Bentley will launch its long-awaited twin-turbo 4.0-litre V8 engine at the Detroit motor show early next month, promising �an instantly recognisable growl�.The engine will initially be offered in special versions of the Continental GT and GTC, identifiable by their black grilles, red badges and �figure eight� exhaust pipes. The models will be positioned below existing 6.0-litre W12 Continentals,