What is it?
It’s the latest offering from Alpina, the slighly left-field family firm from Bavaria that somehow manages to fettle BMWs without annoying the German colossus. Indeed, the D4 Convertible is built, along with every other Alpina, on the main BMW production line - and that means that it has to slot into a niche that doesn’t really tread on the toes of the mainstream company’s products. Talk about walking a tightrope; Alpina’s 70-odd engineers must surely be matched by a similarly strong, and equally intelligent batch of product planners.
Anyway, it will probably not shock you to read that the D4 Convertible gets the same retuned version of BMW’s N57 straight-six twin-turbodiesel as the D3 saloon and estate and the D4 coupé. It’s not short of heft, then, with 345bhp and 516lb ft of torque - enough for a 0-62mph dash in 5.0sec and a top speed of 171mph. Perhaps more important, that wall of torque is available between just 1500rpm and 3000rpm, giving the D4 potentially great real-world flexibility.
For this you pay a smidge under £55k - which is around 10 grand more than BMW's 435d Convertible.
What's it like?
In isolation, the open-top D4 feels not one bit slower than the D3 saloon. The turbodiesel picks up not much beyond 1000rpm and then huge reserves of torque kick in; this is one of those cars where you can think you’re accelerating hard and then find you’ve got another 20% of throttle pedal travel remaining. The Alpina version of the ZF eight-speed transmission is slick, too, with rapid, smooth shifts and quick responses if you switch to the buttons mounted at the back of the steering wheel. You want paddles? Tough - Alpina doesn’t offer them.
The straight six is happy to be revved to beyond 4500rpm and Alpina’s exhaust technology actually helps to give it a curiously appealing note, particularly for a diesel. When you’re cruising, though, it’s near-silent - to the point where you’ll need to keep a careful eye on your speedometer (or the head-up display, fitted to our test car as an option). You will be cruising way beyond the British motorway limit before engine noise becomes really noticeable - although there is a bit of wind rush from around the side mirrors and, as is usually the case on Alpinas, the 20in tyres do come with a bit of road roar.
The stonking powerplant doesn’t seem particularly bothered, then, by the fact that the convertible weighs about 130kg more than the coupé - but the rest of the package isn’t quite so comfortable with it. The D4 still has a suppleness to its ride that is beyond mainstream BMWs - let alone those fitted with any wheels approaching those 20s - but it does feel a teeny bit less deft over bit potholes. Similarly, fast, open roads are dispatched with gusto and real poise, helped by steering that has a satisfying weight to it at speed - but twistier stuff requiring sudden changes of direction demands a little bit more consideration as you factor in the D4's weight. We’re talking tiny percentages here, but there’s no doubt that the D4 Convertible is happier being a fast cruiser than a sports car. The small but noticeable amount of shimmy that you get with the roof down emphasises this.
It’s also worth remembering that the convertible's weight affects not only that acceleration but, perhaps more painfully, the CO2 emissions. On its own, this car’s figure of 156g/km seems impressive enough for something this fast. But the coupé manages 139g/km - enough for it to sit four whole bands lower on company car tax.