It's the new twin-turbo 3.0-liter six found in the E53 and CLS53.
Matt Becker, Aston Martin's chief engineer, recently drove a Mercedes-AMG powered by the company's new turbocharged 3.0-liter inline-six. He told Australia's Wheels magazine, "We got to drive one in Stuttgart at their test facility a few months ago, and it's a very impressive engine for sure."
Becker said that Aston Martin would likely not modify the engine's tune in any way, retaining the 435 horsepower and 383 lb-ft of torque as it does in both the E53 and CLS53. Throttle response and gear ratios could change in an Aston Martin application, though, Becker told Wheels.
Calling the engine "complicated and clever," Becker also said that emissions regulations were a concern, necessitating the switch to a six-cylinder. "Previous Astons have had six cylinders - a long time ago," Becker told Wheels, referring to a period between 1950 and 1969 when all production Astons had six-cylinder engines. "And it's something that could fit with the brand in the future."
It appears that Germans are the go-to manufacturers for inline-sixes. The newly unveiled Toyota Supra will also use a straight-six engine, but though unconfirmed as of yet it is likely it will be a BMW-sourced powerplant.
This would not be the first Aston Martin to utilize a Mercedes-AMG engine. The DB11 sports an Mercedes-AMG-developed 4.0-liter, twin-turbo V8, under technology sharing agreement signed in 2013.