Perhaps the most significant thing about the X6 M50d is that - along with a similarly-equipped X5 - it's the equal first BMW to be fitted with a rather spectacular diesel engine.BMW has quite a history of creating diesels, of course, and there is nothing unusual about the three-litre capacity of this one. What makes it special is that it has no fewer than three turbochargers.Why three? Well, although variable geometry technology has done a good job of improving turbo behaviour, the basic problem with turbocharged cars is that you fit either a small compressor which reacts quickly but doesn't contribute much power or a large one which gives lots of power but takes a while to get up to operating speed, a characteristic long known as "lag".Several manufacturers, BMW among them, have attempted to get round this by fitting two turbos of different sizes, or in some cases one turbo and a mechanically-driven supercharger. With the M50d, BMW has extended the idea by 50%, giving its engine two small (and variable-geometry) high-pressure turbos and one large low-pressure one.The effects are impressive. Since there is nearly always some form of turbocharging going on, there is as near as makes no difference no lag at all. And there is an awful lot of power. At full blast, the M50d engine produces a spectacular 381bhp, giving the bulky M6 a 0-62mph time of 5.3 seconds which puts it well beyond hot hatch territory.Official fuel consumption is noteworthy, too, at 36.7mpg. This equates to 204g/km of CO2 emissions, and although the M50d will be taxed accordingly (£270 annual from year two onwards) I suspect that owners will be lucky to beat 30mpg if they're going to use anything like the available performance.Still, it's a very fine engine with a stirring noise when it's being pushed hard, and I'd like to see it appearing in other BMWs in the near future. The problem, in the current context, is that I wouldn't want the X6 that goes with it.First of all, I think it's one of the ugliest cars that BMW or anyone else is currently building. The first time I saw one on the road, approaching it from the rear, I thought until I got close enough to identify it correctly that I was looking at a 5-Series that had been rear-ended by a bus.The back seems to have been designed specifically to prevent anyone seeing out, and the windscreen pillars are very wide, so in general visibility is dreadful, though the huge door mirrors and optional Top View camera system do help.Standard equipment on the M50d includes self-levelling air suspension with active anti-roll, and this does an excellent job of coping with the power the engine can throw at it. The ride, however, is very fussy, making this a much less relaxing car to drive than the even more powerful, though admittedly much thirstier, supercharged Range Rover Sport.You might find that an unfair comparison between cars in different classes, but they're not really. The Sport is less than £7000 more expensive in standard form, which is only about 10%, and it's quite easy to jack up the price of the BMW. The test car, for example, came with £11,835 of options, some of which are startlingly priced. Does a head-up display really need to cost anything like £1015, for example?As with most BMWs, the interior is very dull compared with that of the opposition. Taking the Range Rover as an example once again, if you sat in the Sport and then in an X6 you'd damn well expect the BMW to be £7000 cheaper at the very least.No X6 for me, then. But I can't wait to see what the triple-turbo engine is like in, say, a 5-Series. That would surely be a car to savour. Engine 2993cc, 6 cylinders Power 381bhp Transmission 8-speed automatic Fuel/CO2 36.7mpg / 204g/km Acceleration 0-62mph: 5.3 seconds Top speed 155mph Price £62,260 Details correct at publication date