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I don't envy the engineers who were ordered to turn the Porsche 911 into a convertible. How do you cut the top off one of the greatest car designs without hurting it? Answer: You don't. An extended test of the latest Porsche 911 S cabriolet provided an opportunity for some spirited driving – the version of the model is the best one yet, with a fantastic motor, a gorgeous exhaust note and Velcro-like grip. But every time I looked at it I thought of Michelangelo's statue of David with its head cut off.

The 911 convertible is a great case study in aesthetics, engineering and human psychology. Like all convertibles, it makes no sense: It is heavier, more expensive and less safe than its hard-top sibling. But we still want it, because convertibles cater to some of our deepest human instincts – vanity and seeking pleasure.

If structural engineers had their way, every car would be a hardtop. But I understand the appeal of the convertible. It allows you to enjoy the elements and put yourself on social display. You are the jewel; the car is the setting. I love driving them, especially on a sunny day. And some cars were born to be topless – like the Lotus 7, the Morgan Plus 4 or the Austin Healey Bugeye Sprite. But in the case of the 911, taking off the top desecrates a classic.

First, a little history. The earliest cars ever built were open-top machines that aped the design of horse-drawn carriages. In 1922, designer Ben P. Ellerbeck created the first convertible by installing a manually foldable top on a Hudson coupe. By the 1950s, convertibles had become symbols of youth and freedom – actor James Dean drove a topless Porsche Speedster (and died in a convertible Porsche 550 Spyder). A convertible top was once synonymous with sports car motoring, thanks to cars like the MGB and the Shelby Cobra. But over time, the convertible was gradually pushed aside by the hardtop as the sports car of choice thanks to its structural superiority. And one of the defining metal-top sports cars was Porsche's original 911, which came to the market in 1963 and became an instant icon.

For generations, the shape of the 911 has spoken to drivers and non-drivers alike. Its profile is semiotic perfection, signaling speed, performance and German craftsmanship. When I was hired as a mechanic in a Porsche shop back in the 1970s, it was like entering the priesthood. I ministered to machines that were not merely cars, but sacred objects that happened to be equipped with seats and a steering wheel. The lines of the hardtop 911 were consecrated. More than once, I sat in the shop after hours and stared at a customer's 911, marveling at the sheer beauty of its form, especially the descending line of the roof as it ran toward the elliptical tail and the bulging rear tires.

Seen from the side, a Porsche 911 coupe is a collection of aerodynamic curves that intersect in exactly the right way, sketching out a dream. So you can imagine how I felt when the first 911 convertibles appeared in the early 1980s: One of the most beautiful shapes ever created had been violated. It was like learning that someone had installed a plastic sunroof and a hot tub in the Sistine Chapel.

Although it offended some of the Porsche faithful, the convertible has been with us ever since. And the 2016 Porsche 911 S cabriolet I tested was an infinitely superior machine to those early 911 convertibles. But this is to be expected. Porsche has done a masterful job of retaining the 911's essence while moving the car into the future. The current model is larger and heavier than its ancestors, yet the shape is still distinct. And the mechanical package is incredible, with smooth power, vastly improved stability and superior aerodynamics that lock it onto the road at high speed.

But not even Porsche's engineers can completely overcome the structural compromises that are created by a convertible top. When you cut the top off a car body, you have eliminated a key structural element. Many classic convertibles exhibit twist so severe that you can actually see the hood twisting out of plane with the dash panel as you drive over bumps or hit the accelerator. In the modern Porsche 911, smart engineering and optimized construction minimize this effect. Underway, the 911 cabriolet feels as stiff and solid as the Brooklyn Bridge. But if you mounted the body on a test fixture, you would find that the 911 cabriolet has less torsional rigidity than its hardtop sibling.

Once upon a time, this kind of thing really mattered. Among the hard core buffs I hung with when I worked on Porsches for a living, cutting the top off a 911 was anathema. We wanted the stiffest possible platform to mount the suspension on, and the best aerodynamics – and that meant a hard top. For us, the appearance of the first 911 convertible marked a tipping point: Porsche's greatest sports car was being gradually appropriated by consumers who saw it not as a driving machine, but a status symbol.

We all know how this particular battle ended. Today, Porsche's best-selling cars are SUVs, and few 911 buyers opt for a manual transmission. The market has spoken, and most consumers want comfort, convenience and status, not an unyielding machine that forces them to learn how to drive with a clutch. I have accepted all this as the way of the world. And seen in isolation, the 911 convertible is still an attractive car. But put it next to the hardtop, and it pales in comparison. As you gaze upon the 911 coupe's curved roof line and its perfect descent, you will see what I did when I sat in a garage back in the 1970s, long after the doors had closed for the day, and was transfixed by true automotive beauty.

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