GT showdown: Mercedes-AMG SL 55 vs Porsche 911 GTS

Nearly 1000bhp between them and a value exceeding £250k - which would you take?

An AMG-focused development for the SL surely signals new dynamics, but can it beat the benchmark?

People like Timo Nordheim have been embellishing the Mercedes-Benz SL story since the early 1990s, piping habanero sauce into dishes that you would never expect to blow your head off.

As engine builders at Mercedes-AMG, for them it’s all part of a day’s work. Nordheim and his many forebears have been behind the rortiest, the most unhinged (hello, Black Series) and occasionally the most sublime SLs, as was the case when the debonair R129 was loaded with the 518bhp 7.3-litre V12 that AMG later supplied to Pagani.

There has been so much to love, and yet in SL lore AMG was never more than a mere flavour. Then, last year, big changes.

The launch of the R232 SL signalled to the world that custodianship of the model name, not to mention the engineering programme, had been transferred away from the mother ship to Timo’s lot.

That’s right: the aristocratic old SL given to potty-mouthed AMG. It’s like leaving your angelic grandma in the care of Noel Gallagher. Which, in fairness, could be hilarious.

It means Nordheim and co will no longer be external contractors roped in to amp up the SL ad hoc. Their work will be fundamental to every SL built, and now the Mercedes-AMG SL 55 suddenly has an even more pressurised expectation to square up to the class benchmark: the Porsche 911 Carrera GTS Cabriolet.

But which one is best? Read on to find out...

*Quick links: Powertrains - Interior - Driving dynamics - Verdict - Winner*

AMG versions of this long-snouted grand tourer will no longer be soft-bellied, Sindelfingen-made machines tickled around the chassis, stuffed with V8 muscle and sent out to dice with more bespoke-built prize fighters from other makes.

Instead, in a radical overhaul of the famous model’s ethos, they will be conceived and developed from the ground up in Affalterbach, meaning that AMG is no longer just a flavour of SL but instead the flavour of SL. It’s a full personality transplant, aimed at turning the SL from topless grand tourer into the drop-top sports-cum-supercar, in the process making the AMG GT Roadster redundant.

Question is, which AMG exactly are we getting here? Is it the one that turned out the gullwinged SLS from scratch and created a modern icon? Is it the one that makes the E63 uber-saloon, in all its softer but still perfectly judged, 600bhp glory? Or is it the one that’s gone off disastrously half-cocked with the four-cylinder plug-in hybrid C63? With the help of one very capable rival, today we will find out.

Note, though, that on the SL’s international launch, AMG CTO Jochen Hermann did juicily preface everything with something of a mic-drop moment.

“When you look back into the history of the SL, you see that it all began with motorsport,” he said. “With the new model, we’ve attempted to make that link again.” Punchy. Never mind the 911 we’ve got in tow; maybe we should have bagged a 911 GT3!

Despite the boulevard aesthetic of the new car, Hermann has a point. This latest SL is the first SL in history to feature back seats for a broader remit, it has returned to a rakish fabric roof and it’s nudging 1900kg at the kerb, which are all resolutely un-motorsporty things, but the AMG-built aluminium platform that it uses is absolutely fresh and will be shared with the upcoming second-generation GT.

Given the wild success Mercedes has enjoyed on track with the GT3 racer, a replacement is surely a given – and, well, there you have a genuine motorsport link to the SL.

*Mercedes-AMG SL 55 vs Porsche 911 Carrera GTS Cabriolet: powertrains *

Thanks to Nordheim, we can also dismiss the half-cocked, downsizing scenario. His signature is found on our car’s engine cover, just above and to the side of the spot where the hot-vee turbochargers nestle, inches from the ram-air intake.

AMG builds only one hot-vee motor, and it ain’t no four-pot. Indeed, few engines are more recognisable either from behind the wheel or from the pavement than AMG’s monster M177 4.0-litre V8, which has been in service for many years but for the SL gains new intake and exhaust plumbing as well as a specific oil pan.

In our SL 55, it makes a relatively tame 469bhp but also a robust 516lb ft of torque, this from only 2000rpm. Not the most exciting figures, but if that’s what you want, the 195mph SL 63 will be your thing. There the same V8 is boosted to 577bhp and 590lb ft. And with electrical assistance, the upcoming PHEV will be beefier still. (Note that there’s a 375bhp, four-pot SL 43 at the foot of the ladder, too.)

As well as their powerplants, the SL 55 and SL 63 share a nine-speed automatic gearbox, 4Matic+ four-wheel drive and a rear-steering set-up, but there the similarities end. 

The suspension of our SL 55 is controlled by semi-active dampers and traditional anti-roll bars, while the SL 63 has a compensatory, cross-linked hydraulic system similar to that devised by McLaren.

It sounds sophisticated, and given the SL 63 costs a Ferrari Portofino-matching £175,000, it should be. Meanwhile, the SL 55 comes in at just under £150,000, for which you also miss out on active engine mounts and a maximum-attack electronically controlled limited-slip differential.

So maybe in this trim, we’re not experiencing this repositioned SL at its most expressive and rewarding. Then again, does a bespoke AMG model actually need the technology toybox thrown at it to compete against something as polished as the 911 Carrera GTS Cabriolet, at which this SL 55 is pretty squarely aimed, with its back seats and ambitious total sports car remit?

What I wasn’t expecting was the extent to which this sentiment applies to the powertrain. Sorry, Timo. Your V8 is magnificent in so many ways, but next to the 911’s twin-turbocharged flat six, with its GTS-specific exhaust, it feels oddly blunt and one-dimensional.

In today’s conditions, it’s not quite as easy to fully uncork the near-500bhp Porsche unit, but do so and its scope is that much greater; its part-pneumatic, part-mechanical aural character that much more interesting; and its combination of shift speed and throttle response that much more invigorating.

I think that in the rush to express our disappointment in the demise of atmo engines for non-GT 911s, we’ve overlooked the heights that Porsche has hit with this 3.0-litre turbo unit.

Only when you put it up against a certified blockbuster like the M177 (more civilised calibration or not) do you realise just how damn good it is. And the performance with which it endows the 911… The Porsche is the faster of the two here, all right.

*Mercedes-AMG SL 55 vs Porsche 911 Carrera GTS Cabriolet: interior *

This Mercedes certainly makes extremely short work of the long, cold stint from London up to the North York Moors.

Atmosphere counts for so much in these kinds of cars, and the SL’s cockpit does the endless-bonnet/high-scuttle thing with conviction but stops short of subscribing to the claustrophobic, pillbox-style environment of the current, soon-to-be-retired GT.

You don’t feel quite so buried within this car, backside cosying up to the back axle, and isolation from road roar is truly in another league not only to that of the thuggish GT but also to the 911.

Surrounded by leather and crisp digital displays (which I could take or leave, although they do declutter the transmission tunnel nicely), you could reel off 500 miles in the SL without thinking, not least because the ride quality in Comfort mode is so well judged and the V8 so unobtrusive. Very nice. But equally, where’s the AMG-ness?

If you keep the powertrain and suspension modes backed off, the new SL does a rather good job of being the old two-seat SL, albeit without the opulently light-infused cabin and the cavernous boot – both of which were among the R231’s defining elements.

Mercedes would reasonably argue that the rear seats provide extra space. However, the ability to keep luggage safely, neatly secured and out of sight is useful.

The 911 has the same problem, of course, but then it’s not trying to cover all the bases of an outright grand tourer. It’s the first sign that this SL might just be a bit confused.

No question, though: AMG’s effort outshines Porsche’s, with its highly sprung, engine-cradling, noise-generating rear axle, when it comes to covering big distances.

*Mercedes-AMG SL 55 vs Porsche 911 Carrera GTS Cabriolet: driving dynamics *

Once on the moors, it also proves itself to be crushingly effective cross-country, that plush motorway damping holding strong in the face of turbulent, rutted road surfaces and the 4Matic+ chassis generating so much traction that even all the V8 beans in second gear fails to unsettle things.

Body control, whether for roll, pitch or squat, is genuinely superb and a fine canvas for the car’s performance.

The showpiece engine duly comes alive in anything more aggressive than the default Comfort mode, casually ripping through the gears. In this application, the M177 isn’t as animalistic as it is in the GT, but a 7000rpm scope and sharp responsiveness for such a burly brute of a motor mean it makes short work of the car’s considerable mass. 

You begin to wonder just how easily the 911 – with 473bhp, ever so slightly more powerful than its foe and also an almost unbelievable 255kg lighter – will be able to keep up…

At first it wouldn’t. Where the SL imparts an easy confidence, the 911 feels tetchy. Sure, these roads are damp and this particular Carrera GTS Cabriolet is solely rear-driven (and optioned with rear steering), but the stiffer car’s dynamic friskiness is down to more than the lack of front driveshafts.

After the SL, whose front tyres are pressed into the road by the weight of its engine, the steering in the 911 seems light and disconnected and the nose more willing to wash wide.

Phenomenal driving position, mind. And the central tachometer is just spot on – refreshing, even, after Mercedes’ pixels. You get a thoroughbred ambience in here that’s sorely lacking in the SL, whose cabin feels in some ways derived from more mainstream cars, which it is. The 911 instantly feels more bespoke and special, if less materially comforting and less adept at dissolving giant mileages.

Back to the driving experience and, as you begin to push the 911 harder on the way into and out of corners, it exhibits more edginess, this time at the other axle, as flashes of oversteer enter the fray before the electronics cut in.

For those first few miles of B-road, you almost hold it at arm’s length, like you would an obnoxious cat. There’s even a world in which this test now ends and the SL edges it, being the finer cruiser and, while far from boring, also that much more reassuring on the charge.

But then the 911 clicks, just as, in fairness, these rear-engined coupés from Weissach almost always do.

Perhaps it has taken longer today on account of the fact that, as a GTS model with 911 Turbo-derived suspension and a lower ride height than standard, this particular 911 Cabriolet is a tad further out of its comfort zone on a cold, wet moors road than it would otherwise be. But boy does it click, and now you’re really dialled into the experience.

You realise that what passes for steering feel in the heavy SL is simply weight and that the new EPAS system is much more heavily filtered than that in the 911.

Neither of these racks possesses the detail of the hydraulically assisted system in the GT, but the 911’s does offer real communication once you’re in fingertip tune with it. The SL offers intuitive gearing and reassuring weight, but that’s it.

In combination with a firm brake pedal that’s full of feel, this means you quickly learn to draw the 911’s nose into corners with grip and accuracy. The SL will do this as well, but there’s much less meaning and reward to it, just a command well executed.

The 911’s natural agility also begins to tell, and this is where no level of locked-down composure or grip is going to help the SL. It’s simply too tubby to be considered a proper sports car, and while the big body never pulls the car off your chosen line or out of shape, you feel as though it might, which is total anathema to any great driver’s car.

On the other hand, once you’re on the 911’s level, it will happily dart this way and that, and it carries pace through a quick S-bend that necessitates a confidence dab of braking in the SL. Where the SL impresses, the 911 enlivens.

Its controls and general manner are that much more vibrant. The SL is hardly stuck in black and white, but the 911 is just so much more vivid when it matters and its synaptic afterimage lingers on post-drive.

The SL might have hit back with a tactic that its forebears have reliably, effortlessly and unrepentantly deployed over the years: on-demand oversteer. This is, perhaps, the very essence of AMG. Yet this car is oddly recalcitrant.

It will tighten its line on the throttle (although not by you coming off it, unlike the 911) but doesn’t do so as naturally as any longitudinally engined roadster from Affalterbach should.

It clings doggedly onto that feeling of stability that seems to be its chosen calling card. It’s all very ‘fast grand tourer’, rather than ‘proper sports car’ and again makes you wonder: where’s the AMG-ness? Meanwhile, the 911 is trickier to balance but rewards precision and is willing to dance. It’s more fun.

*Mercedes-AMG SL 55 vs Porsche 911 Carrera GTS Cabriolet: verdict *

So where does this leave us? It’s pretty clear-cut, I would say. One way of framing the verdict is to ask: does the reinvented SL get closer to being a sports car than the 911 gets to being a long-legged grand tourer? Because if it does, chapeau AMG, as the result would be pretty compelling, even at £150,000. But no.

The AMG is a fine cruiser, but the 911 is no horse and cart and even in Cabriolet form has the space and grace for day-long drives. Then, on those last 30 minutes from motorway to overnight, it will take the SL apart in almost every measurable way, and in several ways you can’t easily measure, too.

That said, there’s clearly quite a bit of potential in the SL. If AMG can give the Mk2 GT the same handling flair as its predecessor but also the road manners of the SL and, say, disengageable front driveshafts? What a thing that could be.

*Mercedes-AMG SL 55 vs Porsche 911 Carrera GTS Cabriolet: winner *

*1st. Porsche 911 Carrera GTS Cabriolet*

Road roar hampers its GT credentials, but it’s stunningly complete in almost every other respect. Subtler than the SL but also more detailed and rewarding.

*2nd. Mercedes-AMG SL 55*

Fantastically capable in isolation but somewhat confused. Loses some GT slickness in an attempt to chase the 911 but never lands any convincing blows.