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

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.