On an otherwise empty back road in Napa Valley, I am momentarily distracted by the appearance of another car in my rearview mirror. It is a small Kia SUV, and it appears to be gaining on me—quite a feat, considering that I am sailing along at a pretty good clip. I am driving a Bentley Continental GTC Speed: a $238,000 special-edition 602 hp high-performance convertible. The soft top is down, the sun is shining, and the tunes are blasting over the 1,100-watt Naim audio system. I have at least twice as many cylinders at my disposal as any compact SUV, but I can’t seem to lose the other driver, and I refuse to be passed by anyone driving a $20,000 Kia. Not on this road, not on this day.

It takes all my skill to keep the 5,478-pound Bentley ahead of the Kia around tight curves, and I manage to maintain my lead on the straight sections of road. The powerful Bentley could not shake its tail; to its considerable credit, the hefty GTC Speed did not shake, period.

"If you take the roof off a car, you need to stiffen up the body," says Brian Gush, the top chassis and power-train engineer for Bentley Motors. Before taking his current position, Gush directed Bentley’s motorsports program, which culminated in 2003 with the EXP Speed 8’s win at the 24 Hours of Le Mans. That victory—reminiscent of Bentley’s early racing victories in the 1920s—set the stage for the Continental trio of coupe, sedan, and convertible Speed models. As a result of Gush’s efforts, the GTC Speed achieves optimum stiffness in a class that includes convertible Ferraris and Aston Martins.

"There’s absolutely no scuttle shake," Gush claims. That’s engineerspeak meaning that bumps in the road don’t transfer to the body of the car (or its passengers). "The springs and shocks absorb bumps, not the twisting of the car," Gush explains. "In most convertibles, you hit a pothole or expansion joint and shake for a while afterwards; but in the GTC, that doesn’t happen. It’s rock-solid."

Part of that solidity is imparted by a lower center of gravity than the regular GTC’s. The Speed model also has a lower ride height, and its suspension and steering have been enhanced for sporting duty. Although almost 50 percent of the body panels are different from the coupe’s, the only items that visually distinguish the Speed from the basic convertible are the dark-tinted matte trim on the grille and lower air intakes, titanic 20-inch wheels, diamond-quilted leather interior, and bigger tailpipes.

Hit the accelerator, however, and those tailpipes still don’t make much noise. There is no boisterous show of power other than unmistakably quick acceleration. "For the Speed versions, we got the extra power through efficiency gains rather than simply turning up the boost pressure," Gush says, "which is the easiest way to increase horsepower."

As a result of the tuning, the Speed’s W-12 engine generates 602 hp. A paddle-shift-operated 6-speed automatic transmission transfers that power to all four wheels, which creates neutral handling that is almost unnerving in a car this big. It also slows in a hurry, with carbon-ceramic brakes that are larger than those on any other passenger car in the world. They also resist fade, which is an important trait when you’re doing your best to stay ahead of some pesky Kia on a twisty mountain road.