We like
Extra low-rpm torque from both 2.5si and 3.0si, extra top-end urge from both engines, 2.5's reduced consumption, six-speed manual and auto transmissions across the board, chassis rigidity, value

Room for improvement
Vague electric steering, 3.0si auto's fuel consumption, styling changes too subtle

AS a compact, rear-drive convertible, BMW’s Z4 has always made a solid argument against both its more expensive Mercedes-Benz rival in the SLK and cheaper Japanese contenders like Nissan’s beautifully sculpted 350Z roadster.

Now, just in time to arrest a significant but not unexpected fall in sales as it approaches middle-age, BMW has blessed its Z4 with more of what it does best: horsepower.

Just as the new E90 325i sedan is a vastly better performer than the 141kW E46 3 Series model it replaces, so too is the entry-level Z4 – now dubbed 2.5si to signify its new engine – a more handy device for doing what it was designed to do.

Along with the extra 19kW of peak power (its 160kW maximum is just 10kW short of the previous Z4 3.0’s output), the 2.5si offers 5Nm more peak torque at significantly lower revs – 2750 versus 3500rpm.

Mated to six-speed transmissions instead of the original Z4 2.5’s five-speed manual and auto choices, that makes the cheapest Z4 more than half a second quicker to 100km/h than before, as well as giving it a higher top speed by about 20km/h.

And the difference is obvious on the road, the least expensive Z4 feeling much more eager at low-rpm as well as offering a new realm of high-rpm performance.

Of course, the extra tractability makes the 2.5 a more relaxed round-town driver than before, while the extra top-end urge gives it a character that’s more like what you’d expect from an open-topped sports car.

The new 2.5si is a real sweetie on the open road, revving with gusto all the way to its higher 7000rpm cut-out and making all the right noises along the way.

With the six-speed manual, it’s a delight to punt hard and builds revs with unexpected alacrity, as well as reveling in high-rev down-changes while its driver heel-toes the well-positioned pedals.

The icing on the cake, despite the extra performance, is lower fuel consumption. While an extensive full-day drive on the poor-quality backroads around Northern NSW didn’t allow us to replicate BMW’s average consumption figure of 8.4L/100km, our 2.5si came respectably close (given the merciless thrashing it received) with an average of just 9.8L/100km/h.

The Z4’s extremely rigid chassis is a good match for the new 2.5, and latest-generation stability control with new braking functions is a welcome addition – as are the two-stage LED brake lights, new-look front bumper and new alloys.

The standard aluminium-look interior trim is also a fitting feature of the 2.5si interior.

But, alas, BMW’s first electrically-assisted steering system remains the Z4’s Achilles heel, despite the addition of a sport button that transforms it from being overly light to just too light.

Best described as twitchy, even in sport mode, the electric steering is far lighter than the conventional, hydraulically-assisted steering found in the Z4 M, and requires constant correction even to maintain a straight line.

It’s less of a problem than in the 3.0si, where the extra power allows one to drive on the throttle more aggressively, but in hard braking situations the lack of front-end feel and straightline stability can be quite disconcerting in a quick-steering, relatively lightweight, rear-drive roadster such as Z4 2.5si.

In short, it’s the weak link in solid chassis that would otherwise make the most of the sub-$80,000 Z4’s new-found performance.

As stated, the electric steering feels even more at odds with the 3.0si, which offers an even greater leap in performance over its 3.0i predecessor.

While its rigid chassis is just as immune to scuttle shake as the 2.5's is, the higher performance makes its driver less confident to explore the limits of adhesion, where it also reveals a degree of steering kick-back.

And while the 3.0-litre auto we drove is a vastly quicker-accelerating, slicker-shifting convertible than the five-speed auto version it replaces (and also gains intuitive steering wheel shift paddles), fuel consumption emerged as another reason to choose the $13,000-cheaper 2.5.

Our 3.0si auto averaged a surprisingly thirsty 16.9L/100km – more than the Z4 M manual we drove earlier in the day, which returned 15.4L/100km.

Of course, the M remains the undisputed king of BMW roadsters – as much for its neck-stretching performance as the lack of the electric steering found in lesser Z4 variants.

But with more power, performance and attitude than the more expensive SLK200 and more poise and panache than the cheaper, gruntier but heavier 350Z, the 2.5si stands out as the thinking person’s sports convertible.

It may now perform even better than it looks and steers but, twitchy steering notwithstanding, the highly polished and vastly improved 2.5si is easily our pick of the improved Z4 range.