Top this, Transformer

Six weeks hence, our collective fore-lobe will be obsessed with the large and loud summer movie "Transformers," a film based on an irredeemable and just plain odd Saturday morning cartoon that ran briefly 20 years ago and was created specifically to help shill toys to children. (Historical note: In Japan the cartoon was called "Fight: Super Robot Lifeform Transformers.") This film has been directed by Michael Bay. Instead of popcorn, I'm thinking a large bucket of hot buttered antidepressants.

So while we're bracing for that awesomeness to wash over us, consider the 2007 BMW 335i Convertible, a car whose quicksilver transformation from coupe to convertible looks like something cooked up by Industrial Light & Magic. Virtually indistinguishable from the 335i Coupe (with a fixed roof), the Convertible can achieve its CGI-like metamorphosis while sitting at a stoplight (23 seconds): The three roof panels levitate apart from one another, shuffle together and sink in a stack under the open deck lid, a minor symphony of servos and levers performed pianissimo. When the rear-hinged deck lid closes and all seals compress, you're looking at a whole different car. Call it Solaris Prime.

Let's dispense with some shopkeeping first. BMW's new convertible comes with a choice of two engines: a naturally aspirated 3.0-liter inline six good for 230 hp (328i, starting at $43,200) and the exquisite twin-turbo 3.0-liter putting out 300 hp (335i, starting at $49,100). That makes the convertible versions of the car $7,900 and $8,300 more than their fixed-roof siblings, respectively. The convertible's structural reinforcement adds 441 pounds to the curb weight of the automatic-equipped 328i and 375 pounds to the 335i. A six-speed manual transmission is standard; the six-speed automatic is a $1,275 option, and worth every penny in this town.

Aside from weight and cost, the big problem with retractable hardtops is packaging. These solid-panel mechanisms don't fold as compactly as multi-bowed, twill canvas tops. That means designers have to make room for them in the boot, and this leads -- as it does in the VW Eos and the Volvo V70 -- to a strange elongation in the rear quarter of the car. Likewise, in both of these cars, the arching curve of the roof requires the rear seats to be low to allow for adequate headroom. Combined with a rising shoulder line, this arrangement leaves rear-seat passengers feeling buried up to their neck in car.

Not so with the BMW. The profile of coupe and convertible are nearly the same -- the convertible does sacrifice the graceful landing the fixed roof makes at the deck lid -- and the rear-seat passengers can look out comfortably, provided the front-seat passengers are merciful in keeping their seats forward.

The other big issue with these tops is cargo space. The Bimmer convertible provides a spacious 12.3 cubic feet of trunk space with the top up (a major improvement over the previous ragtop model's 7.7 cubic feet). The stowed hardtop shrinks trunk space to 7.4 cubic feet, much of it under the folded roof panels. The optional Comfort Access feature ($500) will move the roof panels out of the way to making loading easier.

Cargo cultists will also appreciate that the rear seat backs fold flat to provide a large parcel shelf behind the front seats; and the rear-seat pass-through opening (part of the $750 Cold Weather package) is huge, big enough to accommodate not just skis but a set of golf clubs.

Perhaps there is one more downside: complication. You'd really have to see this top in action -- with its elastic stringers pleating the roof liner and close-tolerance hinges scissoring together -- to appreciate what a bit of precision engineering it is. Can it go wrong? I've already heard from an anguished reader who reported his BMW's super-roof went haywire and it took four weeks to fix. Tsk tsk.

In terms of driving dynamics and performance, I could cut-and-paste what I previously wrote about the 335i Coupe. These are unbelievably refined automobiles, powered with hydraulic smoothness by the company's new twin-turbocharged inline six, complete with direct injection and the Double

VANOS variable valve timing. With a bank of Bosch computers running the show, this engine generates 300 foot-pounds of torque practically everywhere the tach goes. Combined with the glycerin-smooth ZF transmission, the powertrain has a rheostatic, dial-a-speeding-ticket quality, summoning a progressively urgent rush of power from a dead standstill until your necktie is flapping in the wind like a hurricane pennant. And, by the way, can you bottle that sound?

If you feel like changing gears yourself, the ZF provides pewter-like blobs behind the steering wheel (forward for upshift, back for downshift). Like other recent sport auto trannys, the ZF blips the throttle on downshifts to provide rev matching.

With an extra 400 pounds aboard, it seems self-evident that the convertible wouldn't be quite as spry as the coupe, but from the seat of the pants, it's hard to discern. The car still bolts to 60 mph in 5.7 seconds, all while the turbos gargle great warbling streams of pressurized atmosphere. With the $1,300 Sport package girding its loins, our test car disported in utterly shameless and beguiling ways.

I would also note that the car's 20/29, city/highway mileage is downright decent for a 300-hp, 2-ton convertible.

With the top up, the cabin ambience -- quiet and composed, elegant, as solid as a bank elevator -- is indistinguishable from that of the Coupe. With the top down, life is equally good. With the windows up, the seat heaters on and the wind-blocker in place, you could drive al fresco to a Green Bay Packers game. Or a Sun Devils' game. Among the dazzling tech trinkets is an infrared-reflective coating on the leather upholstery that, says BMW, reduces surface temps by nearly 40 degrees.

It's two, two cars in one. It doesn't turn into a death-dealing robot, save the world or liberate millions in box-office receipts, but BMW's little transformer is certainly boffo.

2007 BMW 335i Convertible

Base price: $49,100

Price, as tested: $58,520

Powertrain: Twin-turbocharged and intercooled. Direct-injection 3.0-liter DOHC inline six-cylinder with variable valve timing; six-speed automatic with manual-shift mode; rear-wheel drive.

Horsepower: 300 at 5,800 rpm

Torque: 300 pound-feet at 1,400 to 5,000 rpm

Curb weight: 3,957 pounds

0-60 mph: 5.7 seconds

Wheelbase: 108.7 inches

Overall length: 180.6 inches

EPA fuel economy: 20 miles per gallon city, 29 mpg highway (unleaded premium)

Final thoughts: Tantronicon