There's an important question to ask yourself when checking over a BMW Alpina B10 V8S - what's it for? Sounds flippant. But isn't. Because for ΂£5850 less than the ΂£57,850 Alpina is charging for the privilege of parking a 375bhp B10 V8S in your private, CCTV-guarded space, you could have yourself a 400bhp M5.

You don't need me or anyone else to tell you why the M5's special, which returns us to the question of just where the B10 V8S fits into the grand scheme of things. According to Sytner's Nick Godfrey, wise to all things Alpina on account of selling them for a goodly number of years, this highly modified 5-series is aimed at those customers who actively don't want an M5. Such people exist? Sure, reckons Godfrey. They want power and speed, but they don't want a race car. Ride quality and quiet cruising count to these people. So does an automatic gearbox. Exclusivity is an issue, too, but then I don't cross the road every day afraid of being mown down by packs of unruly M5s.

Alright, enough cheek, although with Alpina now an official satellite of BMW, it's worth clarifying this potential crossover of purpose. And knowing the B10's niche helps establish if it's any good at what it's supposed to do, even if one might suppose that with a 4.8-litre V8 grunting out 376lb ft of torque, it's supposed to go quite quickly.

Speed, and lots of it, is certainly one of the Alpina's more impressive party pieces. Not having to live with the electronic limiter that restrains the M5 to 155mph, the B10 swooshes on up to 177mph. The claimed 0-62mph figure is 5.4sec; knock off a couple of tenths to give you a rough 0-60mph, and it's not so far adrift from the storming 4.9sec evo wrung from the last M5 we figured. The pleasure from the B10 results from being able to repeat the feat time after time, thanks to the efforts of its Switchtronic five-speed auto.

Consistent with Nick Godfrey's assertion that the Alpina is a sort of smoothed-off M5, the B10 goes about its business with a softer tone of voice than its fabled stablemate. Aurally there's no doubting the V8's potency, and when the ZF auto kicks down to release all 375 ponies you get visions of Days of Thunder, but there's never a hint of the cackling beast that bellows beneath the bonnet of the M-powered Five. Yet it's not like you'll be disappointed in the way the B10 sounds or goes, particularly beyond 4000rpm when the revs stack up ever more swiftly and the soundtrack picks up its beat.

Alpina gets its engine blocks from the same supplier that makes them for mainstream V8-engined 5-series, but they're to the company's specific requirements and then filled with its own choice of reciprocating, rotating and electronic parts. In other words, the B10 unit is more 'exclusive' than 'modified'.

Alpina also chooses its own gearbox ratios and has developed a bespoke shift mechanism and change strategies, in the form of its Switchtronic system. As with various other semi-auto 'boxes around, including BMW's Steptronic system, Switchtronic gives you the choice of leaving everything to the 'box itself, or selecting your own gear using either the regular lever or small buttons behind the steering wheel spokes - right for up-shifts, left to change down.

In straight auto mode Alpina has programmed the box to slither up through the gears as quickly as possible, hitting the top cog even faster than a 540i manages. This is to promote the feeling of torquey laziness, and it works a treat. In fact, the gearbox is superb in both manual and auto modes, blending one gear into the next with such silken ease that you often hear the shift more than you feel it.

The 4.8-litre B10 replaces the older 4.6 model and in the brochure the text refers to 'stiffer' suspension this time around. 'Stiffer' is obviously a relative thing, because for a car that does more than 170mph and wears such broad tyres (245s front, 275s rear) of such low profile (35 front, a slender 30 rear), it barely shudders as it crosses urban surface nastiness. Only transverse ridges can rattle the B10's composure, but you'll forgive it because its ride is so excellent the rest of the time.

If there's a drawback to those trademark and preposterously wide Alpina 19in alloys that are now standard on the B10, it's that the front pair dull the steering a touch. More so at low speeds when the turn-in's a bit sluggish than when you're high-speed apex-crushing. Still, you quickly learn to compensate; you have to, really, as the B10's traction out of corners means that the next bend comes along in no time.
Traction control off and you can exit corners smokin', the fat Michelins - developed specially for this car - unable to restrain the muted fury of all that torque and power. Most B10 owners are unlikely to be stabbing the traction button, though, so an overcooked corner will result in a frisson of understeer, an easing of thrust, tightening of line, then a belt of solid acceleration.

Once upon a time the name of Alpina had slightly garish connotations, its distinctive (and unnecessary) sidestripes a crucial accessory to anyone who had, or wanted the world to think he had, a go-faster BMW. By way of contrast, in the B10 V8S, Alpina now builds one of the world's best four-door Q-cars. It isn't an M5 and doesn't pretend to be. It's wickedly quick when you're in the mood for entertainment or feel like blowing off steam, relaxing and refined when all you want to do is mosey home and listen to the news on Radio Four. The B10 won't suit everyone, but if you think you gel with its profile, you may well find it a match made in heaven.