc clutch pack at each wheel, controlled by a separate actuator with ball-ramp device to vary the application pressure. Each pack regulates torque flow through a planetary gearset with two sets of planets, each planet with a different ratio to bias the system.  A ratio change occurs when a clutch pack is actuated.

On command from the skid control module (VDIM—Vehicle Dynamics Integration Management module), the system can add or reduce torque to each wheel, so there is no power loss from use of the ABS/TC operation at any time. Entering a turn, torque is reduced evenly to both rear (drive) wheels for deceleration stability. As the car approaches the apex of the turn, torque is further reduced to the inside wheel to help the car rotate slightly. Closer to and at the apex, the system transitions to positive torque, with somewhat more going to the outside wheel to promote stability. As the car starts to accelerate out of the turn, the torque to the outside wheel is increased further.

When the system operates to guide the car through a turn, it effectively is performing rear-wheel steer.

The TVD can be pre-selected for any of three modes: 1) the default standard, intended to provide a balance for everyday (albeit exuberant) driving; 2) slalom, which increases the responses for more nimble steering and vehicle agility; 3) track, a tuning for a high-speed circuit.

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