BMW has been slotting its deeply impressive twin-turbo 2993cc diesel six into the 6-series cab for quite a while now, but this is the first time we've been able to get our hands on the new 635d Edition Sport.
This is an optional package that takes the £59,705 635d Convertible up to an impressive £63,275, making it comfortably the most expensive way to get this engine fitted to a BMW. The Edition Sport package brings some subtle styling tweaks, stiffer anti-roll bars for 10 per cent firmer front suspension and 30 per cent firmer rear suspension.
Think of it as a long-distance high-speed cruiser and the 6-series cab works a lot better. In fact, that's where the six-pot diesel begins to make sense. Weighing-in at 1935kg it weighs almost 80kg more than a Porsche Panamera, despite being a barely adequate 2+2 convertible, but the 6-series cab is more or less single-handedly rescued by what is possibly one of the greatest engines of the 21st century.
The looks might be a sticking point, however. The 6-series is the high-water mark of Chris Bangle-era design and therefore isn't blessed with the most graceful lines. The best we can say about it is that the design is that it's imposing. It would certainly be hard to call it pretty.
Oh, and it's just a small thing but why, in such an expensive car, is there not a cupholder? Or was it so ingeniously hidden I couldn't find it? Answers on a postcard please...
So the 635d Convertible Edition Sport is a capable, quick, refined and frugal vehicle, whose only real weaknesses are controversial looks and the apparent absence of a cupholder. But does that mean it's any good? It ought to, but that doesn't necessarily follow.
In the end it's really a question of who's actually going to want the 635d Convertible. Image-conscious boulevardiers are hardly going to want a 'd' on the rump of their car, however good the engine is. Those interested in the engine's economy are far more likely to buy it installed in a 3-series or 5-series. And those who actually want a decent-handling diesel 6-series really would be better served with the significantly cheaper coupe.
The 635d Convertible is actually a bit like communism; it's a nice idea in theory, but in practise nobody really wants it. There are precious few better engines out there, but there are plenty more appropriate applications of it.