BMW Z4 sDrive35iS

The current version of BMW's Z4 sports car has not, so far, made it on to this page, which is curious given that it has been on sale for more than a year now.

This second generation of two-seaters to bear the Z4 name is a coupé-roadster rather than a simple soft-top, which is why a separate coupé version is no longer offered. One car now does both jobs.

Previously the Z4 could be had as an M version, honed by BMW Motorsport and powered by a 3.2-litre, 343bhp, straight-six engine also used in the previous M3. Today's M3, though, has a bigger, yet more explosive V8 engine and BMW either can't or won't fit it to the Z4. So ended the possibility of a really rapid, raw-edged, proper sports Z4, or so the pundits thought.

BMW, however, has recognised the void and the car you see here is the response. Mindful of the need for a harder-edged, quicker and altogether more driver-centric Z4, it has introduced the closest thing to a new Z4 M it can readily create without a major re-engineering job. In a grand collision of random capital letters and vanishing spaces, it's called the BMW Z4 sDrive35iS.

The sDrive part is vital, of course, in case you confuse it with the four-wheel-drive version which, if it existed, would be called xDrive. The 35 means that it's a turbocharged version of BMW's 3.0-litre straight-six. And S? That really is vital, because it denotes twin turbochargers and 340bhp rather than the gentler 306bhp of the non-S, single-turbo engine used in several other current BMWs.

That's clear, then. The 35iS also gets its own look, with some silver vanes deftly added to the M Sport nose section's brake-cooling vents and a unique rear valance with an aerodynamic diffuser. There's a sports exhaust with a richer, louder sound than a regular Z4's, and to the M Sport suspension with its adaptive dampers is added quicker-acting steering. A seven-speed, double-clutch transmission is standard.

Roof-up first, we set off on quite an undulating, sinuous test route. The combination of nervous, darting steering – it's electrically assisted and devoid of true road feel – and video-game gear selection makes it hard to slide under the 35iS's dynamic skin at first, because none of its responses are initially intuitive. Soon, though, it starts to make some sense as your brain undergoes a crash course in re-learning the obvious.

It's certainly fast. With virtually the same power as the old Z4 M, it reaches 62mph in 4.8 seconds against 5.0, yet its CO2 rating is an impressive 210g/km against 292. And it sounds delicious, its crisp, deep, straight-six growl building to a proper howl as the revs rise. The gearshift is quick and obedient, but such is the torque on tap (up to 332lb/ft continuously, 369lb/ft in short overtaking bursts) that downshifts tend to be more for acoustic delight than sudden greater accelerative need.

Good so far, then. But there's something wrong with the rear suspension. Part of the steering's dartiness is linked to the way the rear suspension feels as though it is attached to the Z4's structure by a couple of elastic bands. A quick steering movement sends the tail instantly off course, which might sound like a recipe for fabulous responsiveness were the tail's movements not so imprecise. Even on a straight road you can feel the rear wheels steering the car, and you almost can't help exiting every bend with a tail-out flourish.

It's fun, up to a point, but it feels artificial, impure and wrong. Sport mode firms the suspension, slightly solidifies the steering, sharpens the throttle response and stimulates the gearshifts, but the problems remain. Sport Plus just destroys the ride. Opting for 19in wheels over the 18in ones does improve precision with little penalty in ride comfort, so against normal practice, I'd advise doing this.

So, what do we make of the £44,220 Z4 sDrive35iS? It goes as an M-car should, but it doesn't feel like one. It's simply a very fast coupé or roadster, with unsettlingly disconnected handling. Had BMW Motorsport been let loose on it instead of merely allowing its name to be hijacked by the marketing department, the result might have been rather better.

The Rivals

Audi TT RS Roadster: £46,715.

Identical power and torque figures, soft-top, 2.5-litre, five-cylinder engine with four-wheel drive. Should be a thrill but isn't.

Nissan 370Z Roadster: from £30,445.

Raw with a 328bhp V6, BMW-like front engine/rear-drive layout, few electronic interfaces to get in the way. Great fun, great value.

Porsche Boxster S: £42,184.

Mid-mounted, flat-six engine with 310bhp. Fabulous handling, wonderful sound, a car which will get under your skin.