It’s a question we often ask ourselves about models featuring a wide range of powerplants and specifications – which is best? Where is the sweet spot? — the point where the promise of its badge matches the reality of the driving experience.

Our focus this time is BMW’s Z4, a model which from the outside should be the most dynamic BMW of them all, but in truth I’ve always found delivers most strongly when viewed simply as an open-top roadster.

I’m a coupé fan myself, a preference I can no longer fulfil with the latest Z4. In the past I’ve owned a first-generation E85 Alpina Z4 Roadster and E85 Z4 M Coupé – both had their strengths, the Alpina’s 3.4-litre straight six engine was as smooth as silk, while the Z4 M was more sprightly and alert than the equivalent E46 M3 of its day – it also looked awesome if chosen in the right spec.

The second-generation E89 Z4 was introduced in 2009, replacing the soft-top and coupé variants of its predecessor with a folding (metal) hardtop. It’s grown in size and weight, and like other models in the BMW range comes fitted with the company’s TwinPower Turbo engines.

The Z4 was updated in March 2013 with a new entry-level sDrive18i, enhancements to some of the existing engines and minor styling enhancements including white LED corona rings and a white ‘eyebrow’ with the subtle addition of chrome detailing. The profile view now includes a side gill, containing an LED side repeater with chrome detailing.

Standard equipment now includes DAB radio, Xenon headlights, Bluetooth, USB connectivity and a Sport multi-function steering wheel, with Kansas leather upholstery fitted to all models except the sDrive18i.

The range now including three four-cylinder models; the 154bhp sDrive18i, 181bhp sDrive20i and 242bhp sDrive28i, plus two six-cylinder models; the 302bhp sDrive35i and flagship sDrive35is powered by a 335bhp TwinPower Turbo six-cylinder engine.

The Z4 is available with BMW’s Adaptive M Sport suspension (standard on the sDrive 35is), which lowers the car by 10mm and automatically adapts the stiffness of its dampers to suit the road surface or your driving style. Three options can be selected via the Drive Performance Control switch; Comfort, Sport or Sport+ which supposedly gear it better for either relaxed or more spirited driving.

BMW Z4 (UK Model Range)

Model Price
from
Power
Hp
Torque  
Nm
Zero – 62mph Seconds Top Speed
Mph
Combined 
Mpg
CO2 Emissions g/km
Z4 sDrive18i £27,615 154 240 7.9 (8.1) 137 41.5 159
Z4 sDrive20i £29,715 181 270 6.9 146 (144) 41.5 159
Z4 sDrive28i £34,100 242 350 5.7 (5.5) 155* 41.5 159
Z4 sDrive35i £39,935 301 400 5.2 (5.1) 155* 30.1
(31.4)
219
(210)
Z4 sDrive35is £45,815 335 450 (4.8) 155* 31.4 210

The choice is less than obvious when looking purely at their performance figures – at the top of the range the sDrive35is accelerates from 0 to 62mph in 4.8 seconds, delivers an average consumption of 31.4mpg and CO2 emissions of 210 g/km. The entry level sDrive18i packs 181 less bhp, but is 130kg lighter at 1,470kg** and accelerates from 0 to 62mph in 7.9 seconds with an average fuel consumption of 41.5mpg and CO2 emissions of 159 g/km.

So which should you choose? Is ‘more’ clearly better, or is the Z4’s appeal quite separate from the powerplant driving its rear wheels?

Driving them

Starting at £27,615, the entry-level sDrive18i is essentially a detuned version of the 2.0-litre sDrive20i – both offer the same 41.5mpg fuel consumption but with 27bhp less, the sDrive18i is a second slower to 62mph.

That performance gap is noticeable — where the 20i feels sprightly and capable, the 18i feels noticeably more sedate and less confidence-inspiring when overtaking on a B-road. Both corner fluidly, on the M Sport suspension fitted to our test cars, and provide just enough body roll to confirm when you’re pushing on.

Of course neither are sports cars – not in the purest sense, but they do lift your spirits in a way which only a compact open-top roadster can. The Z4’s low-slung driving position provides a real sense of connection with its chassis, although this is slightly undermined by the slight artificial feel of BMW’s Servotronic power-assisted steering.

This is a real bugbear of mine and although the Z4’s steering is well weighted, there’s an inconsistency about its response off-centre making high-speed corners somewhat disconcerting. You sit quite a long way behind the front wheels, so this also makes a difference to how confidently you’re able to press on along twisty B-roads.

The sDrive20i is the most popular model – accounting for 80 per cent of Z4 sales, but that’s not the model we would choose.

The £34,100 sDrive28i delivers the same economy and emissions as the lesser four-cylinder models with around a third more power and torque. The upside is immediately obvious. Zero to 62mph acceleration drops by 1.2 seconds, while the chassis comes alive with a more playful rear-end and a clearer sense of balance. It even sounds better, less like a four-pot with a pleasing growl when driven on part throttle.

BMW-Z4-wireframe

  • BMW-Z4-Review_G2
  • BMW-Z4-Review_G4

The optional M Sport package serves only to stiffen the ride without conferring any benefit on the driving experience, and most of the time you’ll want the Drive Performance Control switched to ‘comfort’ if only to improve the front axle’s turn-in and bite. Based on the car’s in our test, we’d opt for a Z4 with standard suspension, 19-inch double spoke wheels and the optional sports seats.

While the six-cylinder models add more performance they also add weight – around 105kg over the sDrive28i – which turns the agile and responsive roadster into more of a motorway cruiser. But the elephant in the room is the Porsche Boxster S, which for a few thousand pounds more (than the sDrive35is) offers a more engaging drive as well as being nearly 200kg lighter.

And so we begin to understand the ‘sweet spot’ in the range.

The sDrive28i compares rather more favourably against the Audi TTS, offering better refinement with the roof down and the inherent rightness of its more nimble rear-wheel drive chassis. If we were buying a Z4, this is the model we’d choose – there’s sub 6-second 0-62mph performance, 40mpg fuel economy and that sporty exhaust sound piped into the cabin.

Many may choose the sDrive20i, but they’re missing out on the 28i’s mid-range torque that allows the Z4 to reveal the true talents of its chassis.

Buying one

The Z4’s cabin is one of my favourites; snug, flawlessly made and very ‘driver centric’, but some of the older models have been plagued by creaks and rattles from its folding hard-top roof, dashboard and door trim. Some of these may have been resolved in the March 2013 facelift, but it’s too early to say, so pay attention to this if buying used or stick to a new car with BMW’s bulletproof factory warranty.

There’s a special offer on at the moment on the sDrive18i Roadster M Sport, which costs £349.00 per month on a 36 month contract with a £9,655.78 deposit.

There’s also a complimentary M Sport upgrade available on sDrive18i, sDrive20i, sDrive28i, and sDrive35i models if ordered between 1st October and 31st December 2013 and registered by 31st March 2014.

** EU Unladen weight includes 75kg allowance for driver, luggage and 90% tank of fuel.