Mercedes-Benz SLK55 AMG 2012 Review

What we liked
>> Superb engine note
>> Unprecedented economy
>> Stylish interior

Not so much
>> Still not a paragon of handling...
>> ...Nor of mid-corner grip
>> Lifeless steering

The biggest mystery of the stock Benz SLK is why Mercedes decided to make the C-Class Coupe the sparkling handler it is and left the folding-roofed convertible feeling clumsy and stodgy.

The biggest mystery of the SLK55 AMG isn’t so much that (though the stodgy handling remains, in spite of AMG’s best efforts), but how it is so blisteringly quick and still manages fuel economy that puts some fours to shame.

The magic of cylinder deactivation isn’t new, but it is new to the world of the fast, luxury convertible and it allows AMG to shoot the SLK to 100km/h in 4.6 seconds and still post a combined economy number of 8.4L/100km.

While neither number is stunning in isolation, they are astonishing together. The sprinting time is about what you’d expect of a sporting V8 'anything' these days, while the economy number is mid-sized six-cylinder or even inefficient four-pot country.

So exactly how has AMG managed to combine a 310kW, 5.5-litre V8 with 195 grams of CO2 emissions/km in an engine that uses 30 per cent less fuel than the (slower) car that preceded it? It’s gone the same way as Audi’s S8, turning its V8 into a four-cylinder engine whenever all that V8 power and torque is redundant, and then it throws in auto start-stop, just to make sure of it.

It works in a complicated and expensive way, which is why cylinder deactivation has taken so long to arrive in any meaningful numbers and has yet to impact the mass market.

A close relative of AMG’s twin-turbo M157 V8 (that’s the one in the E63, the CLS63, the S63 and the CL63), the M152 uses a different air intake duct, new cylinder-heads and different oil supply and valve drive systems, plus a different crank. Some of those changes have been made for reasons of economy, others were mandated by the chase for economy.

With direct fuel injection running at a stratospheric 200 bar, it continuously adjusts its camshafts and helps itself to a 12.6:1 compression ratio and still spins beyond 7000rpm.

While it will sprint hard, it will also drop out the second, third, fifth and eighth cylinders whenever you’re in C (Controlled Efficiency) mode and you aren’t using a lot of throttle. In a move picked up in Formula One, it simply closes the valves to seal off each cylinder, with the remaining four cylinders still giving at least 230Nm of torque between 800 revs and 3600rpm.Whenever you need the power you paid for, it switches back over to V8 mode in 30 milliseconds.

So, if you’ve suffered through some of Benz’s wheezier, coarser V6 offerings while wondering what all the company’s cleverest engine coves had been up to, now you know. It really is as simple to operate as it reads, though you just know the software code book took years to write.

A rumbling, burbling best of a thing by nature, the V8 starts angry and, quickly, drops into silence as the start-stop cuts it off at the traffic lights. Move off gently enough, or cruise at a constant speed, and an ECO4 light comes on in the instrument cluster. The sound is quieter, the engine no harsher and change to the fuel economy on the instant consumption read out is just astonishing.

The brilliance of all of this is that it has no ill effects when you ask it to do all those things you wanted in an SLK55 in the first place. For starters, you can deactivate the system by asking for either the Sport or Manual modes, both of which crack the whips on all eight cylinders, all the time. That’s the one you want if you’re keen on a popping, slightly uneven burble when you’re sitting outside a cafe or you just want to hear what you’ve paid for.

It’s even better if you’re on the attack, because the engine, hooked up to Benz’s seven-speed automatic transmission, loses nothing for its economy potential. The noise is brilliant, with its burbling idle turning into an urgent, sparkling tone-rich blast at full throttle.

It’s fast, too, with enough power to drive the folding hardtop two-door's 1610kg mass to its 250km/h limit with ridiculous ease.

Full throttle upshifts come with a crackling little bite-sized chunk of audible manliness from the engine before it seamlessly continues surging forward. Every time you lift off the throttle, that same audio system delivers a long burble before settling down and every time you change down a gear, it blips hard to give you yet another opportunity to confirm you are using all eight pots.

It’s as flexible as it is clever, too, with 540Nm of torque arriving at 4500rpm, creating an urgent surge of legitimate performance all the way from just under 3000rpm to just over 7000.

The seven-speeder feels at home here, despite the harder work imposed upon it by the V8’s torque and the start-stop setup. The only downsides are the occasional jerky takeoff on light throttle and the safety-driven downshift it gives the car if you push the throttle through the kickdown button at the bottom of the throttle travel.

The SLK's all good inside, too, with a combination of air vents that verge on sheer art to leather stitching that is exquisite and a shallow dashboard that evokes memories of an earlier age. The seats are strong on lateral grip, the driving position is impeccable and there are places to put (a few) things around the cabin.

Where it falls apart is whenever you point the SLK55 at a corner. It’s not as though it’s incompetent, because it’s clearly not. AMG uses cleverness like torque vectoring and it enhances the SLK’s underpinnings with stiffer springs, stabiliser bars and dampers and adds a new steering system. Somehow, though, it never comes together as a cohesive whole -- in large part because it’s based around an SLK which also, coincidentally, never comes together as a cohesive whole.

A single corner isn’t bad, but a series of them highlight the shortcomings of the original car, as much as the AMG version. Tightening the suspension and lowering the ride height only serve to exacerbate its power-down limitations out of corners, the steering has little more life than it does in the donor car and it still suffers the lateral judders over mid-corner bumps.

It’s never verging on the tautly integrated fun and poise of, say, a Boxster S or the overwhelming technical-governed competence of BMW's Z4. Instead, a series of corners becomes a schedule of individual challenges and you can never wrangle the SLK to treat them as one flowing piece of road.

Indeed, every corner demands a series of inputs, with the steering and the accelerator, to get the best out of it as to first convince the front end to yaw, then convince the back end to stop yawing and then do the same thing to straighten up again on the way out.

The grip isn’t bad, but the car's balance, which feels ordinary in the V6 SLK, is even worse with 187kg of V8 sitting up front. While it feels like the mass governs everything, we know from experience of the standard SLK that the core balance is amiss even in the four-cylinder versions.

For all that, it rides pretty well on its 19-inch tyres, even across the square-edged hits that blighted the stock SLK on its launch roads in the Canary Islands.

It’s also quiet with the roof up (brilliantly quiet for this setup, actually), comfortable with the roof down and the AirScarf blowing away, and supple enough for eight hours of driving in a day.

So, if handling sparkle isn’t right up at the top of the agenda, the SLK55 is definitely a multi-faceted convertible worth considering. And if it is? You'll need to look to another badge.

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