It's the noise. I got into the new Bugatti Veyron Grand Sport feeling pretty cynical, wondering how Bugatti could possibly justify charging $280,000 on top of the Veyron coupe's eye-watering $1.7 million -- plus local taxes and delivery -- for a convertible with less than half the stiffness of the closed-cockpit car, weighing 224 pounds more and a couple tenths slower to 60 mph. Afterthought open tops like this are always worse, right? Floppy, pointless and purely for poseurs?

And then I pressed the starter button. One of the many astonishing things about the Bugatti Veyron is the way it takes such an extreme power output -- way in excess of a modern Formula 1 car -- and puts it into a docile, driveable, reliable package. But you don't realize just how well-insulated the coupe is until you start the Grand Sport with the roof off. The driveline is unchanged, but now at standstill and at low speeds you hear from behind you the constant mechanical hum of that open-air engine's 16 cylinders and 64 valves spinning, by some miracle, in perfect synchronicity. Even at low speeds -- town speeds, under 30 mph -- the hum is punctuated by a loud, fierce hissing from the wastegates of the four turbochargers every time you ease the pressure on the throttle. You might think you're a purist, and prefer the uncorrupted induction and exhaust note of a naturally aspirated engine, but I defy you not to love this noise and not to sacrifice the smooth progress the Grand Sport is perfectly capable of making in favor of a little gentle, deliberate kangarooing to get the engine sounding like a riled cobra.

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And when the road clears and you can push the throttle as far in as your nerve lets you, the sounds of the ancillaries fades out and you hear that 8.0-liter engine itself -- deep, deafening, imperious, and unique, like 16 rubber mallets beating on a barrel.

In truth, Bugatti doesn't need to justify the extra cost of the Grand Sport because its price, power, and performance mean it just doesn't compete with other cars. Only 150 will be made, in addition to 300 Veyron coupes, and Bugatti estimates there are only between 3000 and 6000 millionaires and billionaires in the world with sufficient means and enthusiasm to buy one.

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Bugatti's people like to say the last thing their customers need is another car. On average, they already own 30 apiece and are more likely to be deciding between the Grand Sport and a race horse, helicopter, or yacht than another supercar. So when a car is so dominated by its engine, and when one of the few criticisms you can make of the standard Veyron is that it sometimes seems too refined and aloof, and when cost is almost irrelevant, paying an extra $280,000 just to be more directly connected to that mighty motor starts to look like a bargain.

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So what do you get for your money? When you first see a Veyron, it seems too small to produce and contain its scarcely believable figures. It boasts 987 horsepower, 922 pound-feet, 253 mph, and 60 mph in less than 2.7 seconds, but it's more than a foot shorter than a midsize sedan, a little over waist height, and an unthreatening egg shape. Externally, the Grand Sport gets some cosmetic changes to the lamps and wheels, but apart from that missing roof section it is visually almost identical to the coupe.

Underneath, the front half of the closed car's two-piece carbon-fiber chassis has been completely redesigned to channel crash loads through a reinforced transmission tunnel and the doors, which are now also carbon-fiber rather than aluminum and carry a huge carbon crash member. The A-pillars are now all-carbon to withstand a roll, and there are carbon roll-structures hidden in the air intakes that sprout behind the occupants' heads.

Because almost nothing has changed mechanically, this stiffening and strengthening accounts for almost all the 224-pound increase in weight, taking the Grand Sport to nearly two tons. Bugatti claims the highest torsional stiffness of any open car at 22,000 Nm/degree, (16,300 pound-feet/degree), but that's only two-fifths the figure of the closed car, and the Grand Sport has more mass, speed, and forces to cope with than any other car. With the roof off, the steeper angle adopted by the pop-up rear wing means top speed is drag-limited to around 220 mph; with it fitted it has the same insane v-max as the coupe.

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Inside, the cabin seems too simple: You expect more gadgets for your seven-figure fee, but instead there are just five main dials ahead of you and a simple central console with controls for the audio, the air-con, and the other functions you'd find in any other car. It's all made of flawless, silken leather (the Alcantara used in places in the coupe would stain if it got wet), thick nuggets of aluminum and even magnesium. There are a couple features few other cars have, like the launch-control switch by the simple automatic gearshift, or the slot for the second key between the seat and the door that drops the ride height and alters the aero to let you do the full 253 mph. The Grand Sport gets a new reversing camera hidden in the rearview mirror and a new Puccini audio system too. But there are a few surprisingly cheap touches too, like the thin plastic sliders that control some of the air vents.

The removable polycarbonate roof feels worryingly bendy for something designed to stay in place at nearly a third the speed of sound, but it's simple to fit, easy to handle at 35 pounds, and comes with a Bugatti-branded display stand intended to make the most the elegant, tapering transparent central section. If you've left it behind and get caught in a shower, there's a large, rectangular umbrella in the front luggage compartment; you open it out, slot it into place, and unscrew the handle, which can be slid into a recess by the driver's right knee. Bugatti says it's good up to 80 mph; unofficially you can do 150 mph before you risk it blowing out.

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The Bugatti is surprisingly unintimidating to drive slowly. The redesigned A-pillars gain a quarterlight, improving visibility, and it will just trickle along if you want it to, the seven-speed twin-clutch auto keeping the engine at around 1000 rpm, at which point it still has 540 pound-feet of torque instantly available. You'd expect a car with Can-Am power to bolt ahead with the slightest brush of throttle, but the Grand Sport is a pussycat and the only danger is being taken out by a rubbernecking fellow road-user. The ride is firm and connected but amazingly compliant for a chassis that also has to handle 253 mph, and it is only through the biggest potholes that you get a hint of shudder through its mighty carbon structure.

But this plainly isn't a car designed for town driving, even if it feels like it is. On an open road, it offers you a choice unavailable in any other car. You can, if you're feeling restrained, drive it only as fast as a Pagani Zonda or a Lamborghini Murcielago SV or some other multisyllabic supercar. For this, you might want to use the manual setting and the paddleshifts behind the wheel to keep the 'box a gear higher than it might otherwise kick down to; with all that monstrous torque available between 2200 and 5500 rpm you don't need to stray much beyond 3000 rpm to decimate everything else on the road.

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Driven like this, and with the time and spare mental capacity to think about such things, the Grand Sport feels utterly composed. Its slightly revised damper settings and more progressive tire compound mean the steering is at least as good as the closed Veyron's and is largely uncorrupted by the loss of stiffness. The 400 mm eight-piston carbon brakes are, unfathomably, more than up to the job of hauling 4400 pounds back from three figures at every corner. The car turns in with a single input; if the bend tightens there's an excess of front-end grip to call on, and when it finally runs out there's safe, progressive understeer. It rolls perceptibly but naturally, and its massive traction cannons you out the other side at whatever rate you choose.

Ah, yes -- that choice. When you're done dawdling, you can do what you paid nearly $2 million for and accelerate at a rate that no other car comes close to. For this, you can stick with manual mode or choose the hyper-manic sports-auto mode, which never uses any less than 3500 rpm and is, frankly, utterly pointless and a pain in the ass unless you're either flat on the gas or hard on the brakes.

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Driven like this, the Bugatti is shockingly, almost unmanageably fast. It doesn't matter how often you drive it; its performance never, ever begins to feel remotely normal. If you think you can detect the 0.2-second deficit in the Grand Sport's initial acceleration, you're a neurological marvel. Instead the open car feels faster because it sounds so much better. Under full load, that deep, loud drumming resonates in your chest cavity but stops after a beat or two because you're already at the next corner. When you lift off, the hissing of the wastegates in gentler driving is replaced by a cymbal-crash as they dump full pressure. BOOM-TISH! BOOM-TISH! It's like having John Bonham playing the drums a foot behind your head at full stadium amplification. And if you can find a stretch of road long and straight enough to let the drumming continue, you'll find the absurd acceleration doesn't begin to relent until you're well north of 200 mph. We know. We tried it.

But any old Veyron will do that. With the Grand Sport, as we said, it's the noise. All 30 that have been ordered so far have gone to existing Veyron customers. Some were probably just waiting for an excuse to buy another, but we suspect they'd all acknowledge that it's a significantly more involving car. Worth the extra? Yes. Cynicism gone? Pretty much.

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Veyron Trivial Pursuit: Interesting Bugatti Facts and Figures

-The engine is hand-assembled by two technicians in Germany and makes a minimum of 987 horsepower, even at high altitude or in high temperatures. In perfect conditions its output is closer to 1050 horses.

-With 12 radiators required to cool that monstrous engine and the other systems, the Veyron carries 26 gallons in fluids, excluding fuel.

-The Veyron might do 253 mph, but it can't cover 253 miles in an hour because at that speed its 26-gallon tank will run dry after about 50 miles.

- With a tread depth of just 4 mm, the tires need to be replaced every 10,000 miles and the wheels every 30,000 miles. Total cost? Around $73,000.- The seven-speed twin-clutch gearbox is made by Ricardo in the UK, shifts in less than 150m/s, and costs $172,000.

- The front grilles are made of titanium to withstand bird strikes at 253 mph. "At that speed, the effect could be disastrous," says an engineer. "We need them to arrive as French fries. "

- The ultra-lightweight titanium bolts used to secure major components cost $85 each, and are thrown away if they are tightened or have to be removed for any reason.

- The paintwork is done by the same German company that worked on the BMW Z8 and Porsche Carrera GT, and takes two days to examine in a light tunnel.

- Only one piece of glass in 30 is deemed good enough to be fitted to the car; the rest are stored for spares and future restorations.

- For its 60,000-mile service, the Veyron must be split in half. The work can be carried out in just five locations, or a mobile workshop can be flown to your car. No Veyron has yet done more than about 30,000 miles.

- Three Veyrons have been terminally damaged in accidents: two in the UK, and one in Japan. Another was destroyed in the U.S. when the truck delivering it was involved in an accident. At least one has been repossessed by a finance company in the recession.