Mercedes-AMG has upgraded the flagship versions of the GLE and GLS SUVs with its new flat-plane-crank twin-turbo V8 engine as part of a range of mid-life updates.

Mercedes' performance division has been introducing the new mild-hybrid M117 Evo powerplant throughout its line-up amid a move away from plug-in hybrids to V8 engines.

While the outgoing versions of the GLE 63 and GLS 63 were powered by a previous-generation V8, the upgrade to the new 604bhp engine is intended to improve their efficiency and the driving experience.

Key to the new V8 unit is a motorsport-inspired flat-plane crankshaft, along with a revamped firing order for the cylinders, which is intended to reduce vibration and smooth out the power delivery.

There's also an upgraded fuel injection system, new air intake and exhaust ports, a reworked camshaft and a revised turbocharger housing.

The V8 produces 604bhp and 627lb ft of torque on its own but it is supplemented by a 48V mild-hybrid integrated starter-generator that can add an extra 23bhp and smooths out the delivery of torque.

The new V8 enables both of the 63 SUVs to accelerate from 0-62mph in less than four seconds and they have identical top speeds of 174mph.

Both cars also receive a range of other updates, including a revamped exhaust system that automatically adjusts the flaps to boost the sound of the engine.

In addition, there's an updated active ride control system to reduce body roll, while the front and rear driveshafts have been upgraded to cope with the new V8's high levels of torque. The GLS also features a rear axle locking system to boost agility.

The GLE Coupé and SUV and the GLS gain new front ends with a bespoke AMG grille, larger air intakes and revamped lights. The GLE 63 rides on wheels of up to 22in in diameter, with the GLS's rising to 23in