A Question of Sport #2016
AC Schnitzer endows the #BMW-Z4
with some serious diesel power with a 400hp conversion! A pure sports car with a triple-turbo diesel under the bonnet? Who’d make such a thing? AC Schnitzer – that’s who. And it knows what it’s doing… Words: Auto Bild Sportscars. Photography: Auto Bild Sportscars and AC Schnitzer.
The ACZ4 5.0d has a bespoke interior and many one-off components such as the exhaust which saves a staggering 19kg.
Track tester’s notes
Engine: Because of its nature, it doesn’t rev as sharply as a sporting, normally aspirated petrol engine. The strong torque always leads to a lightning-fast breakaway of the rear end.
Gearbox: Take everything one gear higher than normal, and shift up at 4500 rpm. Steering: Direct, precise, plenty of feedback.
Suspension: Perfectly set up for the Sachsenring, almost no roll tendency in alternating curves, just enough spring travel for small bumps. 1.34g transverse acceleration!
Brakes: Perfectly controllable, no fading, pressure point clear as glass. Brilliant.
Some of our readers may well remember the AC #Schnitzer
99d that the company built back in 2011 which combined BMW’s ubiquitous 2.0-litre diesel engine tuned to 190hp and 310lb ft of torque with the expensively lightened body of a Z4. Thanks to innumerable carbon components, the eco-sportscar was able to slim down to an unladen weight of around 1300kg. It ran on low rolling resistance tyres and returned, on average, a smidgen over 74mpg which equates to a CO² emissions figures of just 99 grams per kilometre. Sadly this technology platform – costing €149,000, but not for sale – remained a highly regarded one-off.
It was also regarded – or rather, watched – by a stubborn interested party who was inspired by the concept of a diesel sports car for rather less noble reasons than saving the planet. For him, it was more about torque. The 310lb ft offered by the four-cylinder diesel wasn’t enough for this customer, so he said to Schnitzer: “If you can make a really powerful diesel, I’ll buy the car.”
So the engine arrived – a freshly donated unit from a M550d. And in a lengthy operation, the AC Schnitzer 99d was transformed into the ACZ4 5.0d. The name is as unwieldy as it is misleading, because the tripleturbo six-cylinder doesn’t have a 5.0-litre capacity – it is, in fact, a 3.0-litre unit. That’s more than enough, though, because straight from the factory this wonder diesel delivers no less than 381hp and 549lb ft of torque and turns the two-ton-plus M550d into a very lively performer.
But what can this oil-burner add to a lightweight Z4, even when on top of everything it’s tuned by software intervention – an increase in injection quantity and, consequently, a rise in boost pressure – to 430hp and 620lb ft? And it gets better: our performance measurement actually recorded 445hp. Will the engine and chassis separate themselves from the bodywork during the traffic light grand prix? Will the propshaft tie itself in knots? Or will the rear wheels simply spin helplessly in every gear?
Somewhat surprisingly none of that happens as Schnitzer transplanted the complete rear axle from the E92 M3 (including the limited-slip diff) and fitted 9.5-inch wide forged alloy rims shod in very grippy 265 Michelins. As a result the Z4 actually transmits all that power and torque to the Tarmac remarkably well. Naturally it is possible, with the driving aids turned off, to transform the rear tyres into small black crumbs with a large dose of the throttle. However, anyone with even a hint of feeling in their right foot should be able to get smoothly off the mark (even in the wet), and rapidly shift up through second and third, and only fully press depress the throttle in fourth gear at the earliest.
The secret of the fundamentally fine controllability and high output of the BMW diesel lies in the complex valve control of the three turbos: a small high-pressure turbo ensures spontaneous response to even the smallest tap on the gas pedal. From around 1500rpm, the large low-pressure turbo joins in and provides plenty of volume and torque. Stage three comes in at around 2700rpm: a bypass line now supplies exhaust gas to a third small high-pressure turbo. From here up to maximum revs at 5400rpm, all three turbos work together to push the huge air masses into the combustion chambers for maximum power. Yet the driver notices nothing of these processes, simply enjoying the lag-free, harmonious but extreme power development up to maximum revs. So on the motorway, eighth gear is enough for all situations. Hectic flips of the shift paddles, kickdown, high revs – why bother? Just engage top gear in manual mode and press the throttle – and enjoy acceleration to a level not experienced before. The speedo needle climbs from 100 to 200km/h (62-124mph) as quickly as it does from zero to 100km/h in other well-powered cars.
The vehement thrust however ends unexpectedly early at a measured 279km/h (173mph). Is this down to the short-ratio M3 rear axle, which was really intended for a high-revving V8 petrol engine? No, because at top speed in eighth gear you’re only at 4300rpm and the diesel has enough breath for a further 1100 revs. Roman Fenners of AC Schnitzer thinks the cause lies in a protective function of the gearbox software, to prevent overheating.
But even 279km/h feels very, very fast in the diesel Z4: the solid hard-top of carbon fibre reinforced plastic, which replaces the standard steel folding top and its complex electro-hydraulic folding mechanism, saving 56kg, produces a noticeable interior noise level. And the very low race-style suspension setup with short spring travel, pronounced negative camber and very wide tyres on the front axle, calls for strong arms on bumpy and grooved surfaces.
When we head off to the track, and specifically the slalom test, what was a disadvantage on the motorway here transforms into an advantage: the slightly nervous agility of the Schnitzer Z4. The pleasantly heavy steering, which feels beautifully taut and extremely precise, gives excellent feedback from the road and allows the coupé to be steered through the cones with millimetric precision. Understeer? Only when the tyres haven’t warmed up. Oversteer? Only when the throttle is used as an on/off switch.
The nose-heaviness is successfully countered by AC Schnitzer with 265 tyres on the front too – instead of the mixed tyres with considerably narrower format on the front which come on the standard Z4 top model, the sDrive35is. That car, with 340hp, weighs in at 1601kg – 123kg more than the Schnitzer with the heavy diesel unit up front. As well as the solid hard top of carbon fibre reinforced plastic there’s also a CFRP bootlid (minus 34kg), a lightweight rear silencer (minus 19kg), CFRP bucket seats (minus 35kg) and forged alloys (minus 25kg) which all save weight.
Our race ace, Guide Naumann, now takes over the controls for our hot laps of the Sachsenring to record a lap time. For this we fitted Michelin Cup 1 semislicks which in the cool autumn temperatures, despite several warm-up laps, never quite reach their optimum working temperature. But the Schnitzer still steers excellently into the Coca-Cola Kurve after the start-finish straight without understeer. The suspension is perfectly set up for the Grand Prix circuit, handling the alternating corners without too much body roll, but was still soft enough to swallow the small bumps of the Sachsenring. For the Nürburgring North Loop we reckon this setup would, however, offer too little spring travel.
When accelerating out of comers, the triple-turbo has too much power especially in conjunction with an only lightly loaded rear axle. If you press the gas on entry to the apex, the rear kicks out suddenly, so you take it in one gear higher than usual, applying the gas late and progressively. But the rear still calls for your full attention, such as in the fast right kink downhill at 180km/h, where it tries to overtake the front! With the diesel roaring loudly at race speed, you can’t hear the rev limiter so you can’t shift based on engine note. Instead you have to keep glancing away from the track and over to the rev counter. The needle, however, should never drift above 4500rpm because higher revs would only cost time and you’ve still got all that solid torque available in the next gear.
The Schnitzer braking system, with six-pots on the front, remains unmoved lap after lap – no fading, no lengthening pedal travel, just a pressure point set in stone, combined with perfect controllability. Naumann’s summed up the ACZ4 5.0d on track thus: “Race-style suspension with very high and correspondingly narrow limit zone. Overall high grip level but the huge torque proves a killer for perfect lines. With a slightly higher exterior temperature or a softer tyre compound, certainly another second could have been squeezed out.”
The comparison with the Schnitzer Z4 99d mentioned initially, which we thrashed around the Sachsenring in spring 2013, is interesting: the 200kg lighter car, which also had 255 less hp, took over five seconds longer. A good time in itself, on a par with a current Audi S4 with 333hp. Or expressed in other words: the six-cylinder diesel is a real powerhouse. For the record the ACZ4 5.0d recorded a time of 1:37.27 on a cold track… a F82 M4 DCT Coupé managed a 1:37.74 under warmer conditions. And that makes the ACZ4 5.0d the fastest diesel we’ve ever driven around the Sachsenring.
And how does the Schnitzer feel in comparison with a standard Z4 35is? Another world away. The softly set up standard BMW, trimmed for comfort and ‘safe’ understeer, feels almost stolid, almost unsporting. Today’s standard, forgiving car sadly can’t offer the sharp handling which you associate with the first generation Z4 (E85).
Overall we’re left with an impression of a machine that really does stir one’s emotions. Emotions? In a diesel? Which occasionally breathes a hint of diesel oil into the interior? Which on starting rattles like the neighbour’s rep-mobile? Which growls darkly at the front but can’t sing melodiously from the exhaust? Yes! Because the baffled looks of a few car nerds who notice that the engine note and car don’t go together, are pure gold. And then there’s the fab feeling of driving something unique, special and exotic.
This unique, special, exotic car could, however, make you curse in everyday use. For example, in the supermarket car park when you have to unlock the carbon fibre bootlid in two places, then take it off completely and put it to one side before loading your shopping. Then there’s the short-travel suspension which the driver has got used to but passengers will never take to. Add to that the always high interior noise level (yes, even the sound insulation has been scrimped on) and that when reverse parking it’s very hard to see the rear extremities… and the former Roadster has now become a year-round closed top coupé. Oh well, you can’t have everything!
It’s not a cheap conversion, though, even if using a secondhand Z4 as a basis. Almost all the internals have been thrown out and the new engine and eightspeed automatic alone cost nearly €50,000. Then it goes without saying that the suspension and brakes have to be uprated to match the huge power gain. The interested party could save a few euros though by skipping the lightweight components.
Either way, AC Schnitzer has come up with a cracking package for this car. A heavy, extremely powerful diesel in a delicate lightweight coupé? We were sceptical, but our scepticism gradually developed into unalloyed enthusiasm during the test – AC Schnitzer has successfully pulled out all the stops to create this extraordinary concept.
Schnitzer has stripped a huge amount of weight from the Z4 thanks to the extensive use of carbon fibre such as these front wings and the new roof.
TECHNICAL DATA #BMW-AC-Schnitzer
ENGINE: Six-cylinder, triple-turbo diesel, 24-valve / #N57S
BORE/STROKE: 90.0 x 84.0mm
COMPRESSION RATIO: 16.0:1
MAX POWER: 430hp @ 4400rpm
MAX TORQUE: 620lb ft @ 2000-2400rpm
0-62MPH: 4.0 seconds
0-124MPH: 12.9 seconds
QUARTER-MILE TIME: 12.31 seconds
TOP SPEED: 173mph
ECONOMY: 20.6-39.8mpg (27.2mpg on test)
ENGINE: Triple-turbo straight-six diesel, retuned
TRANSMISSION: Eight-speed automatic
FRONT BRAKES: 380 mm, vented and slotted, six-piston callipers
REAR BRAKES: 370mm, vented
WHEELS: AC Schnitzer Type VIII lightweight forged wheels ‘BiColor Orange’, 9.5x19 inches
TYRES: Michelin Pilot Super Sport 265/30 ZR 19 Y
ROOF: Replacement of the two-piece, electrohydraulically operated, folding steel roof with a CFRP hard-top saves 56kg, the #CFRP
bootlid a further 34kg. The roof is now fixed and the bootlid can only be opened by removing it fully.
GLASS: The rear screen and rear side windows (which can no longer be lowered) are made of lightweight polycarbonate.
SUSPENSION: AC Schnitzer racing suspension, fully adjustable in compression and rebound stages.
AERODYNAMICS: AC Schnitzer carbon front spoiler, AC Schnitzer carbon sports wings, AC Schnitzer bonnet vents, AC Schnitzer rear spoiler (two-piece), AC Schnitzer carbon rear skirt insert.
INTERIOR: Interior trim elements painted, carbon racing seats with #ACZ4
5.0d logo, AC Schnitzer aluminium footrest and pedal set, AC Schnitzer instrument cluster.
PRICE: €114,000 (one-off build cost)