The 300-horsepower engine in the Jaguar XK, outfitted with variable valve timing and other improvements for both power and fuel efficiency, wouldn't be competitive with other sports cars in this price range if the car had a steel body and 700 pounds more weight. But with only the 3600-pound body and chassis to carry around, the engine is pretty darn good. It sounds especially mean and nasty at full throttle because of some extra valves and plumbing in the exhaust system, especially with the convertible top stowed. In sixth gear cruising, though, it's a near-inaudible pussycat.

The six-speed automatic with manual control, a Sport mode, and shifter paddles on the steering wheel, is about three levels of sportiness better than the outgoing transmission and the old J-gate shifter. The new twin-clutch transmission allows extremely quick, positive shifts between gears, with little or no lurching on upshifts and a nice, growly throttle blip on downshifts. This is an improvement on the similar ZF transmission Ford uses in the Aston Martin DB9, now with better hardware and software. It can sense aggressive driving and adapt accordingly, limiting upshifts in long corners in Sport mode, and giving instant multiple downshifts when conditions are right.

The engine/transmission combination is good for about 5.9-second 0-60 times and 14.4-second quarter-mile sprints, according to Jaguar, with an electronic speed limit of 155 mph. The EPA rates the XK at 16/25 mpg city/highway, thus avoiding any gas-guzzler tax. But Jaguar does recommend premium fuel.

Hugely stiffer than the outgoing car on the basis of its riveted and bonded aluminum construction, almost 50 percent stiffer in bending and much stiffer in torsion, the current XK offers the kind of silky smooth ride and demonic cornering that great sports cars have always had. It's helped by the completely retuned CATS adaptive suspension system with faster-acting, smarter shock absorbers that work with the engine and transmission to immediately react to the situation and driver's intention. The tuning is roughly 10 percent stiffer in the front and 4 percent stiffer in the rear than the old car. There are several assist modes to the CATS system, including completely off, for track days.

Another byproduct of the stiff chassis is the steering accuracy. It's without question the tightest, heftiest, and quickest Jaguar power steering in history, but nowhere near the point of skittishness. Solid, stable, and planted. The traction control system now features a Trac mode that keeps everything on but allows a higher threshold of yaw, letting the car get sideways in corners for those owners who go to Jaguar Club track days.

The new XK models are fitted with a choice of 18-, 19-, or 20-inch wheels and tires. Base tires are Continental P245/45ZR18s front and P255/45ZR18s rear. Dunlop SP Sport 01 asymmetric high-performance tires, 245/40ZR-19 front and 275/35ZR19 rear, are standard on XKR and fitted on XKs with optional 19-inch wheels. Optional on all models (and standard on Portfolios) are Dunlop SP Sport Maxx ultra-performance tires, 255/35ZR20s for the front and 285/30ZR20s for the rear.

The current XK's ABS brakes are much larger than those on the previous model, and much more powerful, considering the reduced weight of the car, with braking starting at the very top of pedal travel, where we like it. No fade whatsoever after a long downhill switchback workout.

Although the weights and balances between the coupe and the convertible versions, which we drove back-to-back, are close, there's really no discernable difference in the way they drive, except that the convertible with the top down generates more sound in the cockpit and more admiring glances from the other drivers than the coupe.

With its Eaton supercharger, the XKR develops 420 horsepower at 6250 rpm, and 413 pound-feet of torque at 4000. It also rides more firmly than the standard model, with spring rates increased 38 percent in front and 24 percent in the rear; and its steering is tuned for higher effort and quicker response. Additionally, the front brake disc diameter is increased from 12.8 to 14.0 inches, and the thickness from 1.2 to 1.3 inches, not only to improve braking performance but also the system's resistance to fade. We haven't driven an XKR ourselves, but Jaguar says its supercharged super-cat can rocket from 0 to 60 mph in just 4.9 seconds. Top speed is still electronically limited to 155 mph. EPA estimated fuel economy dips to 15/23 mpg city/highway, but the XKR still avoids a guzzler tax.

The limited-edition XKR Portfolio takes braking and handling to the next level with standard 20-inch wheels and tires, and an Alcon braking system that includes massive 15.7-inch discs up front with six-piston calipers; and in the rear four-piston calipers squeezing 13.8-inch discs. Crescent-shaped grooves cut into the surface of the brake discs prevent a build-up of deposits on the brake pads and improve braking performance under extreme use.

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