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Supercar power steals the headlines, but is there more to AC Schnitzer's reworked BMW M5 than the hot-rod demeanour suggests?

Nobody behind the wheel of an AC Schnitzer ACS5 Sport could claim ignorance of what they were letting themselves in for.

It’s a brutal thing – based on an already heaving performance saloon, the BMW M5 Competition, only splattered with an industrial degree of carbonfibre bodywork and, in this case, touting conspicuous negative camber because AC Schnitzer’s custom suspension is in its track-day setting. The carbonfibre rear valence, with its Formula-1-style rain light, is almost grotesquely contoured and frames the four canon-style tips of the sports exhaust. It's fair to say that, out on the public highway, there are less obtrusive mid-engined supercars.

This chassis, originally engineered for long-distance refinement and comfort, wears its big body like a glove. The ACS5 Sport turns in to corners like a true sports car

But there's quite a bit more to this car than looking like something you might find at SEMA. If you’ve read our story about the ACS2 Sport, based on the M2 Competition, you’ll know a bit about AC Schnizter’s motorsport pedigree and its approach to tuning BMWs. An additional control unit for the engine piggybacks the existing BWM software and unlocks extra power and torque. Some variation of Schnitzer's Nürburgring-honed suspension is then fitted, invariably with even bigger wheels that leave only an illicit veneer of sidewall rubber.

AC Schnitzer's cars tend to be carefully developed, and so despite the aftermarket aesthetic, the driving experience is usually worth getting out of bed for, or even travelling to Aachen, where the firm has been based since 1987 and where we're picking up this fully loaded but still – somehow – relatively subtle ACS5 Sport for the day.

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As always with Schnitzer, you can mix and match a host of equipment. For the M5 Competition, that means spending as little as £90 for a branded bumper-protection strip or going for the whole £30,500 'ACS5 Sport' party, which apart from an awkwardly dainty carbonfibre rear wing is what you see here. It’s all available from Rossiters in Kings Lynn, which is the point of contact for Schnitzer owners in Britain and manages the fitment of parts and the three-year warranty.

Nobody would mistake this particular ACS5 Sport for the regular M5 Competition, then, although as with all top-end Schnitzer models, M’s carbon-ceramic brakes are left untouched, as are major body panels – such as the doors, bonnet and wheel arches – and BMW’s tuning for the Active M differential.

It’s mostly good, extroverted fun, but what the ACS5 Sport doesn’t quite do is overwrite any preconceived notions you may have about what 710bhp feels like in an executive saloon. 

This is more to do with the donor car’s already red-raw speed than any shortfall on Schnitzer’s part. When we road tested the standard M5, with 'only' 591bhp, it recorded a 0-60mph time of 3.3sec (in the damp and in spite of the malfunctioning launch control programme) and then hauled itself well into triple figures with supercar-grade aggression. 

There was then the 616bhp M5 Competition, the basis of this Schnitzer test car, which was even quicker. Last year, we used one as a subjective benchmark for a head-to-head test involving the Mercedes-AMG GT 63 4-Door Coupé and Porsche’s Panamera S E-Hybrid, and it made each of its offensively heavy compatriots (combined weight is an almost incredible 4.5 tonnes) feel as though they’d each dropped a cylinder. 

It means you wouldn’t necessarily notice the Schnitzer’s extra 94bhp or 70lb ft in the UK, and neither would you really want to. But in Germany, it’s a different matter, where beyond 150mph, the extra grunt is detectable, even if it remains somewhat surplus to requirements. For something with a frontal area more akin to that of an Audi Q7 than a McLaren 720S, the ACS5 Sport thunders its way to 190mph (governed, but "we're working on it") with very little fuss. 

The steering is also good at high speed. Here, you have the same slightly artificial heft but a greater sense of stability than you’ll find in an M5 Competition. This is useful on the autobahn. Schnitzer’s revised suspension geometry is largely responsible, but the driver also benefits from these changes at much lower speeds and on smaller roads, because although turn-in response isn’t any quicker than before, it’s now more crisply defined and gives the ACS5 accuracy that's rare in such a big car.

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Combined with a front axle that doesn’t seem to recognise the concept of understeer, the ACS5 goes down rat-runs looking like a B52 entered into one of Red Bull’s air races but moving like the real deal. Such is the traction that little could escape it.

Much of this extra agility and composure, and ultimately the car's staggering point-to-point pace, owes its existence to the KW spring and damper coil-over units (Variant 4, for those in the know) Schnitzer fits. The hardware is manually adjustable for rebound as well as low- and high-speed compression and is height adjustable by 30mm. Our example was set up for a track session the day after our drive and theoretically brooked no compromise, but the ride was rigid without rifling road imperfections into the cabin – the M5's generously padded seats undoubtedly help – and you could easily live with it.

And the pay-off is that this chassis, originally engineered for long-distance refinement and comfort, wears its big body like a glove. The ACS5 Sport turns in to corners like a sports car, taking up so little slack in the suspension that it requires a moment or two to acclimatise to it. With four-wheel drive and the M5's characteristic balance, there are no trap doors in terms of drivability, except perhaps that the tyres are that much more susceptible to tracking on more corrugated surfaces. For something so big and potent, it's remarkably easy to drive fast: confidence-inspiring, in short.

Naturally, there’s plenty of potential for unsticking the rear tyres, which is true whether you’ve got the front driveshafts disengaged or have the powertrain in its default four-wheel drive mode. The long wheelbase and the engine’s huge cushion of twin-turbo torque make all contemporary hot 5 Series contenders – BMW M5, Alpina B5 and this AC Schnitzer – happy to oversteer, but in the ACS5, there’s more of a ledge between grip and slip, making it less casually playful and less willing to flatter. It's the price you pay for tighter handling within the car's high limits of grip.

Other things to consider are that there are only a handful of options for the interior – the Meccano-esque paddle shifters and branded iDrive rotary control chief among them – so the ACS5 Sport's cockpit feels far less individual than the outside, and probably better for it. The more freely flowing sports exhaust should also be optioned with care. True to the donor car, it's not particularly tuneful but in Sport mode thumps and bangs as though a foul-tempered Tyson Fury was trying to smash his way out of the boot.

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For £130,000, nine out of 10 people in the room would take an Alpina B5 BiTurbo plus a good amount of change, or an M5 Competition and even more change, instead of the AC Schnitzer.

And understandably so. Those works-developed cars ride with genuine fluency are usefully subtle in appearance and handle more freely. On the public road, they are also every bit as stupidly quick as the Schnitzer, despite the long spec-sheet shadow the tuner car casts.

The ACS5 Sport nevertheless has its place and it’s an esoteric but surprisingly well-defined one. At any speed, on any road, there’s more bite to the driving experience than with the others and a welcome sense of connection to the road beneath those epically proportioned wheels.  

The ACS5 Sport is also usefully low riding, with tightly controlled vertical body movements. Together, these go a long way to addressing one of the few 'criticisms' one can have of the BMW M5: that despite the mad performance, it never quite escapes the tall-feeling saloon-body physics.

In this sense, to drive, Schnitzer’s creation feels more of an XL M3 Competition than any traditional M5, which is an impressive feat and something those owners who want one car to drive year round and sometimes maybe even on track days will appreciate.

For my money, I’d leave the engine mods but take the KW suspension with a sensible wheel and tyre package. Too sparing? Perhaps for your average AC Schnitzer owner, but you're still getting something brain-out fast, broadly usable and unambiguously monstrous.

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