BMW Unveils 5.0 Second Z4 Roadster

By Feann Torr - 26/10/2005

Arguably one of the best engines built in the last decade, BMW's highly revered 3.2-litre inline 6-cylinder will be making a big comeback in 2006 under the bonnet of the new Z4 M Roadster.

For something that displaces just 3,246cc, a peak power output of 252kW is still quite astonishing even today, and all this power without the use of forced induction? That's quite extraordinary.

It comes as little surprise, then, that this peaky powerplant has won the Engine of the Year Award for the fifth year in a row at the so-called 'Engine Oscars' - a record in it's own right - and will be standard equipment with the new Z4 M Roadster.

For 2006, the new model Z4 Roadster will get changes across the model range, but we'll get to that a bit later (see panel at bottom of page).

Though BMW is still being cagey about the upcoming (E90) M3 coupe's powerplant - expected to be a small capacity 8-cylinder engine with a high rev ceiling - it has come clean with the new Z4 M Roadster, which will arrive in Australia in early in 2006.

Visually, only the avid BMW aficionado will be able to tell the difference between the Z4 M and a Z4 3.0i. The Z4's sleek shape is still there for all to see in the sporty M variant, with it's swept back greenhouse and short overhangs, while subtle changes to the car's front air dam (larger openings) and power lines on the bonnet hint at the potential power hidden from the eye.

BMW reckons that the new front apron forms a "distinctive X" with the xenon-equipped headlight clusters, which is vaguely evident if stare at it long enough, but a bit of a stretch of the imagination. They could have just said "it looks tough" and then people like me wouldn't be nitpicking for cosmetic updates.

Double-spoke 18-inch alloy wheels are another clue as to the Z4 M Roadster's acutely beefed-up capabilities, while the quad-pack exhaust pipes and modified rear apron - not to mention the classic 'M' badging on the flanks and rear - are perhaps the most obvious changes to the stove-hot rear wheel drive convertible.

Together with the high revving 3.2-litre motor - it has a banzai 8000rpm rev limit - the Z4 M Roadster also gets a 6-speed manual transmission, which, like much of the Z4 M Roadster, has been fine tuned by the M division to ensure it's rock solid reliability under the most extreme of driving conditions.

Furthermore, it was decided that because more than 250kW of power would be finding its way along the driveshaft towards the rear wheels, for what is essentially a relatively lightweight vehicle, the speed-sensitive 'M' diff lock should be employed.

Borrowed from the E46 M3 Coupe, this motor sports-inspired piece of kit ensures that the car maintains appreciable levels of traction even when cornering under high throttle inputs by diverting power to the rear wheel with more grip, by way of a multi-plate clutch.

And it'll need the M differential too, because with 252kW of power and 365Nm of torque of tap, the two-seat convertible is getting unnervingly close to supercar levels of performance, able to accelerate from zero to 100km/h in 5.0 seconds flat.

Take another five tenths off that time and you're looking down the barrel at exotica the likes of which would cost many multiples more than this manic machine from München.

The engine belts out 252kW (343hp) of peak power that hammers the crankshaft @ 7900rpm, with peak torque of 365Nm joining the high-pitched party at 4900rpm.

This leaves a rather fat 3000rpm power band between peak torque and power, helping the Z4 M Roadster on to a top speed of 250km/h. And if it wasn't electronically castrated, one could hazard a guess and suggest a top speed close to 300km/h, or about 185mph.

Like many German super sports cars these days, the Z4 M Roadster's chassis was fine tuned at the famous 20.8 kilometre northern circuit of the Nürburgring, which BMW goes on to label as the "most dynamic race track in the world". No arguments here.

To work in tandem with the severely increased power output of the M division's almighty 6-cylinder mill, and to post solid lap times at the Nürb, suspension changes were a must, and started with a 10mm lowering of the car. Increased negative camber was also applied to the wheels to deliver more traction during high gee cornering, while a new front axle with McPherson spring struts featuring forged aluminium track control arms has helped to reduce unsprung weight.

Seeing as the Z4 M Roadster has no roof cross member to keep the body nice and rigid, the front subframe has been reinforced by a 3mm thick 'thrust' panel that BMW says helps improve the entire subframe's lateral stiffness, which in turn promotes improved handling and power down response.

BMW has also firmed up spring and damping rates somewhat, aimed at giving the vehicle a more decisive on-road character, while fatter tyres will go a long way to increasing grip levels - particularly important considering the vehicle's significantly increased acceleratory ability. Up front, the Z4 M Roadster wears 225/45 ZR18 tyres, and on the rear are fatter and lower profiled 255/40 ZR18s.

Another upgrade to the chassis over the normal Z4 models is the brake system, taken directly from the highly-sought after and appropriately rare M3 CSL. As BMW's best and brightest concluded, "an extremely fast car requires extremely good brakes."

Up front the Z4 M Roadster gets 345mm x 28mm discs brakes, and 328mm x 20mm rotors at the rear, connected to a 10-inch brake booster. The result is that the Z4 M Roadster can decelerate incredibly rapidly, to standstill in just 34 metres (112 feet) from 100km/h.

The steering setup remains trad BMW, with the hydraulic power steering system that can alter it's ratio to cater for both high and low speed manoeuvres. On the topic of steering wheels, potential customers will notice black-faced dials with white numerics and red needles, while other M-specific items include the sporty tiller, gearshifter, door sills and some extra chrome touches here and there.

At the end of the day, the new Z4 M Roadster doesn't look very new, but will indubitably offer fans of the drop top Beemer a totally new driving experience, and history suggests that this car could be a big hit for the German luxury car maker; the 1997 BMW Z3 M Roadster recorded more than 15,000 sales in its lifetime.

BMW's preposterously quick 6-cylinder convertible sports car will make its official debut at the 2006 Detroit Motor in January 2006, and is expected to arrive in Australia in the first quarter of the year.