Mercedes confirms SLK55 AMG’s new 5.5 V8

Published: 21 July 2011

Mercedes has announced details of the new eight-cylinder engine that will power the upcoming SLK55 AMG. It’s a 5.5-litre lump, and AMG promises that its new naturally aspirated engine delivers not just the performance of a V8, but also the ‘economical fuel consumption of a four-cylinder’.

How much power can we expect from the new SLK55 AMG’s engine?

The new hand-built engine (designated M152) is a close relative of AMG’s M157 unit. That twin-turbo 5.5-litre V8 was unveiled in 2010 and now powers the E63, CLS63, S63 and CL63, but unfortunately AMG couldn’t fit the bi-turbo engine under the bonnet of the SLK. So, the new SLK uses a natural aspirated version… 

The displacement (and thus the bore and stroke), the stop/start system and the direct injection system are all the same, but the M152 benefits from a new intake system, different cylinder heads, a revised crankcase and a tweaked oil supply system.

Peak power is 416bhp at 6800rpm, up from the 355bhp at 5750rpm produced by the 5.4-litre engine in the existing SLK55 AMG. There’s also 398lb ft at 4500rpm, up from 375lb ft at 4000rpm. And so you can hear the new engine better, AMG has developed its first sports exhaust system, with flaps in the silencers that remain closed at low speeds and under partial loads, but open progressively (either 15, 30, 50 or 100 percent) when you’re accelerating.

And with great power comes great responsibility?

That’s right, and this new V8 features AMG Cylinder Management, which shuts down cylinders two, three, five and eight under partial loads – provided you’re between 800rpm and 3600rpm, and have selected the transmission’s Controlled Efficiency mode. The instrument cluster lets you know when the cylinder shut-off system is active (flashing up 'ECO4' instead of 'ECO8'), and AMG says even with just four cylinders the 5.5-litre V8 still produces 170lb ft. Plant your foot and you have a V8 again, AMG claiming that at 3600rpm the re-activation process takes less than 30 milliseconds.

The result is that fuel consumption has improved from 23.5mpg to 33.2mpg. And CO2 emissions are down around 30 percent, from 288g/km to just 199g/km. 

Mercedes will unveil the new SLK55 AMG at the 2011 Frankfurt motor show in September. Also expected to be revealed is a diesel-powered SLK, featuring a 201bhp twin-turbo 2.1-litre four cylinder engine.

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By Ben Pulman

CAR's editor-at-large, co-ordinator, tallboy

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