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.
The roof itself is standard BMW fare, so it opens and closes in 20 seconds. With the roof down it’s possible for the front occupants to have a conversation at more than 30mph, but you will need to speak up when you get to motorways. Alpina does sell a rear wind deflector, which does a decent job, and you can also specify the front headrest heaters, which use hot air to keep your neck warm in cooler conditions.
Should I buy one?
If you’re firmly at the ‘keen driver’ end of the Alpina spectrum, you’ll probably want to pass on the convertible and stick with the slightly lighter, more agile and more supple D4 coupé (or, indeed, the jaw-dropping D3 saloon). There is little doubt that the complex metal folding roof brings a few minor compromises to the D4’s package - enough to knock off half a star here from the fixed-head model's rating.
However, that doesn’t mean that the D4 Convertible is a weak link, or an undesirable car. It still possesses the core strengths that need to run through every Alpina - effortless pace and greater comfort than the regular BMW on which it’s based, coupled, of course, with styling additions like those trademark 20in alloys. In many ways, the D4 Convertible is one of the most rounded models the company produces - a supremely fast cruiser, capable of leaving the UK in the morning and arriving on the Cote d’Azur the same evening, roof down and with only a single fuel stop. If that sort of ability appeals, it certainly merits strong consideration.