What's new? Tempting as the 338bhp Z4 M Roadster is, there is an alternative that just might make you think again. Well, two actually. One, inevitably, is the Porsche Boxster S, which undercuts the M Roadster by a hard-to-ignore £4230. The other is a Z4 M stripped of its extra-macho suspension and tyres, M-diff and many other trappings of testosterone-honed hardness. Just a plain vanilla Z4, then?Not exactly. BMW has given the Z4 new front and rear styling, and introduced new engines, including the 2.5-litre straight-six in this 2.5si. The ‘s’ is significant. It adds 23bhp to the power output of the regular 2.5i, making an impressive 215bhp in total. This slashes the 0-62mph time from 7.1 to 6.5sec and increases the top speed from 142 to 149mph. Not quite in the same performance league as the Boxster 2.7 (0-60mph in 6.1sec and 157mph), but this time it’s the Beemer that holds the price advantage: even in Sport specification, the 2.5si costs £30,725 – £1595 less than the Porsche.What's it like? All right, the electric-helmed 2.5si doesn’t get the M car’s meatier hydraulic power steering set-up or trick rear diff for when you simply want to light up the rear tyres on your favourite bend, but what you get in exchange is likely to satisfy more of the people more of the time. The 2.5si comes with BMW’s sweetest six-cylinder engine, a decent helping of urge and the sort of petrol consumption (over 10mpg more than the M Roadster) that won’t put undue pressure on the rather small fuel tank.Should I buy one? The extra layers of comfort, refinement and delicacy don’t do the Z4 much harm, either. In fact, they suit the car’s innate character very well. The 2.5si could just be the optimum Z4.David Vivian
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