BMW M history
BMW M635CSi E24
Based on the BMW E24 6 Series, the BMW Motorsport developed its own version of this already beautiful and powerful GT. This version is known as the M635CSi. Although never labeled officially as an M model in Europe, all North American versions were officially named "M6", both to simplify the name and to bring it in line with the other M models, specifically the M3 and M5 at that time.
|1983-1989 BMW M635CSi E24 Coupe|
M88/3 (S31 B35ME)
210kW | 286HP
255km/h | 158mph
|Fuel Consumption (l/100km | mpg)|
18 | 13
Source for technical specifications: Wikipedia.
Like all 6 Series, the M635CSi/M6 was built on the normal Dingolfing assembly line, with body shells shipped from the Karmann works in Osnabruck. BMW Motorsport used the M88 or S38 twin-cam 24-valve inline-six-cylinder engines first introduced in the M1, tuned the chassis and added a few special cosmetic pieces for a better aerodynamic drag.
There were few major mechanical changes during the five-year period that the M635CSi was produced, though certain items with known mechanical defects (valve springs, two-piece wheels) were updated during the course of production. In addition, the catalyst-equipped S38 engine became available as an option in some left-hand drive European markets starting with November 1986 production (two months after it became the standard M6 engine in North America). Also beginning in 1986, a fixed rear center console with integrated storage compartment and air conditioning unit became available in some markets and Shadowline (dechromed) exterior trim was added to the option list.
The biggest cosmetic changes occurred starting with June 1987 production. At this time, the slimmer chrome-trimmed bumpers were replaced with larger versions featuring body-color end caps. Also, ellipsoid headlights and foglights replaced the halogen units of earlier cars. The latter were part of a reshaped front airdam that was shared with the regular 635CSi. Finally, the shift knob was updated to the ergonomic design introduced a year earlier on the E32 7 Series, and the Highline complete Nappa leather interior, already standard on U.S.-spec M6s, became available as an option in other markets. (Note: All right-hand drive examples of the "big bumper" M635CSi have the Highline Nappa leather interior.) The final change occured in Spetember of 1988 when the power front seat controls were moved from the center console to the base of the seat itself.
Though there were no major mechanical alterations during the production of the North American M6, the are a number of cosmetic differences between M6s built before June of 1987 (1987 models) and those built after that date (1988 models). The 1987 models have large, government-mandated five-mph bumpers with black rubber trim and a unique front air dam that is similar to that of the 1985-87 North American-spec 635CSi, but with extra cooling ducts for the engine oil cooler. The 1988 model features the same updated bumpers, ellipsoid headlights and revised front airdam with flush-mounted foglights as the facelifted, European-spec M635CSi. It also has the ergonomic shift knob. U.S.-specification M6s built to 1988 specification are further fitted with a driver-side airbag with knee bolster, but lack the leather dashboard and upper door trim of the 1987 M6 Highline models (though they retain Nappa leather upholstery on the seats, center console, lower door panels, glovebox door, A-pillars, B-pillars, headliner and sun visors).
The engine of the BMW M635 CSi E24
The M88 and S38 engines are evolutions of the 24-valve inline-six developed for the mid-engine M1 supercar. Like that engine, both the M88 and S38 have a four-valve crossflow cylinder head (with 37mm inlet valves and 32mm exhaust valves) designed for better breathing at high rpm and a pair of 264-degree camshafts. They also share a displacement of 3,453cc via a bore of 93.4mm and a stroke of 84mm. However, for their application in the M635CSi/M6, the M88 and S38 have unique pistons and connecting rods, as well as a more advanced Bosch ML-Jetronic fuel injection system. To save costs, both engines utilize the standard steel M30 crankshaft and wet-sump lubrication system. Many other smaller items, such as the ignition system, are shared with the regular 6 Series as well.
The M88 is the non-catalyst version of the S38 engine. With a 10.5:1 compression ratio and a single-row timing chain, it is identical to the one found in Euro-spec E28 M5 sedans. Power is rated at 286 hp (DIN) at 6,500 rpm with 251 lb/ft of torque at 4,500 rpm. The M88 can be identified by the inscription on its cam cover: the propeller Roundel plus the words "BMW" or "M Power."
The S38 is the "clean" version of the engine, introduced for North America but later offered in other markets as well. It utilizes a duplex (double-row) timing chain and lower 9.8:1 compression designed to work with catalytic converters. This reduced the horsepower to 256 hp (SAE) or 260 hp (DIN) at 6,500 rpm and torque to 243 lb/ft at 4,500 rpm. The S38 has the inscription "BMW M Power" on its cam cover but no Roundel.
All M635CSi/M6s have the Getrag 280/5 five-speed manual gearbox. Gear ratios are as follows: 3.51 (1), 2.08 (2), 1.35 (3), 1.00 (4), .81 (5). A 3.73:1 final drive is fitted to all but the North American M6, which uses a 3.91:1 rear end. All have limited slip differentials with a maximum locking of 25 percent.
All M635CSi/M6s use the MacPherson strut/semi-trailing arm suspension design common to all E24 6 Series. However, it has been tuned by BMW Motorsport in the following ways:
- Ride height reduced by 0.4 inches
- Increased steering caster
- Spring stiffness increased by 15 percent
- Progressive rate rear springs
- Specially-tuned Bilstein shocks
A self-leveling rear suspension was optional in some markets and standard on all North American-spec M6s.
The M635CSi/M6’s recirculating ball steering with ZF power assist is identical to the one used in the regular 635CSi and has an overall ratio of 15.7:1. Like the 635CSi, cars built for the final 1989 model year have vehicle-speed sensitive Servotronic power steering made by ZF.
Brakes and tyres
The M635CSi/M6 has enlarged vented front rotors that measure 11.8-inches (300mm) in diameter. These utilize four-piston calipers. The solid rear rotors, measuring 11.2-inches (284mm) in diameter, are the same as those found on the 635CSi. ABS is standard.
There were two available wheel and tire sizes for the M635CSi. Early European-spec cars used the standard 6 Series metric-sized wheels (390mm x 165mm) with 220/55VR390 Michelin TRX tires. However, optional from the start of production (standard in some markets) were larger, two-piece alloys in a cross-spoke design measuring 415mm x 195mm (16.3-in. x 7.7-in.). Cars with these wheels carry 240/45VR415 Michelin TRX tires. After the two-piece wheels showed signs of cracking, BMW switched to a similar one-piece cross-spoke design in the same size as part of the June, 1987 facelift. However, these wheels were also fitted to all North American M6s from the beginning of production.
The M635CSi has few visual clues to distinguish it from the normal 6 Series. Almost all body panels are shared with other E24s, including the body-color rear spoiler with black rubber lip. The only exception is the deeper front airdam fitted only to pre-facelift models, which is ribbed on the lower portion. There are also two M badges, one on the trunklid and one on the grille, though these could be omitted upon request. In addition, the rear-view mirror housings are painted in body color instead of the chrome finish used on all other pre-facelift 6 Series. Finally, cars with the BBS-style cross-spoke wheels (in either the early or later version) have body-color fender lip extensions. After the mid-1987 facelift, the front airdam was changed to the same one fitted to all 6 Series, so the M635CSi could only be identified by its M badges (where fitted) and 415mm (16.3-inch) cross-spoke alloys, now standard and in a one-piece design. As with the 635CSi, Shadowline (dechromed) exterior trim became optional on the M635CSi during 1986 but only with limited exterior colors.
Cosmetically, all North American-spec M6s have unique grille and decklid badges with the "M6" logo. They also have a rear spoiler (not fitted by the factory to other North American 6 Series models) and the later one-piece metric-sized cross-spoke alloy wheels. Cars built for the 1987 model year (before June, 1987 production) have the bulky five-mph bumpers used on all 1978-87 6 Series in North America. The 1987 M6 also has a unique front airdam, with additional cooling slats. Cars built for the 1988 model year (6/87 and later) have ellipsoid headlights and a slimmer 2.5-mph bumper design that is visually identical to the European-spec M635CSi of the same vintage. The front airdam on these cars is also identical to the one used on all 1988-89 6 Series. All North American M6s have chrome (non-Shadowline) exterior trim.
The interior of the M635CSi is much like that of all other 6 Series models. The only truely unique item is the instrument cluster, which contains a 280 kph/170 mph speedometer and a special tachometer with the M logo instead of an economy meter. In addition, every M635CSi is fitted with the three-spoke M-Technic steering wheel (unless equipped with the optional driver's airbag) and the same sport seats available in other 6 Series models. In some markets, these could be replaced by the wider "comfort" chairs, also shared with other 6 Series models. Finally, the battery of the M635CSi is mounted in the trunk for better weight distribution.
The M6 interior differs little from that of the M635CSi. Like the European model, North American examples have the unique instrument cluster with a 280 kph/170 mph speedometer and a special tachometer with the M logo. However, they also have a generally higher level of standard equipment, including leather sport seats with power controls and driver’s side seat memory, power sunroof, cruise control, air conditioning, rear window sunshade and an eight-speaker am/fm cassette sound system. U.S.-spec cars are further enhanced by Nappa leather upholstery with either the Highline (1987 models) or Halfline (1988 models) interior trim and a rear center console/air conditioning unit. On 1988 models, a driver’s side airbag is also standard for U.S.-spec examples. Heated front seats were standard on the Canadian-specification M6 and the sole factory option on U.S models.
To add additional luxury to the M635CSi late in its product cycle, BMW introduced a Highline interior option as part of the mid-1987 facelift. It is worth noting that all left-hand drive facelifted examples are not necessarily Highlines, as they are often incorrectly referred to. In other words, there are a small number of left-hand drive facelifted cars without the Highline interior option.
The Highline interior includes hand-stitched Nappa leather on the seats, door panels, center console, glovebox door, headliner, sun visors, door pillars, upper door trim and dashboard. This interior was standard on all U.S-specification M6s built before June 1987, as well as all "big bumper" UK-specification M635CSi examples. In some other markets, the Highline interior could be ordered as an extra-cost option.
A special variation of the Highline linterior was introduced on all U.S.-specification M6s beginning with June 1987 production (1988 model year) after BMW of North America was forced to replace a significant amount of 1987 M6 leather dashboards that had been warped by the sun in warmer parts of the United States. Unofficially known as the "Halfline" interior, it included all the Nappa leather trim of the Highline interior except for the leather dashboard and leather upper door trim.
In general, the level of standard equipment increased on the M635CSi from year to year. Depending on the market and production date, options included: cross-spoke alloy wheels, metallic paint, Shadowline (dechromed) exterior trim, headlight washer/wipers, leather upholstery, air conditioning, rear air conditioning and storage unit, power front seats with driver's side memory, heated front seats, power sunroof, rear window sunshade, rear headrests, and various audio systems.
There were no official special editions of the E24 M635CSi or M6. However, BMW of Great Britain offered an unofficial
"Motorsport Edition" of the right-hand drive M635CSi in three special color and trim combinations: Misano Red (236) with Black
Nappa Highline leather upholstery, Nogaro Silver metallic (243) with Black Nappa Highline leather upholstery and Macao Blue metallic (250)
with Lotus White Nappa Highline leather upholstery. Produced in one consecutive batch of 23 examples between October of 1988 and January
of 1989, the "Motorsport Edition" included all of the standard UK M635CSi equipment, plus Shadowline exterior trim and M stripe
lapels on the front and rear seats. (Note: Production of the standard RHD M635CSi then resumed for one additional month.)
- ECE (LHD without cat): 3,283 built from 12/83 thru 12/88
- ECE (RHD without cat): 524 built from 09/84 thru 02/89
- ECE (LHD with cat): 117 built from 11/86 thru 12/88
- NA (LHD with cat): 1,767 built from 09/86 thru 09/88
- JAP (LHD with cat): 164 built from 09/86 thru 03/88