When you're driving a style statement like the BMW 6-Series, you need firepower to back up the aesthetic salvo. To that end, BMW offers three tiers of performance: the wallflower six-cylinder, the lusty twin-turbo eight-cylinder, and the excessive hand-built V8.
Dropping the straight-six into the 6-Series keeps the price down and delivers adequate performance. But the truth is that for car of this stature, the 315 horsepower it delivers is far from satisfying. This is a flavor of automobile that drips with over-the-top goodness; a powertrain that's merely adequate simply falls short. Zero to 60 mph may come in a tick over five seconds, but a 6-Series with this engine feels as wrong as Usain Bolt in hiking boots.
The 4.4-liter twin-turbo V8, however, is much more in line with the character of the 6-Series. With 445 hp and 480 pound-feet of torque, it's a burly hulk of a motor that delivers the effortless power we expect of a car like this. Zero to 60 comes in just over four seconds as it sprints toward a top speed of 150 mph. Backing this engine is an eight-speed automatic transmission that pairs well with the car's intended mission. Shifts aren't lightning quick like a dual-clutch unit, but it's smooth and composed, resolute in its decisions and never second-guessing itself. Not only does this powertrain offer prodigious power across the entirety of the tachometer, it's delivered with the smoothness of a James Bond pickup line. Of the three available motors, it's easily our top choice.
If 445 hp isn't enough, there's the Alpina B6. For about 30,000 more than the 650i, the Alpina brings along a hand-crafted V8 making 600 hp and 590 lb-ft of torque. Running to 60 in just 3.6 seconds, it's the ultimate in overkill. But as nice as it is to have 600 ponies on tap, you can't help but think how the V8 in the 650i is nine-tenths of the Alpina for three-quarters less. There's just not enough additional audacity here to merit paying the exorbitant sticker price.
Even in B6 form, the 6-Series isn't a sports car, but rather a large gran-tourer. That means carving corners is off the table, but it'll cruise with a certain sublimity absent in many lesser cars. It'll whisk you purposefully to your destination and do so in utter comfort over even the most pockmarked roads.