Now, quite plainly, these cars are not natural rivals - we've not entirely taken leave of our senses post lockdown. Those who covet a Ford Mustang are not particularly interested in comparable coupes because, really, nothing else compares; it's as American as July 4th, which makes a Mustang, if not perfect, then certainly unique as sports cars go.
To those eyes, the S5 will look like just another hotted-up repmobile, ready to take on all comers in the fast lane of the M40. But the Audi, too, brings with it a level of intrigue thanks to its unconventional powertrain. Here's a model that's evolved through V8, supercharged V6 and turbocharged V6 engines in its lifetime, now powered by a mild hybrid turbodiesel with Audi's Electric Powered Compressor on the side. It is technology first seen in the SQ7 - a car which, alongside the SQ8 TDI, has already met its maker just three years after launch - and is ostensibly identical to the unit found in the S6 and S7. The compressor helps the V6 along while the turbo spools up, while the hybrid tech (a belt-alternator starter and storage battery) enables coasting, fuel saving and more efficient stop start. The Mustang has a naturally-aspirated petrol V8, featuring pistons which go up and down. Two more different approaches to a sports coupe at circa £50k you could hardly imagine.
But that's sort of the point. As more and more cars are forced to share vital parts, as homogeny spreads its tentacle-like across the industry, to have two such different takes on a similar remit is pleasing. Variety is the spice of life, after all, and it really would be dull if we all liked the same thing. In case you hadn't heard before.
Engine bay diversity has certainly made pigeonholing the S5 quite difficult; when both S and RS A5 were V8 powered, it seemed the S5 might have been a junior RS5; when the V6s came in, it seemed closer to the standard models. Now is the first time an S5 has an engine without any link to its siblings, beyond the mild hybrid technology also found in the four-cylinder A5s. Now, more than a decade after its original launch, might be the time for the S5 to properly define what 'S' means.
That it is an agreeable place to spend time will not be a surprise to anyone. Be it a late night dash along the A11 or a trudge through a city or chasing a Mustang around the countryside, the S5 is an exemplar of the Audi way. The button-less infotainment irks as it did in the RS5; otherwise the combination of cool metals, sharp displays and swanky materials creates just the right aura. Whatever the journey, you're happy embark upon it in the S5. And that matters, plain and simple.
Furthermore, while a mild hybrid diesel may not yank at the heartstrings, there's no arguing with its effectiveness. As the unit has proved elsewhere, its capacity for reducing lag while further enhancing torque has produced a mighty engine. It isn't viceless, yet feels about as good as a TDI can hope to get. Whether or not the black pump is living on borrowed time in this kind of installation remains to be seen - but if we are witnessing the last act, the V6 is quite some way to sign off.
Serious performance is essentially on tap from idle; certainly it will start rocking and rolling before 2,000rpm. That's before peak torque, too, at 2,500rpm, by which time there's no stopping it - least of all a chunky 1,845kg kerbweight. The powerband feels wide, the V6 doesn't tighten up as revs (and speed) pile on. That it also manages to be pleasant enough to listen to, delivers a realistic 40mpg and cruises near silently only furthers its case. Indeed the only noticeable blot in its copybook is the mildly dim-witted auto it's attached to. For an engine that appears to need no more than three gears, its habit of choosing one that feels at least a couple of cogs too low is annoying. Best to take manual control with the paddles, where it will pull top gear from no revs at all.
Refinement, efficiency and huge overtaking potential make it a consummate autobahn car, and of course that's by design. Where Audi has previously struggled is in marrying straight-line speed with tangible driver reward. The S5 is no rebirth on that account - but it does take advantage of the low bar set for it by its predecessors, and delivers slightly more entertainment than the S badge can previously claim credit for.
A point of note, however, before we go any further: the S5 was fitted with some significant chassis options - including sports suspension, the sport differential and dynamic steering. And we've not had chance to try a car without, so it's impossible to be definitive on whether they're worth another £3,500 or not (even if the diff has proved handy in previous installations). Or what the £2,500 20-inch wheels might do for the ride that a smaller rim might not...
What can be said, though, is that this particular S5 is pleasingly direct, accurate and more thanwilling to be driven fast - more so than the Mustang and despite a significant weight penalty. Moreover, compared to the S6, it feels better suited to the job - perhaps due to a conventional suspension setup and lack of rear-wheel steer. Whatever the case - and despite a familiar disdain for feedback - the S5 inspires considerable confidence thanks to its consistency and measured response to input. It is very rarely unsettled, particularly if left to its own devices in 'auto'. The is less temptation here than in the RS to delve into the drive settings and tailor a bespoke mode because you're not attempting to eke out every last bit of driver reward. Because there are no revs to chase - or at least none that are more enlivening than those in the mid range - you end up just letting the car get on with it. This hardly earns it legendary status - but is nonetheless indicative of its ability to worm its way under your skin. Critically, you are not bored; just sated. The S5 has an assured and understated way of doing everything well enough for you to not crave anything that would likely be beyond its capability. It's a masterclass in strait-laced likeability.
It is left to the Mustang to show you what you are missing. Ford sent one in Twister Orange with the big wing option and the questionable stickers that come with the 55Edition - last year's V8 special to mark 55 years of Mustang. It is a consummate crowd-pleaser. People point and smile. You smile back. Everyone knows. Everyone gets it. The Mustang is fancy dress in car form.
Inside it is as distant from the Audi as Nuremburg, Pennsylvania is from Nuremburg, Bavaria. The plastics are tacky, the space vast, the driving position harder to get just so and the displays - even following a facelift - are wilfully primitive. In the S5, the infotainment benchmark is Apple IOS; in the Ford, it's a Sega Dreamcast.
But then you crank the V8 and all those concerns melt away, right? Well yes, actually, that's exactly what happens. Because as 5,036 cubic centimetres rumble, burble and snort at idle, a cloud descends over the Playmobil toggle switches and Primark leather and everything else. The cloud is made of nostalgia, and the primeval business of air and fuel being turned into sound and vibration. It is mood-altering. But it is not fast. Not in the real world, not like the Audi is. If the S5 is like watching Netflix Ultra HD in Taiwan, the Mustang is like going to Blockbuster in Leatherhead. Never has the V8 felt so thin and lacking at low revs. Its gear ratios don't help - first goes to 55mph - but realistically it just doesn't have an answer to the V6's bells and whistles. Below 4,000rpm the uberdiesel minces it.
One solution to this is simply to drive more slowly. Seriously. The Audi has you rushing about because doing so is thoughtlessly easy. In the Mustang, this isn't possible or necessary for enjoying yourself. The V8 ensorcells at idle - at slow speeds it positively hums with atmospheric glee. There is no synonym for the way it dawdles along a high street because it's doing it like a muscle car, like in the movies.
The other solution is to give it death. It still won't respond like the S5, which is quite capable of going quickly when you're not looking, but it does unlock the kind of facial expressions that Audi's face monitoring camera would surely decipher as a stroke. Punch the six-speed manual about its gate (which feels the best way, despite an oddly vague clutch), keep the revs high and the 55Edition feels every inch the thoroughbred. Peak power isn't until 7,000rpm, which isn't far off twice what the S5 needs, but what a treat it is to experience a naturally-aspirated V8 reaching its crescendo, thundering through its powerband and speed building all the way. It's even better with the horizontal rev counter of Track mode, too...
In many ways the comparative handling ability mirrors the performance of the engines. The Audi is as drama-free as a German airport; the job gets done with the minimum of fuss, impressively if not memorably. The Mustang, on the other hand, requires a more nuanced approach to extract the best from it, a sort of delicate bullying if you will - sometimes it needs gentle treatment, sometimes not.
Surprisingly given it has 100kg on the Audi, the Mustang feels a bit less willing to turn in and a little longer to settle - that does improve with the optional Magneride dampers made tauter, though it compromises the ride. That sprawling bonnet ahead of you and glassy steering doesn't help confidence, either, even if you soon learn to trust the traction and composure on the way out of bends.
The S5, seemingly about a foot closer to the ground, possesses the sort of body control that the Mustang can't hope to emulate - even if there is a certain comfort zone that it likes to operate in. You know that come rain or shine, morning or night, it's going to deliver stacks of grip, massive traction (with the Sport Differential just occasionally felt lending a hand) and immense performance. The lack of what you might call interaction means less in the S5 because you don't spend your time looking for it.
As an enthusiast, the joy of the Mustang experience is that it rewards the effort, encourages you to learn its idiosyncrasies and embrace its quirks. Do so and it's a treat; matching road speed with engine speed on the way into a bend, judging turn in correctly and not being too greedy with the throttle on the way out isn't complex entertainment, but it's entertaining nonetheless. Yes, it's big and occasionally cumbersome, though that engine and gearbox combination ensures the driver is never less than fully immersed in the Mustang driving process - in a way that the S5 driver will never, ever know.
Should that equate to jealousy? Or is it all horses for courses? Certainly it's hard not to cherish a day spent in the company of two such tremendous combustion engines. They could hardly be more different - but it is by virtue of their distinctiveness that you appreciate their relative strengths. It is reductive to simply label one as sophisticated and smooth-talking and the other as rough and ready, yet there is no question that each car's personality is indivisible from the engine which powers it. And with an all-electric future looming large ahead of us, that's heartening, because somewhere down the line there will likely be no fun at all in weighing the respective merits of an Audi battery-powered electric motor against a Ford one.
That's a quandary to solve another day. For now, know that the Audi S5 boasts one of the best diesel engines on offer today, and the Ford Mustang GT one of the best V8s - that the cars they power are also adept and capable in their own way makes both recommendable. So although the average S5 buyer probably won't have a great deal in common with your stereotypical Mustang driver, that both cars meet and surpass expectations for them means each is going to be pleased with their purchase. And at the end of the day, personal contentment with a car purchase sounds better than any objective validation. No need to change your mind, then - the one you already like is really good.
SPECIFICATION | FORD MUSTANG 55 EDITION
Engine: 5,038cc, V8
Transmission: 6-speed manual, rear-wheel drive
Power (hp): 450@7,000rpm
Torque (lb ft): 391@4,600rpm
0-62mph: 4.6sec
Top speed: 155mph
Weight: 1,743kg (to EU, including 75kg driver)
MPG: 23.9
CO2: 269g/km
Price: £46,185 (price as standard; price as tested £48,985 comprised of Magneride suspension for £1,600, large rear spoiler for £350 and Twister Orange paint for £850)
SPECIFICATION | AUDI S5 TDI
Engine: 2,967cc V6 diesel (with electric compressor)
Transmission: 8-speed Tiptronic auto, four-wheel drive
Power (hp): 347@3,850rpm
Torque (lb ft): 516@2,500-3,100rpm
0-62mph: 4.8 seconds
Top speed: 155mph
Weight: 1,845kg (EU, with drive)
MPG: 40.9
CO2: 180g/km
Price: £52,950 (price as standard; price as tested £69,310, comprised of 24-litre Adblue tank for £60, Sport Differential for £1,400, Front dn rear view camera for £1,180, Head-up display for £1,025, Red brake calipers for £350, Matrix LED lights with Audi laser lights for £870, Storage package for £195, Extended LED interior light pack for £125, Driving Assistance Pack, Tour for £2,700, Dynamic power steering for £1,025, Sport suspension for £1,000, Panoramic glass sunroof for £1,400, Black styling package for £570, Star matt titanium 20-inch alloy wheels for £1,460, Gloss carbon inlays for £475, Bang&Olufsen 3D sound system for £850.)
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