Original plans for a model in the mould of the GT four-door were first aired in a series of patent applications filed by AMG in 2012. At the time, plans centred around a long-wheelbase version of the SLS featuring two gullwing doors and a pair of small, rear-hinged rear doors.
More recent plans for the performance saloon subsequently became more closely linked to the GT. But while the latest addition to AMG’s line-up will share key styling cues and elements of its mechanical package with the company’s headlining sports car, sources say it will be based on a different platform from the bespoke aluminium structure used by the GT.
The basis for the GT four-door is parent company Mercedes-Benz’s modular rear architecture (MRA), as used on the C 63, E 63 and S 63. It has been specially modified for the four-door GT, with what AMG insiders describe as 'unique' wheelbase and track width measurements.
Other dedicated features include the greater use of lightweight materials within the floorpan and body structure, most notably aluminium and hot-formed high-strength steel, the adoption of which is aimed at lowering the weight of the new model to below that of Mercedes-AMG’s existing MRA-based models in the search of greater performance potential and enhanced handling characteristics.
Power for the 2019 model will come from the latest evolution of AMG’s twin-turbocharged 4.0-litre V8, already confirmed for the new E 63. Among its developments is the adoption of new twin-scroll turbochargers mounted between the cylinder banks. These new units are claimed to provide improved induction qualities over the more conventional ones used in the existing version of the M177-designated engine.
The new engine is tipped to be offered in two states of tune from the start of sales, with both standard 4Matic+ and S 4Matic+ models. In initial range-topping S 4Matic+ guise, the GT4's engine output is expected to top the 603bhp of the new E 63 S 4Matic+.
It is also understood that AMG is considering coupling the engine with a newly developed integrated starter generator (ISG). The move would not only provide it with the ability to deliver short periods of added performance via a 20bhp electric motor mounted within the gearbox, but also to offer instant start-up, with what one engineer with knowledge of the new system described as 'seamless' stop-starting.
The ISG operates via a 48V electrical system, suggesting the GT4 could adopt a moderate-sized lithium ion battery as part of its arsenal of high-tech developments.
The engine will also feature dynamic mounts designed to suppress load changes during hard cornering and be combined with the latest version of AMG’s Speedshift MCT automatic gearbox.
This transmission will be coupled to the latest version of the 4Matic four-wheel drive system, which features a fully variable apportioning of power between the front and rear wheels. It will also incorporate a ‘drift’ feature that allows the driver to channel power exclusively to the rear wheels, in a move aimed at promoting oversteer.
Also likely is a less powerful six-cylinder variant of the GT four-door powered by Mercedes-Benz’s new turbocharged 3.0-litre in-line engine, likely in combination with the aforementioned ISG.
The Panamera rival will have a largely bespoke suspension system that is being developed to provide unique dynamic characteristics, with greater camber adjustment and wheel control than those delivered by the suspension used on other MRA-based AMG models, along with revised steering rates. It is not yet clear whether the GT four-door will include the three-chamber Air Body Control air springs used on the latest E 63.
AMG is on a roll at the moment, with booming global sales. Moers, who was made chairman in 2013, said the Mercedes performance division is approaching the size that Stuttgart neighbour Porsche was in 2010 before the Macan SUV was launched.
In recent years, AMG sales have boomed, with sources suggesting that the launch of models such as the turbo 3.0-litre V6-powered C 43, E 43 and GLC 43 will help to push AMG volumes to well over 80,000 units this year.
The AMG line-up currently comprises 31 models, with plans to expand to 48 models.
Additional reporting by Julian Rendell