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AMG keeps it natural with SLK55

2011-07-21 23:01

KEEPING IT NATURAL: One of AMG’s lesser-known models will be the only one to retain its naturally-aspirated engine configuration.

Vehicle Specs
Manufacturer Mercedes
Model AMG SLK55
Engine 5.5-litre V8
Power 310kW
Torque 540Nm
The Mercedes-Benz SLK55 AMG has always been the red-haired stepchild of Affalterbach’s range.

Whereas its larger CLK and SL AMG siblings have traditionally garnered the bulk of open-top Mercedes-Benz hot-rod sales, the SLK55’s always offered a unique (and unheralded) roadster experience – as the only German compact executive two-seater with sun-seeking roof convertibility and V8 power.

When Mercedes-Benz revealed its third-generation SLK in January of 2011 a new SLK55 was expected in due course.

The bothersome question, though, was whether AMG’s seminal compact hot roadster would retain its fabled naturally aspirated V8 configuration, or switch to forced-induction – as nearly all AMG’s products are expected to do by 2014.

Well, the good news is that the new SLK55 AMG, due to be revealed at the 2011 Frankfurt auto show in September, will gain power and reduce emissions while keeping its 5.5-litre naturally aspirated V8.

Considering the S-Class, E-Class and CL-Class AMG models have already converted to Affalterbach’s new twin-turbo, 5.5-litre V8 (internal codename M157), how’d SLK manage to stay “natural?”

The answer is F1.


Although AMG is not intimately involved with Mercedes-Benz’s F1 team’s engine construction (that responsibility falls to Mercedes-Benz’s British-based high-performance engine division, formerly Illmor engineering), Affalterbach’s engineers have found F1-style cylinder deactivation technology the technical sollution to ensure its naturally aspirated engines conform to current emission regulations.

The new SLK55’s V8 is essentially an M157 specification engine sans its two turbochargers – a crucial design change as the SLK’s engine bay architecture would (allegedly) not be able to accommodate the 5.5 V8 with its associated turbochargers and elaborate exhaust.

With the forced-induction plumbing removed, a new cylinder head and intake manifold have been bolted into place, with mechanical drag reduced by a slicker oil system and redesigned crankcase.

Statistically the un-blown M157 engine (internal code M152) is good for 310kW (at 6800rpm) and 540Nm (at 4500rpm), bettering the current SLK55’s 265kW and 510Nm. Beyond the notable increase in power output, new SLK55’s expected to emit 30% less emissions and be (slightly) lighter on fuel thanks to start/stop technology and the ability to run on only four cylinders when peak performance is not required.

If you are a complete engine design anorak, the new V8’s trump card trivia is its direct-injection system’s 200-bar pressure rating – the highest of any naturally aspirated production engine…

SOUNDS RIGHT: SLK55’s new M152 V8 is the first AMG engine to feature dual exhaust flaps, operational above 2000rpm…


Best of all, though, is that the SLK55 will debut AMG’s new exhaust flap technology, a first for any Affalterbach machine.
Although an affront to automotive elocution purists, exhaust flaps are a necessary technical evil to ensure dramatic acoustics are balanced with contemporary European Union-mandated noise regulations.

The SLK55’s flaps are located in each of the 65mm diameter dual exhausts and open at varying increments (15, 30 and 50 degrees) depending on the duration of full-throttle driver input.

When the flaps open, exhaust gases cover a shorter distance and that characteristic V8 sound is writ large in a bellowing tone.

As more and more performance car engines make the switch to forced induction (or hybridisation) naturally aspirated V8 options are becoming few and far between. With the new SLK55, AMG’s guaranteed its customers something unique in the compact roadster market.

Much like the original SLK55, though, one gets that nagging feeling that the decision not to allocate sufficient engineering resources to ensure a successful fit of the twin-turbo 5.5 V8 (much as was the case with the M156 6.2-litre V8), has more to do with preserving the SL63’s status than actual technical feasibility…

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