Mercedes-Benz E400 Cabriolet 4Matic AMG Line 2018

Conventional wisdom has it that when buying a convertible you must go for the sweetest, most splendid engine you possibly can.

It’s not hard to understand why. Roof down you’re going to that much more exposed to its charms – or otherwise – and because such cars champion laid-back luxury you really should have a powerplant of commensurate depth and refinement. Unless you’re talking about a carbon-tubbed supercar, convertibles are also heavier than their coupe counterparts, so going with a punchier powerplant is of practical benefit, too.

For the Mercedes-Benz E-Class Cabriolet, that engine is the 2996cc twin-turbo V6 tested here. It crowns a range mainly consisting of turbocharged four-cylinder diesel and petrol options and does so because unlike the more compact C-Class and barge-sized S-Class Cabriolets, with the E-Class there is no Affalterbach-built beast badged ‘AMG’ to delight in. It’s an engine that touts 328bhp and 354lb ft, with the latter arriving usefully early at just 1600rpm.

Going for the E400 obliges you have AMG Line trim (which includes larger wheels, a dotted front grille and more aggressive bumpers and skirts) and 4Matic four-wheel drive. Meanwhile the only gearbox option is Mercedes’ 9G-Tronic unit, which we know to be superbly smooth in the coupe.

A bit of a behemoth, in truth. This generation of E-Class Cabriolet is longer than its predecessor and nearly 70mm wider of track. Not that the main controls aren’t suitably undemanding, it’s just that the size of the footprint and weight of the strengthened chassis are palpable from the moment you cast off the E400’s moorings and it slips gently into motion.

To get to that stage you’ll have swung open a long, immensely heavy door, slid aboard and probably marvelled for a moment at the loveliness of the interior. With the addition of some metal-dipped, roof-related switchgear at the front of the central armrest, it’s identical to the tin-top coupe, with an array of crisp digital displays juxtaposed against more conventional sweeps of wood, leather, and some plastic. It’s wholly becoming of a big-engined Riviera dandy with a price that’ll surpass £60,000 once you’ve ticked a few options boxes.

And it is a fine engine. With twin-turbocharging comes hearty torque at low engine speeds – ideal for stroking the E400 along with minimal fuss – and yet extend those half-dozen cylinders and you’re rewarded with a serrated chorus that’s unusually crisp for a forced induction setup. Supposedly, it'll fire the car to 62mph in 5.5sec, which is up there with the very quick hot hatches. Proceedings never feel quite that frenzied, mind, which we suppose is the intention. Meanwhile the claimed 39.2mpg is a bit ambitious in mixed driving, though you’ll manage 35mpg at fast cruise – far from an embarrassing figure.