CONVERTING a two-door, two-seat coupe into a soft-top roadster is one thing converting it into a credible, sporting, two-seat roadster is another.

Too often the soft-top version of an existing coupe looks like an obvious late arrival in the model lineage – something that was not factored in when the original shape was created. Not exactly a backyard chop-top, but often less cohesive, in a design sense, than it could be.

So when Nissan unveiled the roadster version of its strong-selling 350Z coupe, the expectation was that the open-top would probably be less of a sports car and more of a lazy, smooth cruiser.

The reality comes as something of a pleasant surprise. In fact, in some ways, the roadster 350Z is more capable than the coupe. Part of this is due to Nissan’s retuning of the suspension to remove some of the harshness that so far has been all too evident in the 350Z.

The result is that the roadster’s suspension compliance enables it to perform better on chopped-up roads, absorbing a lot of the stuff that unsettles the coupe and tending towards a more faithfully held line through a corner.

It also means the roadster is more comfortable on the road. With some of the harsh edges honed off, the soft-top is more pleasant to live with. Yet it remains a focussed sports car, with the extra compliance coming in spite of its bigger, 18-inch wheels – the same as used on the even harder-edged Track version of the coupe.

The larger wheels tend to underline the fact Nissan didn’t want to go soft with the roadster. Importantly, they don’t introduce any bump-steer into the road behaviour (the suspension re-think changes the car in such a positive way that Nissan will adopt the same damper settings on the coupe as well).

Importantly, the basic 350Z essence remains undisturbed in the roadster it is still a sharp-edged, on-and-off sort of car. The engine has a deliberately contrived rawness that suits the driver’s-car aspect, and the quick-shifting manual gearbox requires a certain expertise if it’s to be operated smoothly.

The car has the same, slightly heavy (speed sensitive) steering as the coupe, and a decisive turn-in well suited to its sporting nature.

The four-wheel discs, all ventilated, do a good job of hauling the car down from high speed and are aided by the expected four-channel anti-lock complete with brake assist and electronic brakeforce distribution.

The roadster also gets traction control and a viscous-coupled limited-slip differential although, because there will be no Track version, electronic stability control doesn’t feature – even on the options list.

The roadster’s structure has been toughened up to ensure rigidity. A centre crossbar connects the side sills, while reinforcement is provided at the front via an A-shaped cross bar. There’s added reinforcement at the door openings as well as a triple member under the seats which connects the sides to the floor structure.

The result is an open-top car that is not afflicted with any noticeable scuttle shake, but weighs more than its coupe sibling.

The weight (a substantial 1548kg in manual transmission form) doesn’t seem to unduly affect performance though. The all-alloy, multi-valve twin-camshaft V6 is so torque-rich (no less than 363Nm from its 3.5 litres) that it will respond eagerly even at low revs, yet still loves to be wound out.

It has what feels like the flattest of torque curves. There is no sign of a peak anywhere and it simply pulls strongly whatever speed, or whatever gear it’s in.

With all this engine punch, and a chassis that is more compliant and easier to live with than has been the case with 350Z so far, the roadster ends up being a genuine sports car that could be quite happily ranged alongside the more purpose-built Honda S2000, or BMW Z4.

Perhaps the only real difference when compared with these cars is that the Nissan feels rather more substantial. It certainly carries more weight. But the engine can cope with it.

And it’s hardly less comfortable to live with than the coupe. The roof powers down, in about 20 seconds, into a neat, concealed space between boot and interior, and has a glass rear window with demister wires.

It encourages laziness less than some recent powered drop-tops because the driver first needs to release a catch at the top of the windscreen. The Nissan also lacks the full interior lining seen in some luxury convertibles but that’s hardly a problem because it cruises pretty quietly.

With the roof down, the interior remains generally unruffled, partly due to the high door sills, partly due to the clear glass wind-blocker between the seats that prevents the inflow of air from the back of the vehicle. Tall passengers will feel their hair ruffling, but that’s about all.

The boot isn’t bad either - wide enough to hold a golf bag after a little manipulation and certainly more secure and passenger friendly than the open, unrestrained space in the back of the coupe.

The interior is generally identical in both 350Zs, with the driver’s seat specially sculpted to provide a little more general support, and a couple of sliding-door bins behind the seats to make up for the lack of a conventional glovebox.

The lidded cubby in the centre of the dash has a redesigned lid less likely to break if used insensitively. The three-pod dials confronting the driver move up and down with the steering column to ensure a constant, clear view of the gauges.

Driver and passenger get dashboard and door-mounted airbags although, obviously, the roadster mises out of the side headbags installed in the coupe.

The seats are operated by small inboard toggle switches and provide just enough adjustment for 185cm passengers. Cleverly, the left-side seatback tilts forward slightly when the roof is being opened or closed to ensure it doesn’t brush the passenger’s head.

This is less of a problem on the other side because the driver is already leaning forward anyway, to reach the switch mounted low on the dash to the right of the steering column.

Equipment is basically the same as the coupe, with a powerful, seven-speaker Bose sound system (including a sub-woofer and ambient noise sensitive volume control), power-adjusted, heated leather seats (sports net seats are optional), climate-control air-conditioning, cruise control and trip computer.

Like the coupe, the roadster is optionally available with a five-speed sequential automatic transmission.

Apart from the net sports seats, that’s about it for options.

The roadster price premium, at $10,000 above the base coupe, is pretty reasonable for a good-looking, lively sports car offering total open-air freedom without any real compromises.

Unlike some coupe-based convertibles, there’s no significant loss in performance, while there are some gains in both handling and ride qualities.

To many, the extra $10,000 would seem a mere drop in the ocean compared to what you’d have to pay for a similar-performance Euro sports roadster.