Apr 17

Apr 17 Guide: M-tuned Secret - a Historical & Technical Appraisal of the BMW E31 850 CSi

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BACKGROUND

Following the launch of BMW’s range-topping E31 8-series at the Frankfurt Motor Show in September 1989, the Munich firm’s Motorsport department began work on a pair of high performance variants.

Created as a more expensive, more upmarket replacement for the much-loved E24 6-series that had been around in one shape or another since 1976, the 8-series was initially offered exclusively with BMW’s all-alloy five-litre Type M70 V12 which had made its debut in the new E32 7-series Saloon back in 1986.

The first twelve cylinder engine produced by a German manufacturer since the onset of World War 2, the M70 unit offered massive tuning potential which BMW Motorsport was keen to demonstrate.

In standard trim, the M70 featured single overhead camshafts, two valves per cylinder, wet-sump lubrication and an 8.8:1 compression ratio. With a 4988cc displacement it developed an already impressive 300bhp, but the possibility was there for major gains in every department.

With this in mind, BMW Motorsport set about developing two suitably enhanced variants with a brace of enlarged engines: the radical 6-litre M8 and somewhat more refined 5.6-litre 850 CSi.

Both models adopted BMW Motorsport’s tried-and-trusted approach to production models with a high output engine dropped into a suitably tuned chassis. As usual, the underpinnings were draped with a customised body and the interiors kitted out with selected enhancements.

Impressively, the M8 took this philosophy to a more extreme level than seen on any BMW Motorsport road car yet. However, on account of several wickedly expensive details that would have pushed its list price to nearly twice that of a Porsche 911 Turbo, the M8 was ultimately deemed commercially unviable because, since the 8-series’ debut back in 1989, a severe economic recession had taken hold in most major markets.

By contrast, the 850 CSi was given the green light and went on to make its salon debut at the Frankfurt Motor Show in October 1991 and revived the CSi badge first seen on the three-litre E9 Coupe of 1971. Since then, the CSi moniker had been adopted for various iterations of the E24 6-series to include the 633 of 1976, the 635 of 1978, the 628 of 1979 and the legendary M635 of 1984. Uniquely though, the 850 was the first BMW Motorsport product to wear the CSi badge.

Production began on the regular 8-series assembly line at BMW’s Dingolfing plant in August 1992.

CHASSIS

Each 850 CSi started life as a standard E31 galvanised steel monocoque with 2684mm wheelbase.

As usual, a 90-litre fuel tank was located underneath the rear seats.

Suspension was via a compact MacPherson strut arrangement up front while the back end ran a five-way multi-link system. An anti-roll bar was fitted at either end.

CSi-specific enhancements included reinforced front hubs, stiffer shocks and shorter springs thatdropped the ride height by 10mm.

Instead of the engine speed-sensitive power assisted steering system found on the normal 8-series, the CSi got a road speed sensitive ZF Servotronic set up with a quicker rack.

Hydraulically-controlled Active Rear Axle Kinematics (dubbed AHK for Aktive Hinterachs-Kinematik) was another standard CSi feature (and subsequently offered an an option for other E31s). Depending on conditions, the AHK system enabled the rear wheels to turn at up to 1.8° for enhanced cornering dynamics.

The CSi’s brakes also came in for attention: the 324mm vented front and solid rear discs with single-piston calipers found on the 850i made way for 345mm front (now with two-piece floating calipers) and 328mm rear items.

Standard issue wheels were the same 17-inch M System II five spoke alloys fitted to the contemporary E34 M5. They measured 8-inches wide at the front, 9-inches wide at the back and were shod with 235/45 and 265/40Z R17 tyres respectively.

ENGINE / TRANSMISSION

Instead of the regular M70 B50 60° V12 fitted to the 850i, the CSi came with an enlarged S70 B56 unit that featured an array of performance upgrades although, unlike the all bells-and-whistles M8 motor, it did retain retained single overhead camshafts per bank with two valves per cylinder and wet-sump lubrication.

To begin, the cylinder bores were enlarged from 84mm to 86mm and stroke was lengthened from 75mm to 80mm which yielded an additional 588cc (5576cc as opposed to 4988cc).

BMW Motorsport then added lightweight aluminium pistons, more aggressive valve timing, a forged crankshaft, a separate engine oil cooler and the latest Bosch DME 1.7 engine management. Compression was hiked from 8.8:1 to 9.8:1. An uprated exhaust system was installed with larger diameter pipes and a pair of trick catalytic converters further helped reduce back pressure.

Also new was an electronic engine performance control system (EML) that, via a central control panel-mounted toggle switch, enabled the driver to choose between two modes (Komfort and Sport) that varied throttle sensitivity. EML also managed the CSi-specific gear-dependent rev limiter (6400rom in first and second, 6200rpm in third, 6100rpm in fourth and 6000rpm in fifth and sixth) along with the adaptive idle point detection.

The S70 B56 engine was identifiable on account of a central cam cover between the cylinder banks that bore a ‘Powered by M’ inscription.

As a consequence of these changes, peak output went from 295bhp at 5200rpm to 380bhp at 5300rpm and 332lb-ft at 4100rpm to 406lb-ft at 4000rpm.

Unlike the 850i which could be ordered with a four-speed automatic gearbox, the CSi was offered exclusively with a six-speed manual (a Getrag Type E unit with custom ratios and separate oil cooling system).

Transmission was through a single-plate clutch, a limited-slip differential, a CSi specific separate differential oil cooler and rear axle with custom 2.93:1 ratio.

Automatic Stability Control plus Traction (ASC+T) was fitted as standard as per the regular 850i which, in the event traction was lost, enabled the brakes to be applied and / or engine power to be reduced

BODYWORK

In typical BMW M car fashion, the 850 CSi came with a number of subtle exterior details that were unique to this high performance derivative.

At the leading edge was a new bumper assembly with de-vaned brake ducts either side of the licence plate and a deeper, flatter front apron with a new rectangular intake to feed the CSi-specific engine oil cooler.

Down each flank, BMW Motorsport added deeper sills and aerodynamically-profiled rear-view mirrors.

As per the front, the rear bumper assembly was subtly modified to incorporate a deeper apron along with enlarged cutaways for the big bore exhausts and a diffuser.

An 850 CSi badge was mounted on the right-hand side of the trunk lid, but no BMW Motorsport badges appeared anywhere on the exterior.

Body panels were fabricated exclusively from steel with the exception of the bumpers, sills and mirror housings which were fashioned from plastic composite.

INTERIOR

Like the exterior, the 850 CSi’s pillar-less cockpit featured some new equipment that was specific to this model.

Behind the four-spoke airbag steering wheel was a customised instrument display with red needles and a rev counter with graduated redline to reflect the variable limiter. The 300kph / 180mph speedometer and smaller read outs for fuel and oil temperature were lifted from the standard 8-series. Located underneath the dials were a couple of digital screens and an array of warning lights.

Upon opening the doors, occupants were greeted by sill plates engraved with the BMW Motorsport logo.

Each car was automatically built to a very high specification with black leather upholstery, electric heated front seats, a three-position memory function for the driver’s seat, an electrically adjustable steering column, electric windows, electric mirrors, dual zone automatic climate control, burr walnut wood trim, an onboard computer, cruise control, central locking and a radio cassette player fitted as standard.

OPTIONS

The list of optional extras available on the 850 CSi included the cross-spoke BBS alloy wheels from the 850i, a rear spoiler, exterior model delete, retractable headlight washers, BMW Individual paint, Nappa leather upholstery, extended leather upholstery, two-tone upholstery, contrast seat piping, a choice of alternative wood trim inserts, extended wood trim, a rear window sun blind, an electric sunroof, an in-car telephone, remote central locking, a ski bag with through loading bay and a choice of superior audio systems.

WEIGHT / PERFORMANCE

On account of its enlarged engine, the 850 CSi weighed in 75kg heavier than the 850i (1865kg as opposed to 1790kg).

Top speed was once again limited to 250kph / 155mph.

The 0-62mph time dropped from 6.8 seconds to 6 seconds flat.

PRODUCTION CHANGES

In June 1993 BMW began production of right-hand drive versions and the original four-spoke steering wheel was switched to a three-spoke item with coloured BMW crest.

From September 1993, a four-spoke M design steering wheel with tri-color stitching on the rim and matching tri-colour stitching for the gear knob was introduced.

USA VERSION

In October 1993 BMW began production of a US version of the 850 CSi which featured a number of detail changes to Euro spec. cars.

Most significantly, US variants did not get the engine oil cooler, the differential oil cooler or active rear axle and came with standard 850i brakes and exterior mirrors.

Other details included amber instead of clear indicators, BMW Motorsport-branded door handles and extended yew wood trim fitted as standard. Most US-bound examples of the 850 CSi also came with the original plain four-spoke airbag steering wheel.

LATER PRODUCTION CHANGES

In January 1995 BMW began to offer larger 18-inch diameter M Parallel Spoke alloy wheels with wider 9.5-inch rears as an option. These were originally shod with 245/40 and 285/35 ZR18 tyres.

At the same time, there were some minor changes made to the brake, security, audio and onboard computer systems lin line with the rest of the 8-series range.