Alpina B4 Biturbo Convertible

Alpina's appealing BMW M4 alternative arrives in convertible form with a more relaxed nature than BMW's M car but still impressive performance and handling

What is it?

It has been almost two years since the Alpina B4 Biturbo – Alpina’s alternative to the BMW M4 – arrived. But although we have driven, and liked a great deal, the coupe variant, a convertible version of Alpina’s take on the 4 Series has never crossed anybody’s path. Until today.

Alpina is a curious, fascinating car company. It’s completely independent of BMW, whose models it modifies and rebadges for public consumption, yet BMW not only tolerates it but also encourages it. Alpina has access to prototypes and sometimes - as with the 2.0-litre diesel D3 a few years ago - BMW even allows Alpina variants to roll down its own production lines. 

In terms of vital statistics, the current B4 Biturbo is remarkably similar to BMW’s own M car but its character is quite, quite different. An M4 is a relatively hardcore coupé and convertible that is as happy on a track as it is on the road. Alpina largely forgets the track part of that description and concentrates on making a softer, more rounded sports car.

The base model for the B4 Biturbo, then, is BMW’s 435i, upon which Alpina’s 70 engineers get to work. The single, twin-scroll turbocharger is removed from its 3.0-litre engine and Alpina puts in two turbochargers – a small one to get things going and a larger one for once they are. The resultant power and torque peaks are an M4-troubling 404bhp and 443lb ft. And whereas the M4 uses a dual-clutch transmission, the B4 Biturbo retains the 435i’s ZF eight-speed automatic, with its software tuned for the B4; and instead of flappy steering wheel paddles, there are nipple-sized buttons behind the steering wheel spokes. The steering wheel itself is thinner-rimmed than BMWs own, part of an interior and exterior makeover that also includes a distinctive front grille and standard 20in alloy wheels with bespoke – non-run-flat – Michelin tyres. The suspension gets Alpina’s own tuning, too.

What's it like?

Although the B4 Biturbo Coupé weighs 43kg more than BMW’s M4 Coupé, because of M division’s obsession with weight saving, the difference when it comes to the convertibles is rather less. There’s only 15kg in it, and that’s because folding metal roofs are heavy and, frankly, there’s not a lot you can do about it. A B4 Biturbo Coupé weighs 1615kg; the Convertible 1840kg.

It’s worth remembering because it makes a considerable difference to the way the B4 goes down the road, and around any corners that come with it. The B4 Coupé has a poise and fluency to its ride and cornering ability that has been blunted in the B4 Convertible. It has been a while since I drove a B4 Coupé but I think the Convertible’s ride is less settled, and I’m sure the body control isn’t as good.

That isn’t a criticism per se, just an inevitable consequence of adding nearly a quarter of a tonne to a car. For a start, that’s a lot of extra mass for the suspension to cope with and, for the most part, it’s additional bulk that arrives placed quite high around the rear quarter, which is not where you’d want it. Inevitably, it raises the centre of gravity, which in turn places greater strain again on the suspension, which faces a bigger struggle here to control the B4’s body movements. 

The B4 Biturbo attempts it well enough – for a convertible – but the caveat remains. If you want the best-handling B4, go for the Coupé.

But let’s forget comparisons from which the Convertible is never going to emerge well, and assume instead that you are hell-bent on having a four-seat, open-top performance car. Well, in the broader context, the B4 is still a very likable thing. It’s still fast: its 0-62mph time changes from 4.2sec to 4.5sec, which is still brisk in any language, and the slushmatic’s entirely smooth nature is quite in keeping with a convertible’s more relaxed character. The Alpina steers sweetly, with reasonable weight and a good amount of self-centring and straight-ahead stability that’s the hallmark of fast German cars, and as with the Coupé, a limited-slip differential remains optional. In hard cornering, you’d want it – more body roll means a lightly loaded inside wheel, and power that threatens to spin away harmlessly – but if you’re not going to do hard cornering, it’s unlikely to be worth the (near-£1900) expense.

Then, finally, there’s the interior, which is something Alpina always does well, bringing subtle highlights and lovely leather surfaces to a cabin that is already steeped in fine ergonomics and solid materials.

Should I buy one?

That interior alone may be enough to tip the balance in the B4’s favour over similarly priced and specified rivals. 

Then there’s the fact that its engine is sweetly responsive, its gearbox similarly slick and the whole caboodle oozes sophisticated, relaxed performance driving. 

No, dynamically the B4 Convertible is not as adept as the Coupé, but that’s about as surprising as finding out that it gets dark at night. This is a rarefied area of the market. There aren’t many cars that set out to do what the B4 does, and those that do are all charismatic and, in some way, compromised by the fact that their roofs fold (quickly and leaving minimal buffeting, in the Alpina’s case). Of them all, the Alpina remains solidly appealing.

Alpina B4 Biturbo Convertible

Price £62,950; Engine 6 cyls, 2979cc, twin-turbo, petrol; Power 404bhp at 5500-6250rpm; Torque 443lb ft at 3000-4000rpm; Gearbox 8-spd automatic; Kerb weight 1840kg; Top speed 187mph; 0-62mph  4.5sec; Economy 35.3mpg; CO2/tax band 186g/km, 34%