BMW’s Z4 convertible has reached middle age and, in true BMW style, the raucous roadster’s midlife makeover entails a healthy dose of Viagra rather than a phony cosmetic facelift.

Of course, the headline act of the revised Z4 range is the return of an M version for the first time since Z3, but BMW hopes new features, new technology, subtle styling updates and new 2.5 and 3.0-litre magnesium-alloy engines will help lift the garden variety Z4’s 57 per cent sales plunge so far in 2006.

With almost 1200 examples sold in Australia since its release in 2003, BMW says Z4 is "unquestionably the class leader" - despite the fact it’s claimed to have no direct rivals here.

Mercedes’ four-seater CLK coupe and convertible may be this year’s top-selling sports car under $80,000 with a dominant 38.6 per cent segment share, but Z4’s 19.3 per cent share of the category so far in 2006 has come under attack from the other major dedicated two-seater roadster, Benz’s more expensive SLK (17.8 per cent).

However, the Bavarian brand hopes more power and performance, better fuel economy, subtle exterior changes and more features will return Z4 to its traditional sales rate of about 300 per annum.

Borrowed from 3 Series and 5 Series, the new N52 magnesium-alloy straight sixes are lighter, more compact and more flexible thanks to the employment of Valvetronic and a higher 7000rpm redline (versus 6500rpm).

To be available in Europe alongside the current engines – hence the "si" suffix - both engines are now mated to either a six-speed manual transmission (2.5i buyers previously made do with a five-speed manual) or a six-speed Steptronic auto, which replaces both variants’ five-speed self-shifter and now features steering wheel-mounted shift paddles.

The fitment of dynamic drive control also provides more aggressive auto-shifting and firmer steering at the push of a button, while the latest-generation dynamic stability control brings extra functions like brake standby, brake drying and brake fade and rollback resistance.

Again expected to attract about half of all Z4 sales, the 3.0-litre version benefits from the same new magnesium-alloy, Valvetronic-equipped straight six that powers the E90 330i sedan and facelifted E60 530i sedan and wagon.

Delivering 195kW at 6600rpm (5kW more than the 330i/530i and the same as the 130i Sport) and 315Nm of torque from just 2750rpm, the new engine propels the 1310kg (1340kg auto) Z4 3.0si to 100km/h in a claimed 5.7 seconds (auto: six seconds) and on to an electronically limited top speed of 250km/h.

That makes the 3.0si 0.2 seconds quicker than the 170kW/300Nm 3.0i in both manual and automatic guises, despite the fact that combined average fuel consumption is down slightly, to 8.6L/100km (9.0L/100km auto).

The Z4 2.5si, meantime, is powered by the same 2.5-litre six that’s now found in the E90 325i sedan and E60 525i sedan. It offers almost as much as the outgoing (170kW) 3.0-litre, with 160kW available at 6500rpm and 250Nm of torque on tap between 2750 and 4250rpm.

That compares to the previous Z4 2.5’s outputs of 141kW and 245Nm (at 3500rpm).

Weighing in at 1285kg (1320kg auto), the 2.5 blasts to 100km/h in a claimed 6.5 seconds (6.9 auto) and to a top speed of 240km/h (238km/h auto), while consuming 8.4L/100km (auto: 9.0L/100km).

While the 2.5is’ top speed is up a big 20km/h, it’s also half a second quicker to 100km/h than before as a manual and 0.6 seconds quicker as a (six-speed) auto.

There’s more to the revised Z4 than extra performance and economy, however, with new bumpers and lighting giving it a subtle but more modern look.

The new front fascia features a wider front air intake with integrated fog lights and wheelarch-mounted side indicators, while LED tail-lights with two-stage adaptive brake lights constitute the biggest change at the rear.

New 17 x 8.0-inch alloys for the 2.5si and new 18-inch alloys (8.0-inch front, 8.5-inch rear) for the 3.0si complete the Z4 facelift, while alloy interior highlights are standard on the 2.5si and 3.0si buyers have the choice of either alloy or woodgrain trim.

Complimenting the quicker, better-equipped and more frugal are pricetags that actually undercut the original launch prices of its predecessor, with the 2.5 dropping $900 and the 3.0 costing $1200 less.