Merc SLK55 AMG's new 5.5 V8

Mercedes’ new SLK55 AMG for the Frankfurt motor show will be equipped with a new 5.5-litre V8 engine.

The hand-built 5461cc engine in the new Mercedes SLK55 AMG is a normally aspirated version of the twin-turbo 5.5 V8 found in the new CLS63 AMG and E63 AMG models.

Removing the two turbos is a necessity for packaging the all-aluminium 187kg V8 in the SLK, but as one insider pointed out: “It also wouldn’t do to have an SLK that’s faster than the SL.”

Power from the new 5.5 V8 has risen by 60bhp over the old 5.5 V8 in the outgoing SLK55 AMG to 416bhp, while torque has risen from 376lb ft to 398lb ft.

Advances have been made in fuel economy and CO2 emissions, too, the former improved by 30 per cent in the new SLK55 AMG over the old model. Combined economy is 33.2mpg and CO2 emissions are 199g/km. Stop-start is as standard.

The engine features an innovative F1-inspired AMG Cylinder Management shut-off system that allows the engine to run on just four cylinders under partial loads to help boost economy.

Read Autocar's first drive of the Mercedes SLK 250 CGI

The SLK55 AMG will also feature a new sports exhaust system with two independent rear silencer flaps. These stay closed at low loads and engine speeds below 2000rpm, and progressively open as the driver applies more throttle for a louder exhaust note.

Alongside the SLK55 AMG at Frankfurt will be a diesel version of the new SLK, the first time an oil-burner has features in the roadster.

The Mercedes SLK 250 CDI will be powered by a 201bhp four-cylinder turbodiesel that is already used in many of the staple Mercedes models.

A spokesperson explained: “There seems to be the necessary demand for diesel cabriolets. We’ve seen good sales of diesel E-class cabriolets and other manufacturers have had a lot of success with similar models, so it only makes good sense to add it to the SLK.”

Both models will be in showrooms by early 2012. Though not confirmed, expect the SLK 250 CDI to sit between the base 200 and the 250 petrol models at roughly £34k and the new SLK55 AMG to top the range at close to £55k.

Vicky Parrott, Mark Tisshaw