01 · Overview
What was left of the M8
After BMW halted the real M8, the 850CSi became the fastest series production form of the 8 Series: the S70B56 — developed at M GmbH, identified by the “S” in the engine code — extracts 380 PS and 550 Nm from 5.6 litres of V12, available exclusively with a six-speed manual gearbox.
This was joined by a firmer suspension, lower aprons, four tailpipes and, on request, active rear-axle steering. With 1,510 units, the CSi is the rare collector’s item of the model line — prices have long since decoupled from the rest of the E31.
02 · History
Where it fits
The 850CSi is not an upgraded 850i. It is what remained of a project BMW stopped. The timeline shows how the planned M8 turned into the flagship of the range — and why it ended in 1996.
- 1989 The 8 Series arrives as the 850i with the M70 V12, 300 hp, optionally with a six-speed manual. The first twelve-cylinder ever offered with a manual gearbox.
- 1990 BMW M builds one running M8 prototype with the S70/1 engine — lightweight construction, well over 500 hp. It remains a single car; the project is stopped.
- August 1992 Production of the 850CSi begins in Dingolfing. The S70B56 makes 380 hp, and the six-speed manual is the only choice.
- Late 1992 ALPINA launches the B12 5.7 Coupé based on the 850CSi. 57 cars are built through 1996.
- 1993 From the S70, BMW Motorsport and McLaren develop the S70/2 for the McLaren F1 — 6,064 cc, 627 hp.
- 1994 The 850Ci receives the 5.4-litre M73 with 326 hp, automatic only. The CSi remains the only 8 Series with a manual gearbox and more than 350 hp.
- November 1996 Production ends after 1,510 cars. Tighter emissions and noise rules from January 1997 would have forced further development that could not pay for itself at these numbers.
03 · Model overview
The S70 family
The engine code S70 covers a small family reaching from a production coupé to the fastest road car of the nineties. Only the first line went into series production.
Technische Daten anzeigen — komplette Modellmatrix
| Modell | Motor | Hubraum | Leistung | Production |
|---|---|---|---|---|
| BMW 850CSi | S70B56 | 5,576 cc | 380 hp | 1992–1996 |
| BMW M8 (prototype) | S70/1 | approx. 6,000 cc | approx. 550 hp | 1990, one car |
| ALPINA B12 5.7 Coupé | S70-based | 5,700 cc | 416 hp | 1992–1996, 57 built |
| McLaren F1 | S70/2 | 6,064 cc | 627 hp | from 1993 |
| McLaren F1 GTR | S70/3 | 6,064 cc | 635 hp | from 1996 |
04 · In detail
In detail
Coupé
A two-door coupé without a B-pillar, with frameless windows that drop slightly as the door opens. Lowered by 15 mm against the other 8 Series cars, with a revised front spoiler section, a changed rear diffuser and M mirrors. There was never a convertible E31.
05 · Engines
The S70B56
Not a new engine, but a thoroughly reworked M70. What makes it an M unit is in the detail — and in the letter S of the engine code.
S70B56 — the only engine the 850CSi ever had verified
A larger bore and a longer stroke turned the 5.0-litre M70 into a 5.6-litre. Add higher compression, revised valve timing and lighter pistons. The exhaust system received downpipes enlarged in cross-section and two metal catalytic converters that reduce back pressure. As on the M70, two separate engine management units work bank by bank — the engine is essentially two straight-sixes. Two valves per cylinder, timing chain, Alusil bores.
Technische Daten anzeigen
| Layout | V12, 60 degrees, OHC, 2 valves per cylinder |
| Displacement | 5,576 cc |
| Bore × stroke | 86.0 × 80.0 mm |
| Compression | 9.8 : 1 |
| Fuel system | 2 × Bosch DME 1.7, one per bank |
| Power | 380 hp (280 kW) at 5,300 rpm |
| Torque | 550 Nm at 4,000 rpm |
| Firing order | 1-7-5-11-3-9-6-12-2-8-4-10 |
| Gearbox | Getrag six-speed manual, no automatic |
| Rear axle | 2.93 : 1 final drive, 25 % limited slip |
e31.net, bimmerarchiv.de, auto motor und sport — power and torque consistent across all three
Where the sources disagree
Two figures for this engine appear in two versions. Power is usually given as 380 hp, but the model archive bimmerarchiv.de states 381 — most likely a rounding question from 280 kW. For kerb weight the data sheets give 1,865 kg to DIN and 1,955 kg to the EU standard; both are correct and simply follow different measuring rules. Auto Bild weighed 1,970 kg in a 1993 road test.
06 · Weak points
Reliability and weak points
- The engine is not the problem. The S70 is considered robust and covers high mileages with regular servicing. One known issue: the bolts of the camshaft bearing caps can work loose — accessible once the valve covers are off.
- Throttle bodies and ignition. Twelve cylinders means everything twice over. Throttle bodies are expensive, and the EML warning light reports faults in the electronic throttle control. Plugs, leads and distributors all come in pairs.
- Clutch. Premature wear is a known CSi topic — and replacing it on a twelve-cylinder is involved.
- AHK hydraulics. If the rear-axle steering warning light comes on, it gets expensive. After thirty years the pressure accumulators for brakes and steering often no longer work reliably, and specialists for the system are rare.
- Rubber bushings. The many bushes of the elaborate suspension are at the end of their life on most cars. Vague steering or knocking noises usually start here.
- Electrics and interior. Pop-up headlamps with scorched wiring, failing window lifts and central locking, dead matrix displays in the on-board computer and radio, climate control panels, electric seat adjustment. The headlining comes loose and there is no off-the-shelf replacement.
- Sunroof. The optional tilt-and-slide roof likes to jam in the open position with age.
07 · Engineering
Chassis, brakes, four-wheel steering
Active Rear Axle Kinematics (AHK)
Suspension
Wheels and brakes
Cooling
Interior and markers
Technical data — final drive ratios & gearboxes
geprueft
08 · Identifikation
How to spot a real CSi
The 850CSi has become valuable enough that converting an 850Ci pays. Five markers cannot be retrofitted, or only at considerable cost.
- VIN
- It begins with WBS, not WBA. WBS is the manufacturer code of BMW M GmbH — the CSi was not built by BMW AG. Many registration documents therefore list the type as M8.verified
- Engine code
- S70B56 on the engine. The S marks it as an M unit; the 850i and 850Ci carry M70 or M73.verified
- No automatic
- The CSi was only ever built with the six-speed manual. An 8 Series with a torque converter is not a CSi, whatever the boot lid says.verified
- Instruments
- Red needles and a dial face of its own, plus the „BMW Motorsport" lettering on the door handles and the M badge in the lower door frame.verified
- Brakes and hubs
- The uprated brakes with their wider discs require CSi-specific hubs. A cosmetic conversion usually leaves them out.plausibel
- US cars
- US versions lack the AHK, the M mirrors, the engine oil cooler and the differential cooling, and use the smaller 850i brakes. A US import is technically not the same car.verified
09 · Special models and tuners
The M8 that never came — and what ALPINA made of it
In 1990 BMW M built a single running M8: a lightweight body and the S70/1 engine with roughly six litres and well over 500 hp. Ferrari and Porsche were the intended targets. The project was stopped before it could reach production — the market for supercars collapsed at the start of the nineties.
More than a museum piece came out of that work. The engine of the McLaren F1, the S70/2 with 6,064 cc and 627 hp, traces back to the same line and made the F1 the fastest road car of its time.
The 850CSi is the part of it you could buy: the same engine family, the same WBS manufacturer code, but clearly reined in. Anyone driving a CSi today has M8 in the registration document and the car that was built in its place in the garage.
At ALPINA in Buchloe the CSi became the B12 5.7 Coupé: 5.7 litres, 416 hp, six-speed manual only. 57 were built — they are included in the 1,510 CSi and do not add to the total.
10 · Motorsport
What remains
1,510 cars in four years, 1,125 of them left-hand drive for Europe. Until the range ended, the CSi was the most expensive model in the entire BMW line-up — DM 180,000, around 99,000 euros. It remains the only BMW twelve-cylinder with a manual gearbox and more than 350 hp.
The registration document says M8. What was built is the 850CSi.
11 · Buying guide
Buying in brief
The CSi is not a car for a tight budget. It is not the engine that costs money but everything around it — suspension, hydraulics, electronics. A well-kept car with unbroken history is almost always cheaper than a bargain with an open past.
Rust is a secondary concern on this Dingolfing-built body. Where it does appear: jacking points, edge rust on the front wings, the upper windscreen frame, the lower door edges and around the sunroof.
More common than corrosion are badly repaired accident damage and lost originality. Both hurt the value more than a little edge rust.
Check the papers first: WBS number, service book, a traceable chain of owners. Then the AHK — a test drive with an eye on the warning light and on how the car steers at speed. Only then the bodywork. A car without history can demand a five-figure sum for suspension, hydraulics and electronics.
12 · Ersatzteile
Parts supply
Much of the 8 Series is shared with the 7 Series E32 and is still available through BMW channels — better supplied than average, and more expensive than average too. It gets tight with anything unique to the CSi: brake discs with their hubs, parts of the AHK hydraulics, the M interior, the two-piece forged wheels. For 8 Series body and interior parts a small remanufacturing market has grown up. Owners who plan to keep a car tend to build a stock of spares early.
13 · The competition
What the CSi was up against
Mercedes-Benz SL 600 (R129)
The other German twelve-cylinder of the period, 394 hp from 6.0 litres — but a roadster, automatic only and tuned for comfort.
Porsche 928 GTS
The direct rival in concept: a large front-engined GT with a manual gearbox, 350 hp from a 5.4-litre V8. It ended in 1995, a year before the CSi.
Jaguar XJS V12
The twelve-cylinder GT of the old school — a much older design, cheaper, but without the CSi engineering.
Ferrari 456 GT
From 1992 the benchmark for a fast two-door with room for four: 442 hp from a 5.5-litre V12, six-speed manual — at a distinctly higher price.
Aston Martin V8 Vantage
It even shares the CSi fate: there too, tighter regulations ended production rather than a lack of demand.
14 · Galerie
An 850CSi in detail
Photographs of a Hellrot 850CSi built in 1993 and first delivered to Switzerland — body, interior, the S70B56 and the document folder. The images are by Rune Haaland, who owned the car for seven years; his story is in the magazine.
15 · Sources
Sources & Further Reading
The technical figures on this page come from the sources below. Where they contradict each other, the text says so.
- e31.net — model overview, production figures and the list of standard CSi equipment for Europe and the USA
- BMW M — „Der BMW 850 CSi E31", article in the BMW M magazine (manufacturer source, among others on the AHK)
- bimmerarchive.org — model archive, engine code S70B56
- auto motor und sport — „BMW 850 CSi: 12-Zylinder mit 380 PS" and the 850i/850CSi buying guide
- AUTO ZEITUNG — buying guide BMW 850 CSi (E31), weak points
- E31Wiki — Active Rear Axle Kinematics, development and availability
- Factory build record of an 850CSi dated 24 September 1993 (first delivered to Switzerland), provided by Rune Haaland