What is it?
It's the new Alpina B5 Biturbo, the latest creation from the BMW-based manufacturer and the first chance weve had to sample a 'performance' version of the new 5-series.
Recent Alpinas have avoided using the same engines as M division products, but the B5 will come pretty close to going head to head with the forthcoming M5. It shares the same basic 4.4-litre V8 engine (the inlet set-up is different), equipped with two retuned turbochargers.
In Alpina tune the unit produces a flat 500bhp and 516lb ft. That's enough, says the firm, to take the car from 0-62mph in 4.7sec and onto a gloriously unrestricted 191mph.
Unlike the M5, which is likely to use a dual-clutch transmission, the B5 gets an eight-speed automatic. But the unit has been developed by Alpina and ZF, and it uses a trick way of speeding up shifts when you're in one of the car's sportier modes.
Instead of merely retarding the engine timing, the B5 uses its direct injection tech to merely cut off a cylinder or two. Alpina says the system cuts gearshift times in two, to around 180ms.
What's it like?
Jaw-droppingly fast. We drove the B5 from Alpina's base in Buchloe to the UK - in time for its appearance at the Goodwood Festival of Speed - and with an empty stretch of autobahn in front of it, it's supercar quick.
Alpina's claim of 0-62mph in 4.7sec feels conservative - and as for the top speed, well, we saw an indicated 190mph with three and their luggage aboard, and it was still pulling strongly.
Truth is, though, that making a 5-series with this engine feel strong on a motorway was never going to be the biggest challenge faced by Alpina's engineers. More testing is the task of making it handle on poor surfaces and twisty roads, and here the firm's modifications - lower suspension, non-runflat tyres and comprehensive reprogramming of BMW's adjustable dampers - pays off.
We wouldn't say the B5 has precisely the same level of agility and subtlety as Jaguar's XFR - but it's not far off it at all, and that means it's very good indeed. Revised steering programming makes the wheel feel lighter at low speeds than a 5-series, and weightier when you're pushing on, and it's a more satisfying set-up than BMW's.
With the car in Sport or Sport+ set-up the ride is firm but still compliant (no runflats, remember, just Michelin Pilot Sport 2s, or Pilot Super Sports on the final cars) and body control is excellent.
That trick transmission works well, too; we can't think of many dual-clutch units that beat it on speed and smoothness. The engine has massive urge from beneath 2000rpm, and if you push on it makes a great Nascar-esque soundtrack. It's perfectly docile for everyday use, though; cruise along at 80mph (barely 1900rpm) and it's silent.
Should I buy one?
At around 70k, the B5 will sit above the Jaguar XFR on price, but beat it hands down on exclusivity. Even our car - production prototype number one - was beautifully finished, the spec is relatively generous by BMW standards and Alpina can also offer a personal service matched by no mass manufacturer.
Of course, you could be tempted to wait for the M5, which will offer a more hardcore experience but, almost inevitably, more compromises on ride quality to boot. But if you're more sold on a 5-series than an XF, we'd be sorely tempted to put down a deposit now. Even by Alpina's relatively high standards, this is the firm's most complete package ever.
[autocar]
I'd have sex with that thing all day everyday 
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