The development version of the BMW 8 Series convertible was spotted near Munich. Picture: MICHAEL TAYLOR
The development version of the BMW 8 Series convertible was spotted near Munich. Picture: MICHAEL TAYLOR

Soon after the debut of the BMW 8 Series coupe, we caught the Bavarian brand’s engineers testing the 8 Series convertible.

Its rough shape has been seen before in patent filings but we spotted a prototype on the autobahn near Munich, with its engineer enjoying the sunshine while testing the four-seater.

The big daddy coupe has already begun production at Dingolfing in Germany, and made its dynamic debut at the Goodwood Festival of Speed last week (Motor News took a ride up the famous hill in it), with deliveries due to start at the end of 2018 in some markets.

But the convertible has been harder to spot. Alongside the 8 Series Gran Coupe four-door, the convertible has been absent from BMW’s public statements.

The Gran Coupe, closely related to the M8 Grand Coupe Concept that stunned the 2018 Geneva Motor Show, is slated for production late in 2019. The two-door convertible may even beat the four-door coupe to production, but not by much.

While visibly identical to the coupe from the front end to the A-pillar, what we saw was a convertible that is unique from there to the back end. Where the coupe is long and sleek, the convertible moves to a flatter tail.

It has its own A-pillars, strengthened internally rather than externally to preserve the windscreen angle of the coupe, and sources suggest it has thicker windscreen glass, too.

The boot lid is flatter, without the need to accommodate a sloping rear glasshouse, though it finishes off with the same overbite flourish as the hardtop.

It retains the wind blocker system its predecessor also used, eschewing such wind diversion devices as Mercedes-Benz’s Air Cap to divert airflow up and over the cabin. This inevitably means the rear-seat occupants will find it fairly windy at speed.

There will also be a different silhouette from the coupe, with the folding cloth roof sliding back into a permanent slot beneath the boot lid and fixing in the up position with cloth buttresses joining the rear quarter panels on either side.

The car we stumbled on used two fat exhaust pipes, indicating it was running the M850i xDrive powertrain, with 390kW of power and 750Nm of torque from its new 4.4l, twin-turbo V8. It’s claimed to hurl the hardtop to 100km/h in 3.7 seconds, so the convertible should reach the same in less than 4.3 seconds.

The diesel powertrain option of the new coupe, the 840d, should also sneak inside the cabriolet, with 680Nm of torque from the in-line six-cylinder motor, and fuel consumption should rise marginally from the claimed 6.2l/100km in the lighter hardtop form.

Both variants use eight-speed automatic transmissions hooked up to a variable all-wheel drive system that can switch quickly from a pure rear-drive stance into a grippier torque spread as needed.

Unlike the 6 Series convertible it replaces, the 8 Series version will sit on its own standalone platform, rather than rely on the 5 Series architecture.

It still employs a four-link front suspension and a five-link rear axle, riding on 20-inch wheels and tyres. It is expected to use the same suspension hardware as the M850i, right down to its variable damper control, electronically controlled dampers, adaptive power steering and limited-slip differential.

The coupe is 4,843mm long and the convertible will either shed or gain only single-digit millimetres from that figure, and will retain its 1,902mm width. It is slated to lose some of the coupe’s 420l boot capacity.

Inside, it uses the coupe’s 12.3-inch digital instrument cluster and stand-up 10.25-inch infotainment unit, while there are LED head and tail lights.