Indecent exposure - Audi TTS v BMW Z4 v Merc SLK 350 v Porsche Boxster S

Surely, even in Germany, this couldn't be called convertible weather. Down on the flatlands it's a sunny 15 degrees. Up here in the Snowies, somewhere near Cabramurra, it's closer to four. The wind is howling, clouds are whipping past at road level, noses drip and drizzle stings. If it wasn't for windscreens, foot heaters and hats, this merry band of road testers would be frostier than a penguin's pecker. And that's one very cold beak...

Sensible drivers would raise the roofs to keep the elements out. But then, does any of this quartet look like the kind of car a sensible type would consider owning? Each is priced on the wrong side of six figures; none has more than two seats and fairly token luggage capacity. And even though the individual cylinder count stops at six, healthy power outputs promise performance aplenty. As our topless testers rip past with ears flapping in the wind, you can bet the farmers and hydro workers passing in their white Landcruiser utes think we're just a little bit crazy.

Funny, too, that this lot are all from Germany, a country often accused of lacking a sense of humour. It's difficult to not see the funny side of the rest of the world queuing up to buy the Teutons' ever more powerful and outrageous artworks on wheels.

The reason for our congregation is the new BMW Z4, with its long roadster bonnet, folding hardtop roof and, in the case of the sDrive35i, a twin-turbo six and seven-speed double-clutch gearbox. Fortuitously, each of its rivals - from Mercedes-Benz, Audi and Porsche - are also new in their own ways. The TTS arrived late last year featuring a beefed-up four-cylinder turbo engine delivering 200kW. Just before that, Mercedes waved the wand over its SLK, nipping and tucking the styling and adding a more powerful V6. Not to be outdone, Porsche recently slotted a seven-speed dual-clutch and direct-injection into its re-engined Boxster S.

While all four are two-pedal cars, only the Mercedes uses a conventional torque converter auto, albeit the clever 7G-Tronic now fitted to almost all of Benz's rear-drive cars. And where two pedals might once have been seen as the default choice for poseurs whose driving ambitions rarely extend beyond tooling about town in D-for-Dawdle - the spec sheets for all four suggest they have the potential to deliver some serious sports-car thrills.

But before getting too carried away, we need to visit a dealership and part with our hard-earned. It's the Audi that inflicts the least pain in this regard, the TTS coming in at a snip under $100K, or $103,265 with the dual-clutch transmission. That's about five grand more than the TT coupe, or a similar amount above the V6 quattro Roadster. Depending on how you look at it, that's either plenty of choice around the one price point, or grounds for some confusion about which is the best TT to buy.

The SLK 350 and Z4 35i are much closer, and a fair bit more expensive, at $119,000 and $120,400 respectively for their two-pedal variants. The mid-engined Boxster S raises the bar again at $146,000 with the PDK twin-clutch. As if to rub salt in the wounds of recession-shy Aussie buyers, the Porsche came optioned with 19-inch wheels, adaptive suspension and a Sports Chrono package among other items, lifting its price to an eye-watering $162,300. More cosmetic additions to the TTS, Z4 and SLK meant that in the form you see them here, they cost $108,000, $125,480 and $139,800 respectively.

For that sort of money you might reasonably expect more than just pose value, so it's a good thing that all four deliver in their own ways. The 2.0-litre, direct-injection turbo four fitted to the TTS gets a significant power hike over the standard TT - the kilowatt count rises from 147 to 200, and torque is up from 280 to 350Nm.

This makes the TTS the least powerful car on test, which would be okay if it were also the lightest. But it's not, tipping the scales a good 120kg heavier than the Boxster S which boasts the most power, all 228kW, from its 3.4-litre flat six. At least the Audi is not the heaviest on test, that honour goes to the 1580kg BMW Z4 35i. However, with its twin-turbo in-line six delivering a competitive 225kW and a benchmark 400Nm (from as low as 1300rpm), the BMW backs its beef with muscle. Finally, with its rev-range extended and its power output lifted by 24kW to 224kW, the SLK 350's 3.5-litre V6 promises to make the 1485kg roadster decently quick.

For the mathematicians among you, that's a combined 877kW storming up Kosciuszko, climate control and tachos in the red, each car flaunting its remarkably different character; zinging, howling, popping. It makes for wonderful open-air theatre, provided you're not a mountain goat, or a late breaking possum.

Take the Z4: drop the hammer in first or second gear, and there's electrifying performance backed by a deep and menacing exhaust note that pops and grunts between upshifts of the automated gearbox. And the flexibility and torque application of the twin-turbo six is stunning; throttle inputs in sixth or seventh gear from low rpm don't produce coughs or splutters, only a long, steady rush of acceleration.

Audi's smaller four-cylinder engine also loves to rev, and although clearly less meaty than the BMW, it's full of characterful whooshing and whistling. And if the higher-boost turbo indicates a degree of peakiness to the power delivery, there's no indication of that in practice. It pulls strongly across the mid-range, spins hard to its redline, while the quattro all-wheel-drive system easily looks after traction.

Then there's the Merc. Like a choirboy singing in the wrong key, it's ousted as the aural weak link of this orchestral quartet. Yes, it has plenty of performance, but the V6 sounds grainy and feels unwilling to rev. Its seven-speed auto is also slower-shifting than the dual-clutch 'boxes.

Finally, there's the Boxster's donk - an absolute gem. With a guttural, spine-tingling wail at full throttle, it has a delicious ability to rev hard and fast, delivering ever-increasing scales of acceleration. Yet the flat six is just as happy to pull big loads from low revs and, when left to its own devices, the transmission shifts quickly into seventh gear. Manual changes are effected by buttons on the steering wheel residing alongside the driver's thumbs, or via the console-mounted shift lever.

The Porsche's handling, too, is sublime. The addition of optional active suspension and the Sports Chrono package gives a much greater degree of adjustment to gearbox and handling modes. With it all turned off, the ride is acceptably supple, gearshifts smooth and precise, and the driver is left wondering why any adjustable modes are needed. The steering's also deliciously accurate and well weighted, chassis balance close to perfect, and grip levels enormous on the larger optional tyre package. Hit the 'sport' button and the suspension stiffens, gears are held longer, and cornering speeds increase. Go to 'sports plus' and there's another level of gearbox and suspension performance, but the trade-offs in ride and gearshift harshness (not to mention the barest skerrick of available ESP functionality) mean this mode is best left for racetracks or truly challenging smooth roads.

Back in the BMW, and the impression from the driver's seat is more classic roadster. The Z4's two seats are located almost as far back as the rear axle, with a long speedboat-esque bonnet stretching out ahead. It looks cool but can be disconcerting as the steering acts almost too sharply for the rear end to catch up, requiring continual re-adjustment even in higher-speed corners. Hit the three-stage 'sport' button and the steering nervousness disappears, the gearbox goes into a more serious mode and the whole equation feels much handier. Retaining ESP functionality on slippery surfaces is advisable, however, because with 400Nm on tap the rear end is easy to break loose and the steering lacks the communication to give full confidence in recovery.

The TTS is less dramatic, but hardly less effective. It's a more nimble car than the Z4; lighter in the steering but with more feedback, it changes direction quickly and with impressive stability. Even the dual-clutch gearbox shifts more smoothly, the two paddles behind the wheel perfectly positioned for manual shifts. It does tend to understeer in tighter corners, at which stage the ESP kicks in to kill the power, but with the stability system switched off there's more than enough AWD traction to maintain safe grip.

And even with the magnetic dampers set to the harder of their two settings, the ride of the TTS is surprisingly supple, which works well with the quietness of the tyres and engine to give the car refinement as well as punch. This is no highly strung, pumped-up version of the TT, but a capable and comfortable sports car in its own right.

Chasing the TTS, and tracing the same safety-yellow shoulder lines, the SLK 350 immediately feels less composed, the ride more knobbly and the body more flexible. It also has a big steering wheel, and while the steering itself seems quite good initially, when the pace quickens it feels more the convertible, less the sports car. The brakes, too, feel spongier and less aggressive on retardation than its rivals.

If fuel consumption is an issue then the Boxster performed the worst, although its overall figure of 12.2 litres/100km hardly makes it dipsomaniacal, and barely more thirsty than the Audi (12.1 L/100km), the BMW (12.0 L/100km) or the tall-geared, atmo Mercedes-Benz SLK 350 at 11.5 L/100km.

With no back seats, bodies designed for looks rather than utility and, of course, folding roofs that have to be stored somewhere, none of these cars is built as a family hauler. But it is interesting to note the difference between the BMW and Mercedes' solid hardtops and the TTS and Boxster's fabric items: the latter two are comparatively quick to disappear while the SLK and Z4's require more time. Boot space is also compromised by the hardtops (roof down, the BMW and Mercedes end up with luggage slots instead of boots) and attention has to be paid to pulling a luggage cover forward or the folding operation ceases to function.

So the Audi is left with a very useful boot able to swallow a decent amount of luggage (or extra wind cheaters) and the Boxster S also splits quite an acceptable amount between the shallow compartment behind its engine and the deep bay between its front wheels. The TT also has the more attractive and user-friendly interior, possibly in the latter case because it resorts to good old-fashioned buttons for chores like cranking up the heat after snapper Brunelli demands another top-down cornering shot. The Z4, on the other hand, has the latest iDrive menu-based system which can be distracting and takes a lot of learning. The SLK's COMAND version, which also relies on a vast array of buttons, is hardly better. Porsche's touch-screen is the best compromise of the three.

Like many recent BMWs, the Z4's lack of space for oddment storage seems, well, odd, as does the fact its sun visors can't be swivelled laterally. Minor annoyances in the others include the Merc's 12-volt power output being hidden in the passenger's footwell, the lack of iPod connectivity in either the Audi or the Porsche, and the latter's cacky beige interior treatment and slabby dashboard - far less classy than the Audi's black cockpit, or the BMW's sweeping instrument panel. But then, compared with the SLK's gorgeous, arrow-sharp wedge shape, the Z4's Wacky Races-style long bonnet or even the Boxster's double-ended version of Porsche's ubiquitous styling theme, the TT's high sills and bubble-shaped roof possibly make it the least attractive of the four cars.

Fashion may well dictate which of these cars will sell to a particular person, but in terms of functionality there can only be one winner - the Boxster S. Not only is it fast, it possesses the best iteration of the dual-clutch transmission in the PDK, and is balanced to perfection for quick driving.

But it's also 40 percent more expensive than the cheapest car here. Pricing does not separate the Z4 35i and the SLK 350, but the BMW's sublime engine, along with the quick gearbox and the option of steering and gearbox performance modes, helps make it the better of the two. The SLK looks fabulous and up to a point is fast and capable - it just lacks that final 10 percent of sharpness.

Which leaves us with the TTS. Offering a terrific blend of poise and performance for less than any other car here, it also has less packaging compromises, yet surprisingly high levels of refinement both in terms of ride quality and noise suppression.

Heading back towards the big smoke, woollen scarves and beanies finally removed and temperatures returning to something approaching sanity, the decision's clear: for the ultimate in performance, the Porsche is the pick of this bunch; weigh everything up against price and the TTS is a winner. Bet the guys in Ingolstadt get a chuckle out of that.

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