National Road 124 runs through Portugal's Montes Novos region, and it's a near-perfect stretch of pavement for car testing. Located 45 minutes to the northeast of Faro, it not only offers gorgeous mountain and valley views, but more importantly, a hugely entertaining mix of straightaways and corners that will put a vehicle through its paces. It's the sort of road you'd dream about driving in a Mazda MX-5 Miata, Porsche 718 Boxster or some other nimble, topless sports car. Yet my steed for today's test is something much, much larger.

The 2019 BMW M850i Convertible is a big car. It's long and wide, and weighs in at a substantial 4,736 pounds (258 more than an M850i Coupe). Managing editor Steven Ewing praised the 8 Series Coupe's on-track performance last year, but I have my doubts about this large-and-in-charge convertible on such a technical backroad. Happily, my skepticism is put to bed after only a couple of corners.

It's a runner

With the M850i Convertible in Sport Plus mode, cranking the steering wheel in either direction is met with near immediate turn in, nice weight and actual feedback through the wheel, letting you know exactly what the Bridgestone Potenza S007 tires are up to. Frankly, the communicative steering is downright refreshing -- many recent BMWs have been lacking in this area.

In almost every situation, the convertible hardly feels out of sorts thanks to its rock-solid chassis, the Adaptive M suspension only giving way to a touch of body roll. There's plenty of grip with my test car's 20-inch wheel and tire package, helping to keep things surefooted. That's not to say the big convertible never plows through corners -- it will if pushed, or if you're driving like a buffoon. But for most spirited road driving, standard hardware like rear-biased all-wheel drive, rear-wheel steering and a torque-vectoring rear differential do an almost magical job of snuffing out understeer and masking the hefty curb weight.

Power is another reason the M850i feels lighter than it is. A 4.4-liter twin-turbocharged V8 brings 523 horsepower and 553 pound-feet of torque to the party. The latter is available between 1,800-4,600 rpm for serious, lag-free giddy-up off the line, or while exiting bends. The M850i Convertible will hit 60 miles per hour in a BMW-estimated 3.8 seconds. That's a touch slower than the (much lighter) Coupe's 3.6-second sprint, but one of the benefits of the convertible is that the pleasing, bassy exhaust growl is easier to hear with the top down. That clearer soundtrack is worth the tradeoff of a slightly slower acceleration time.

A ZF-sourced eight-speed automatic transmission bolts to the V8, providing excellently timed gear changes. The manual shift paddles are fairly responsive, not to mention satisfying to use. Together, the drivetrain combo is estimated to return 17 miles per gallon in the city and 26 mpg on the highway.

The drop-top life   

That the M850i Convertible is still more than up to the task of confidently blazing through tricky stretches of road is great, but let's be honest, this thing's more of a grand tourer. For that, it's still a winner. The multilayer fabric roof does a great job insulating the cabin from wind and road noise when up, and creates a still attractive silhouette to boot. In just 15 seconds, the top goes down at the push of a button, and can be operated at speeds up to 30 mph.

With the top stowed, the big 8 looks stunning, with its wide stance and flowing body lines. There's some visual attitude, too, with aggressive bumpers, a steeply raked windshield and shapely decklid. Inside, everything feels premium, with soft leathers on the dash, door panels and seats, and there's a tasteful slathering of flashy bits like a glass shift knob. There's not a cheap looking surface anywhere.

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The seats themselves feature both adequate side bolstering for hard driving, and support in all the right places to keep people comfy for long hauls. Wind buffeting isn't an issue at speed, either, with the fold-up deflector allowing me and my co-driver to converse without screaming at each other. The only interior hiccup is the tighter backseat: it's easy enough to get in and out of (with the top down), but legroom is in very short supply.

For cruising, putting the M850i in Comfort mode is the ticket to simmer down the drivetrain and soften the chassis. Throttle response isn't as jumpy, the steering lightens and the suspension is more forgiving. It's certainly not as cushy as, say, a Mercedes-Benz E-Class or S-Class convertible, but it's perfectly compliant for daily driving.

Top-notch tech

Completing this modern grand tourer package is a heavy helping of technology. Quarterbacking infotainment is the latest BMW iDrive 7 software controlled by a responsive 10.2-inch touchscreen or redundant controls on the center console. The system features crisp graphics and intuitive menus for navigation, a 12-speaker Bowers & Wilkins surround sound setup, Wi-Fi hotspot and Bluetooth. One year of Apple CarPlay capability is standard issue if you aren't an iDrive fan, though BMW will make you pay a subscription fee to use it afterward, which remains a head scratcher. (Android Auto, meanwhile, is nowhere to be found.) Gesture controls are also included, enabling you to adjust volume or accept and decline phone calls with hand motions. I know, it's a gimmick, but it works surprisingly well.

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A robust list of driver-assist tech is available bundled into a $1,700 Driver Assistant Professional Package. Checking that box on the build sheet adds adaptive cruise control with stop-and-go functionality, lane departure warning with lane-keep assist, and BMW's Extended Traffic Assistant system for partially automated, hands-on-the-wheel driving. If you don't pony up for the package, all 8 Series Convertibles still come with collision and pedestrian warning with automatic braking, automatic Parking Assistant, a head-up display and a 360-degree camera.

A do-it-all GT convertible

Like the Coupe, the 2019 BMW M850i Convertible doesn't really have a direct competitor. Its $121,400 base price, not including $995 for destination, makes it more affordable than the larger, more luxurious and more expensive ($134,300) Mercedes-Benz S560 Cabriolet. From a sizing standpoint, the 8 Series is closer to the E-Class Cabriolet that, in its most potent E53 AMG form, isn't as powerful or expensive, with 429 horsepower and an $80,350 price tag.

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One could possibly look to the new 2020 Porsche 911 Carrera 4S Cabriolet to match the Bimmer's sportier personality. The issue there is that the $134,650 base price is a fair bit richer for a car that doesn't pack the same level of standard content, power or luxury fixings.

With that said, the M850i Convertible truly does exist on its own island in the automotive world. It packs a brutal level of power, top-shelf cornering reflexes and a fantastic, luxurious ride. On top of that, it's attractive and offers a bevy of tech equipment. It might just be the most complete GT drop-top available this side of the Bentley Continental GT.