The Mulsanne Speed is the most powerful four-door in the world, right?Well, that's what Bentley claims, although I suspect Brabus might have something to say about that. Either way, this is a muscular, powerful machine. But it needs to be if it's to hurry along 2,685kg of body mass.How much more potent is it than a standard Mulsanne?Before I get started on that, I should point out that there's not much intrinsically new about this Mulsanne - it was facelifted last year, the biggest alteration of which was the adoption of the prison bar grille and a gentle realignment of the headlights.Right, so the Mulsanne is Bentley flagship - it's biggest, costliest motor. It's over five and a half metres long, and it's a rolling palace\: decorous, stately, dignified and aloof.This Speed version's 6.75-litre twin turbo V8 is lifted from 505bhp and 1020Nm to 530bhp and 1100Nm, boosts of just five and eight per cent respectively.Bentley claims the Speed lops 0.3secs of the 0-96kph (now 4.8secs) and adds 10kph at the top speed (now 306kph). Not massive gains, it must be said.Difference in the detail, then?That's about it. The Speed is more a change of ambience than a gain of speed. So instead of a leather gearknob, here it's knurled metal. The wheels are 21s, not 20s, the headlining is diamond quilted, the pedals are alloy, the rear lights are dark tinted and the exhausts are rifled, which just sounds cool.The suspension is lowered and 'sports tuned', but it's still not exactly a driver's car. Even if you press the handily-located manual gears button on the steering wheel and twist the driver settings knob to Sport, this is not a car that you can hustle - and, more importantly, feels right being hustled.Two things prevent you barreling about\: 2,685kg, and your own sense of propriety. Instead you 'make progress'. You can do this at what we might describe as a 'fair lick', but to be frank, once you get a sense of how the prow rises and surges under load, of the sheer physicality of this object and of how leisurely the brakes are, you soon get a sense of what real momentum is. And it's almost alarming. And you don't want to be alarmed in a Bentley.So it's not a driver's car, then?I'm assuming you're thinking of words like feedback, tactility, response, precision? No, none of those apply. Muscle it along a country road and you'll get heave, the steering will go woolly and the car will feel awkward and confused. You won't have got the engine to hit hard and fast, nor the brakes to bite properly and the suspension won't be keeping roll and motion in check.Instead the best way to view this is as a driving 'experience'. There's nothing else quite like it. You guide it with your fingertips, you allow the gearbox to make the decisions, you ignore S-for-Sport and leave it in B-for-Bentley mode, and pretty soon you'll be having a ball.The car will moderate your attitude for you, because that is what Bentleys do. They calm and relax you in a way that top end Mercs and Audis just don't.But it must be fast in a straight line at least?I'd rather describe it as 'not slow'. Fast these days is sub-four second to 100kph I reckon, and the Speed's a mile off that. Because I could, I ran some numbers on it. 4.7 seconds to 96kph was the result (nothing as uncouth as launch control here), and on past 160kph in 10.9 seconds. The standing quarter-mile took 13.19 seconds with a finishing speed of 175kph.This is all broadly similar to a Ford Focus RS, but think of the work its driver will be having to put in, rowing the gears, smashing the pedals, while you just let your great steamship lunge at the horizon, engine rumbling and churning, but otherwise entirely unstressed. Like I said, it's the experience that matters here, not the numbers.What's the engine like?Sort of marvellous. There's something to the way it accelerates from 1,500rpm - you hear turbos spin, then there's this deep cross-channel ferry thrumming that's less about cylinders going up and down and more about the crankshaft spinning. No real pulses from the exhaust, just a smooth rumble. And the urge, this endless, deep, burgeoning expansion of power.If you do demand kickdown from the gearbox, you can feel the transmission do some shuffling, and the turbos begin to respond, but almost before it's finished doing that, you look down at the speedo and are aware you've already piled on another 50 or 60kph. It's peculiar, a distortion the audio/visual continuum.And the slowing down?I've alluded to this already. The Mulsanne Speed doesn't like being stopped. It prefers momentum, so be careful how much you build up, as shedding it takes colossal effort and the dissipation of vast amounts of energy. I got it stopped from 160kph in 94.91 metres, which is almost 10 metres better than a Bentayga.But if you're doing it because someone has ill-advisedly filed out of a side turning into your path, you are instantly very aware of every kilogram you're carrying and just what a mess that high, bluff grille is going to make of the back of their Fiesta.You wouldn't even notice the impact inside, though...Well, provided you were looking out the windscreen I think you'd struggle to miss it. Although it must be said there's plenty to distract you in here, mainly because it's quite distracting. Bentley has tried to integrate elements of Audi's virtual cockpit in here, but without much success. The screen between the dials is too small and why do those dials run upside down, 0 at the two'o'clock position?Then there's the USB connection. There's only one and it's in a pop-out draw above the heating controls. So ergonomically, it's flawed. But you don't care (much) because of the quality and craftsmanship and materials that swaddle you. You feel unworthy to be in here.Especially the back seat which is, let's face it, the one to be in. But then if you're not going to drive, you might as well be in the Extended Wheelbase model...Choices, eh? Precisely that. I came away from the Speed liking the cosmetic enhancements of it, but feeling that the more sporting looks are writing cheques the dynamics themselves can't quite cash.Is this a problem? Nah, any Mulsanne Speed owner who actually minds about driving will have a garage full of tasty stuff that helps them get their rocks off. This one's for when they wants to feel lord of all they survey.