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.