So we've had the S6, now it's the turn of the S4 to prove that diesel has definitely not had its day in a performance-based setting. For Audi's money - and it will have made the bet long ago - the S4 provides the buying public with a genuine rival to the newly overhauled BMW M430i and AMG C43 rival; one that uses a mild-hybrid diesel powerplant to produce 347hp/516lb ft while also offering a claimed 40.9mpg combined. The numbers, it will venture, do not lie. But cars aren't all maths and money - the old petrol V6-powered S4 could also claim to be a moderate amount of fun. For an oil burner, that description is like gold dust.
In fact, the only real drawback of the old machine was arguably its thirst for hydrocarbons; PH's old long-termer would only just edge the right side of 30mpg over a fair distance. But it was keen revving and light on its nose, two traits that aren't normally associated with the running gear of an MHEV diesel. The stats suggest little has been lost in a straight line, as the 48v electrified S4, complete with a belt-driven starter generator for added torque and electric compressor to keep the turbo spooled up, can sprint from 0-62mph in 4.8 seconds. That's just one tenth off the old 354hp petrol, and the instantaneous responses gifted to an electrified car ought to give the new S4 an advantage once rolling as well. Still, the question as to how all that extra kit will affect on road handling remains. More on that shortly.
First, let's look over the revamped S4's broader specifications. Audi's gone full kitchen sink with the equipment of the whole 2019 A4 range which the S4 sits atop. Every UK variant of the line-up gets the full Virtual Cockpit suite, including the 12.3-inch digital instrument cluster and a wider, dashtop-mounted 10.1-inch MMI display as standard, which is a massive win across the board. The touchscreen has crystal clear graphics and it's intuitive to use; if we're searching for flaws it's a shame the update has removed a transmission tunnel rotary dial, because that made for the simplest operation while on the move in the old car. A small price to pay, though, in exchange for what remains comfortably the best and most impressive in-car infotainment tech of the segment.
The new S4 is especially well-trimmed, too, its new Matrix LED lights and sharpened details giving the car a typically digital look that matches the cabin architecture. Our test car received black leather sports seats, contrasting red stitching and carbon fibre trim, creating a premium performance look that suits the S4's cause. The feeling is enhanced by a fairly low seating position low and a steering wheel that can be pulled close; there's even a gravelly tone which hints at the performance on offer.
Get going and the new S4's broader character is instantly obvious. When trundling along in Comfort mode, engine noise is almost inaudible and road noise remains very hushed too, while the adjustable dampers slacken off so the ride on the 19-inch wheels is supple and there's little encouragement to shatter the peace. You could very well leave the engine in this mode and live life leaning on the S4's vast reserves of torque, were the eight-speed ZF torque-converter not a little hesitant when a sudden increase in pace is requested. This annoying trait only slightly improves in Auto mode, so much so that you're often forced to pull the left paddle for a lower cog should you need an instant shove of acceleration - which seems odd given the extent of hybrid tech on board. Presumably it's evidence of the new S4's increased focus on efficiency; a theory backed by the fact the engine will seamlessly switch off when you're coasting with the 48v system busy recharging as soon as you apply the brakes.
The gearbox feels far more appropriate for an S4 when the powertrain's in Dynamic mode, where it offers swift changes up and down the ratios and even a satisfying little thud during full throttle upshifts. There's absolutely no doubting the unit's grunt in this mode as it hauls the S4 from corner to corner, making full use of its all-wheel drive traction in tight corners to the point that you needn't worry about regulating throttle inputs at all, even on bumpy Italian roads. On flowing Bolzano tarmac, the S4's front end ensures swift progress with keen and responsive reactions, characteristics that significantly make up for the lack of feel provided by the steering. The chassis is happy to tighten its line on a trailed brake or even open throttle if you're more aggressive with the steering. It's no four-door Fiesta ST, but there's a neutral balance to be exploited in the S4.
With the torque vectoring Sport Differential optioned, there's even a modest degree of throttle adjustability if you can believe it. The S4 is capable of sending up to 85 per cent of available torque to the back axle, and its interaction is noticeable if you accelerate hard out of a hairpin. Like the mid-corner stuff, it never gets beyond a slight kick of oversteer, but it's amusing enough and of course its bolstered by a huge margin of built-in stability. Heck, we'd even go so far as saying it makes driving along a technical route rather fun.
Do we miss the petrol? In pure performance terms, no; quite the opposite, actually. But subjectively speaking, the diesel's lack of thrust after 4,500rpm does limit the excitement factor somewhat. You operate it through the tractable mid-range so while it feels perfectly capable, it's never going to be as thrilling as something that asks to be worked. But that was a given in the switch to derv and must have been considered a necessary evil as far as Audi was concerned. What's left is a broadly capable car and a half way parsimonious one, too - not to mention something completely different to the traditional alternatives from AMG, BMW and Jaguar. An MHEV diesel S4 won't be for everyone, but at worst the new car does now boast some distinguishing characteristics that its predecessor did not.
SPECIFICATION - AUDI S4 TDI
Engine: 2,967cc, V6 plus starter generator
Transmission: Eight-speed automatic, all-wheel drive
Power (hp): 347@3,850rpm
Torque (lb ft): 516@2,500-3,100rpm
Top speed: 155mph (electronically limited)
Price: £48,000 (est)