YOU can never find a road tunnel when you want one.

Why, you might ask, would the driver of a sun-soaking open sportscar want to disappear into the shadows of a fume-filled tunnel?

Simply to bathe in the glorious baritone bellow of the big-bore V8 exhaust note punched out by the twin tailpipes of Mercedes-Benz's SLS AMG Roadster, of course.

The German brand’s hot shop has long had a handle on beautiful burbles from its normally aspirated 6.2-litre V8, but experiencing it in all its naked glory without the cocooning, insulating roof of a sedan or coupe takes it to a whole new level.

Drop the top, drive into an enclosed space, press the loud pedal to unleash 420kW and presto – automotive Beethoven.

Who said modern cars were sterile?

The shame of all this is that so few people will get to enjoy one of the great motoring pleasures, with perhaps only 20 or 30 Australians able to stump up the loot for a car that will set you back about $500,000 on the road – and way more if you start ticking boxes for the outrageously priced options ($30k special paint, anyone?).

Even those who can afford this sumptuous motoring masterpiece are unlikely to experience the full range of the SLS AMG Roadster’s tricks, unless they want to spend time at Her Majesty’s pleasure.

Such is the raw power, sheer pace and terrific terminal velocity – 317km/h, apparently – that few drivers will get to more than tickle it around the edges, even on a race track. Still, they will have fun tickling.

The triumph of the open-top version of the SLS is that it does pretty much everything its esteemed gullwing coupe SLS sibling can do, but with more fresh air and sunshine than your average nudist camp.

Although we can’t personally vouch for the manufacturer’s claim that the rigidity of the topless version matches that of the roofed coupe original – both have a high-tech aluminium-and-steel spaceframe at their heart, with a little extra strengthening for the soft-top version – the Roadster shows nary a niggle of the dreaded scuttle shake over big bumps, unlike so many sportscars of old (any Toyota Supra cabrios still out there?).

Only when tortured in undulating corners, with the suspension well loaded, did we feel a hint of twist that might be absent in the coupe.

Nevertheless, this two-seat convertible punches through bumpy bends with all the class of the thoroughbred it was designed to be in-house at AMG.

Employing a large slice of race car design and technology, the SLS twins – gullwing coupe and roadster – carry almost as much wallop as a V8 Supercar, but with much more comfort and style.

Bumps are ridden as well as some luxo barges, yet the driver always feels in total control.

The AMG engineers stuck with the hydraulically assisted rack-and-pinion steering arrangement, and we can see why. Steering turn-in and mid-corner grip are what you would expect for the money – excellent – and steering feel and feedback are significantly better than lesser Mercedes models.

The seven-speed dual-clutch transmission is so competent that we rarely felt the need to move into manual mode, but therein lies another of the SLS Roadster’s aural pleasures, with the downchanges blipping the throttle and the exhaust crackling like a DTM race car.

Weaving the big SLS along the sinuous roads of hilly central Victoria, with the booming V8 echoing off the giant roadside trees, we never felt that sense of unease that some supercars can engender – like, where is the edge of this extremely fast performance envelope?

The SLS talks to you constantly through your hands and backside, like your personal driving coach, making for a swift but safe and enjoyable drive.

The hulking composite brakes are exemplary, adding to the all-round sense of driving ease.

With the roof down, conversation is possible without shouting while cruising at highway speeds, helped by a little glass wind blocker between the headrests that contributes to cabin calm.

In the warm summer sun, we did not try the Airscarf neck warmer that blows air from a vent in the seat behind the head, but we are assured it makes all the difference for drivers who want to go alfresco on cool days.

However, we did get to test the SLS’s roof on the run, raising the soft-top at 50km/h while cruising through city traffic.

That manoeuvre can be completed in just 11 seconds, which is good, because it quickly blocks out the jaundiced glare of jealous fellow road users.

Unfortunately, it also blocks out much of the vision around the rear of the vehicle, and it is no wonder that Mercedes has added a blind-spot warning system to the roadster to help make lane changes a little more assured.

With the roof up, you could be riding in any luxury coupe, as the three layers of fabric do a great job of blocking out noise and heat.

Because the fabric roof folds compactly behind the seats on top of the rear bulkhead, boot space is not defiled. Mind you, 173 litres is not exactly cavernous, but it is sufficient for a weekend jaunt, which is this car’s natural habitat.

The sports seats cocoon the occupants, but are typically firm under the butt – a Benz trait. Some people like this, but we would prefer a little more ‘give’.

For Australia, customers get the Designo Exclusive leather upholstery – which is soft to the touch – in a variety of colours and combinations.

The aircraft-inspired cockpit, with its carbon-fibre centre console, has alloy-faced speedo and tacho, which not only look sporty but are strangely effective, catching the light to make for easy reading.

Unfortunately, the speedo has a dearth of numbers, missing Australia’s most common speed limits, 60km/h and 100km/h, which is irritating.

All the switch gear is typical Benz, scattered over the central console, steering wheel and steering column stalks, but once familiar the operation becomes second nature.

If the SLS has one glaring shortcoming, it is fuel economy, or lack thereof. Benz claims 13.3 litres per 100km, but your average SLS driver will find that difficult to achieve unless they keep the right foot feather-light.

In this car, that will be almost impossible.