You'll already be familiar with BMW's M850i xDrive formula because we drove it as a coupe last October, and rather liked it. Now though, with summer fast approaching, it's time for its drop-top sibling to shine. Mechanically, the newcomer is much like the tin top; structurally though, the introduction of a folding fabric roof - alongside the prerequisite thickening of the sills to compensate - has made the convertible 125kg heavier.
Despite this weight gain, BMW says it has gotten no slower. An identical 3.7 seconds is claimed to 62mph, its brevity earned by the same marriage of 530hp, 553lb ft of torque twin turbo V8 and standard-fit all-wheel drive. Moreover, with BMW's carbon rich CLAR structure, fewer changes to the car's suspension have been required to counter the shift in strong points - which means that its maker is claiming a McLaren-like similarity in performance whether you buy an 8 Series with a roof or not.
Low speed driving through an Oxfordshire village would, you might think, reveal any obvious defects in the 8 Series convertible's rigidity (I've memories of the i8 Roadster and its creaking carbon tub, for example, on similar routes) but the far larger 8 Series Convertible has no perceivable chassis flex or scuttle rattle, even over the harshest of speed bumps. Pick up the pace and it's the same story; this thing is as stiff as you like - and that inevitably helps with ride, because the car's steel coil suspension setup glides over intrusions with confidence. Any thud you do register is a result not of the architecture, but rather the downside of fitting run-flat equipped 20-inch wheels at each corner.
There's little wind and road noise at normal pace either. Pick up speed and things remain impressively quiet, so much so that we'd hedge the soft top's cabin is quieter than those of most regular coupes and saloons. It's not Rolls-Royce-silent, of course, and there is some audible road noise on coarse surfaces - but mostly it's the bass of the 4.4-litre engine from up front that floods the cabin. In Comfort mode it's an ever-present force which doesn't insist on being loud or brash to compress time and space.
Click into Sport and even Sport Plus, and things don't dramatically change like they do in red-blooded M models. Sure, the V8's now more vocal - gravelly at low revs and thickening to a deeper, more aggressive tone higher up - but you're a far cry from the thunderous belch of an AMG eight. The way BMW's twin-turbo engine pulls, however, is right up there with anything shy of a supercar. It's only 23hp short of the BMW M5, after all, and makes use of the same eight-speed automatic to provide you with lag-less grunt above 2,000rpm. It's rapid, even once rolling, rest assured of that.
As ever though, straight line muscle doesn't necessarily translate into all-court athleticism. With its standard-fit variable steering, there's nothing in the way of feel at any speed or when the car's loaded up. It's easy to place the 8, but your palms are none the wiser about the surface beneath - and your right foot fares no better because the brake pedal is equally deadpan. That said, the car's xDrive hardware and those 245 front, 275 rear wide tyres provide so much mechanical grip that exploring the M850i's limits requires licence-threatening commitment. On a warm spring morning it's not possible to break traction, even at the back, which means you never really get a sense of the xDrive system favouring the rear axle. The front end does respond eagerly, but you're always aware of the convertible's mass as the body pitches and rolls slightly before the dampers step up to the plate.
Still, you can carry serious pace along a B-road, top up or down, and in a way that allows you to take real pleasure in the elasticity of a V8 that only sounds all the better for the open air. It'll feel too disconnected for anyone who prefers to tackle such a section enthusiastically, but that just confirms what the two-tonne kerbweight tells you from the outset: that this is a luxury soft-top grand tourer. Temper your driving accordingly and it feels much more natural; not with the mass-defying physics of the S-Class convertible, perhaps - but big and powerful and old school. In a good way.
It's possible that this outcome is a result of the M850i's position in the 8 Series line-up; with the upcoming M8 - which will also spawn a drop-top model - there's plenty of room for something which takes its comfort levels seriously. That won't make it everyone's cup of tea, of course - but there's still much to like, the engine and gearbox for a start, not to mention BMW's intuitive digital cabin technology and the convertible's purposeful good looks. A pleasant old waft, then really. We can wait for the M8 to blow us away.
SPECIFICATIONS - BMW M850I XDRIVE CONVERTIBLE
Engine: 4,395cc, V8, twin-turbocharged
Power (hp): 530@5500-6000rpm
Torque (lb ft): 553@1,800-4600rpm
0-62mph: 3.7 seconds
Top speed: 155mph (limited)