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Wednesday 23rd November 2011


DRIVEN: MERCEDES SLK55 AMG

Does a big V8 make the SLK a credible driver's car?



The Mercedes SLK55 AMG - it's a hairdresser's hot rod, right? Well, the last Affalterbach example I drove was the original Black Series; an evil-handling thing with a fixed roof and 400hp. This new, non-Black Series SLK55 AMG is based on the third-gen car and deploys no less than 422hp.


In other words, this SLK55 AMG isn't necessarily a posing pouch. After all, as the only compact roadster fitted with a V8 engine it is potentially quite the tool. Admittedly there is also a slight danger you'll look a bit of a tool driving it, but with 0-62mph done in just 4.6 seconds, perhaps you won't be hanging around long enough to care. There's no denying that folding hard-top adds a certain cosiness to the open-air motoring experience, too - and we haven't even mentioned the torque vectoring brakes or the cylinder deactivation yet. Both of which are derived from Formula One.

Engine first. Given the familiar 5.5-litre capacity you get no extra fuel tokens for figuring out this new 'M152' V8 is closely related to the 'M157' bi-turbo that's been replacing the old 6.2-litre NA in bigger AMG Mercs since last year. Based on the same block, the M152 obviously gets a new air intake arrangement, but also new cylinder heads, modified valve drive, changes to the oil supply system and an 'optimised' aluminium crankcase.


It keeps the biturbo's stop-start system. Dull, we know, but it's tech like this that allows AMG to keep on banging out the big-cube V8s while maintaining Mercedes' relatively clean corporate conscience. This engine emits just 195g/km with 33.6mpg, some 30 per cent better than the previous SLK55 in spite of an extra 82hp, and quite astonishing given the performance.

In fact, it makes the M152 the most efficient V8 engine in the world right now. Yes, AMG does world-class efficiency.

Clearly, there's more than an upgraded alternator and starter motor assembly at work here. Which is where AMG Cylinder Management comes in. This cylinder deactivation trick, as previously seen on some American gas guzzlers, shuts down cylinders two, three, five and eight in a light-load situation between 800rpm and 3600rpm. Modern F1 cars do similar to save fuel in the pit lane, when stuck behind the safety car, and apparently even during slower cornering.


Up to 169lb ft is available at half capacity - it makes 398lb ft at 4500rpm total - but more pertinently, the engine snaps back to the full enchilada in just 30 milliseconds. The change is imperceptible, to the point where there's a modest "ECO4" indicator light to let you know when you're going green. Alternatively, disable all the economy aids by switching the car out of "Controlled Efficiency" mode (remember when "C" used to stand for Comfort?) and into Sport or Manual.

You're probably wondering what a V8 running on four cylinders sounds like, and to a certain extent so am I - because AMG has also taken the 'emotional sound design' a stage further with the SLK's exhaust system. For the first time on an AMG it includes "variably activated" flaps in the rear cans, which actually make the car damn near silent at a motorway cruise, even with the roof down. But listen hard when ECO4 is aglow before nudging the accelerator until it becomes ECO8 (something else that goes away in S and M), and you'll detect a deep drone becoming a deep burble.


You don't have to trouble the right pedal very much at all to wake up the V8 completely, however, and Controlled Efficiency does nothing to hold you back when you properly stamp on it. Eco mode or not, the SLK55 will rip the air asunder and send those of a more sensitive disposition running for the hills. At minimum flap the V8 has a loud, bassy bark, complimented by the usual AMG crackles and pops on the overrun - leaving little doubt about its natural tendencies.

But find a twisting road - and these were surprisingly plentiful along the Californian launch route - and some unexpected things happen. The engine isn't quite the dominant force in the SLK's package you might imagine; with no twin-turbo torque it needs working hard to really flatten you into the seat back, which, while arguably in keeping with the roadster spirit, does occasionally leave you wanting for decisiveness.


AMG also seems to have gotten a touch carried away with that emotional engineering on the exhaust - come on and off the throttle several times in quick succession and you'll realise it makes the exact same overrun sound on each occasion. It's like a drum roll, and so apparently artificial it becomes anodyne. Hmmm.

The AMG Speedshift Plus 7G-Tronic 'box does a generally decent job, but it's the chassis that should have you raising your eyebrows. Stacks of (dry) grip, a flat attitude that's unfazed by alternating cambers, and a notable degree of compliance make short work of switchbacks and sweepers - AMG's "Direct-Steer" modifications to the front end complimented by "Torque Vectoring Brake" at the rear. This tweaks the inside back wheel to deliver sharper turn-in rotation; McLaren has a similar system on the MP4-12C.


What the steering lacks in feedback it makes up with accuracy, while the ESP Sport Handling mode almost inevitably gives you just enough rope to hang yourself with if you're clumsy with the throttle exiting slower corners - and that's in the dry. The reality that you're driving a short-wheelbase car with a massive engine and some interesting weight distribution is unlikely to ever go away, but it doesn't particularly get in the way, either. And there is an AMG Handling package for more serious shenanigans, including stiffer suspension, composite front brakes and a slippy diff, alongside some additional trimmings.

The SLK55 AMG may not prove to be the most enduringly engaging roadster on the market. But it's now a very well-rounded product, spreading itself across a range of talents instead of just letting rip with a big gun. Progress - but here's hoping AMG doesn't take this new-found finesse too far.





 

Author: cjhubbard