V Is For Volatile

General Motors Corp.'s Cadillac luxury division moves a step closer to matching the high-performance luxury image of its German rivals with the introduction of the new '06 V-Series STS sedan and XLR roadster.

Aimed squarely at niche vehicles produced by the in-house tuning efforts of Audi AG (Quattro GmbH), BMW AG (M GmbH) and Mercedes-Benz (AMG GmbH), the new V-Series cars return an element of performance, style and exclusivity to the vaunted Cadillac name at prices unseen by the brand until now.

Cadillac jumped on the performance bandwagon in 2004, when it unveiled the CTS-V. It has been successful, selling about 4,000 units annually.

With the STS-V and XLR-V, Cadillac plans to shed some of its track-ready luster in favor of a more refined performance package. The two models share many similarities. Under each vehicle's bulging, sheet-molded-composite hood is an intercooled and supercharged 4.4L version of GM's DOHC Northstar V-8.

In production since 1991, the all-aluminum Northstar is rejuvenated in supercharged trim for V-Series duty through a smaller cylinder bore, strengthened internal components and revised camshafts with variable valve timing.

Nearly half the engine's components are new compared with the standard 4.6L.

The engine, which is assembled by hand at GM's Performance Build Center in Wixom, MI, also uses the auto maker's proprietary, roots-type supercharger. Rated at 12 psi (0.83 bar) of boost, the unit combines the intake manifold, supercharger and intercooler assembly into a single unit mounted between the cylinder heads.

Due to intake and exhaust differences, output is Society of Automotive Engineers-certified at 469 hp and 439 lb.-ft. (595 Nm) of torque in the STS-V and 443 hp and 414 lb.-ft. (561 Nm) in the XLR-V.

Cadillac says both cars are electronically limited to 155 mph (249 km/h) and can accelerate from 0-60 mph (97 km/h) in less than 5 seconds, a claim that seemed reasonable after extensive flogging on Southern California's deserted highways.

Handling the power in both vehicles is GM's new 6L80 6-speed automatic transmission and limited slip differential. The Driver Shift Control system can be activated for quick, manual gear changes.

However, the unit's Performance Algorithm Shifting and Liftfoot features do a commendable job of finding and holding the proper gear when the selector is left in Drive and sport mode is engaged.

Overall, the feel from the new V-Series powertrain is one of refined urgency, not brutal excessiveness. The Northstar SC lacks the low-end grunt of GM's overhead-valve V-8s and Mercedes-Benz's larger AMG mills but still provides more than enough usable power in the upper range when the supercharger begins to spool up.

The increased power necessitated a revision of both models' suspension, which Cadillac fine-tuned on Germany's famous 13-mile (21-km) Nurburgring Nordschleife racetrack.

Specific enhancements on both vehicles include stiffer springs and anti-roll bars; performance revisions to the Delphi Corp.-supplied Magnetic Ride Control system (XLR-V); recalibrated monotube and ZF Sachs-supplied Nivomat self-leveling shocks (STS-V) and larger suspension bushings and quicker steering gear ratios.

The result is a composed, comfortable chassis that provides substantial feedback, minimal body roll and plenty of lateral grip. Both cars feel nimble for their size and planted to the road at high speed.

Additional equipment includes huge Brembo disc brakes and larger 10-spoke aluminum wheels: 18-in. front and 19-in. rear on the STS-V and 19-in. on the XLR-V. Both vehicles are shod with Pirelli high-performance run-flat tires.

Other performance-enhancing features include throaty, less-restrictive exhaust systems; strengthened drivetrains; numerous fluid coolers and a revised aero package for improved brake-system cooling.

The XLR-V's exhaust uses vacuum-operated baffles in the mufflers designed for the Chevrolet Corvette ZO6.

Under light-load situations, the valves route the engine's gases through the normal exhaust system, emitting a smooth and refined burble, with little hint of the car's potential. However, when the throttle is firmly depressed, the exhaust valves open and unleash the engine's full baritone roar.

The STS-V, on the other hand, sounds quiet and refined for a 470-hp muscle sedan. But under hard acceleration, the vehicle's conventional stainless steel exhaust emits a wonderful bellow along with a seductive supercharger whine.

Both cars feature significantly improved interiors, with touches of exotic woods, metal and suede trim scattered about the cabin.

Drexel Meyer, interior supplier for Mercedes' Maybach ultra-luxury unit, handles the integration of the upgrades. The STS-V benefits from dark, Olive Ash Burl wood trim on the center stack, shifter and armrests, while suede leather inserts grace the door panels and seats and prevent sliding around.

A thick, nicely weighted, leather-wrapped steering wheel provides ample road feel when piloting the car. The 2-seat XLR-V features many of the same appointments found in its 4-door sibling, but adds unique Zingana wood trim on the center console.

Cadillac says the XLR-V is its fastest and best-handling model ever, yet the Corvette-based roadster ironically suffers from an oversized steering wheel and seats that lack lateral support.

On the road, both cars lack the aura of refinement and panache of some of the European heavyweights.

Nevertheless, the new V-Series cars, brazen with power and bold American-styled bodywork, are exceptional drives and illustrate GM's efforts to return the Cadillac brand to greatness.

Viewed in the context of its rivals, the XLR-V's base price of $100,000 undercuts the 493-hp Mercedes SL55 AMG ($125,775) by a significant amount and is slightly more expensive than the less-powerful Maserati Spyder ($91,415).

The STS-V is Cadillac's most-powerful model ever and with a base price of $77,090 commands nearly a $30,000-premium over a standard V-8 STS.

Cadillac says the vehicle's major competitors include the faster, yet equally powerful, Mercedes E55 AMG ($82,575), and the F1-inspired, 500-hp BMW M5 ($81,895).

The STS-V is built at GM's Lansing, MI, Grand River plant with the rest of the Sigma-based Cadillacs (SRX and CTS). The XLR-V shares assembly lines with the regular XLR and the Chevrolet Corvette in Bowling Green, KY.