There's essentially three tiers of performance from the 6-Series: the relatively sedate six-cylinder, the more brutish V8, and a hand-built eight-cylinder that's all muscle. The base 3.0-liter twin-turbo six-cylinder is, well, the base engine. While its 315 horsepower is enough, it's missing that selfish authority which makes a car like this so enjoyable to drive. When passing prowess on the highway is merely on par with a family sedan, your impressive luxury sedan suddenly doesn't seem so impressive.
This won't be a problem in the V8-powered 650i. The 4.4-liter engine is smooth and fast, all 445 hp ready to be released by a single shove of the gas pedal. Whether around town or on the highway, there's enough gusto here to make any sort of acceleration effortless. It's the easy favorite of the three choices here, thanks to the relative affordability and the smooth-operator performance.
The Alpina B6 model uses a hand-crafted version of that 445-horse V8. In this application, the magic makers at BMW's tuning house coax out 600 horses and 590 pound-feet of toque. As one might expect, it is the most potent engine here, and will run to 60 mph in only 3.6 seconds. Though it might turn this big four-door into a gazelle, the B6 does demand a hefty $30,000 premium over the lesser V8 in the 650i. Unless money isn't an issue, we feel that the marginally better performance of the B6 isn't quite worth the big leap in sticker price.
All versions of the 6-Series have multi-adjustable suspension, and toggling between the three different settings bring noticeable differences in feel and response. Overall, the hardware underpinning the 6-Series does an impressive job of trying to be all things to all people, though it's still no sports car.