BMW E24 633/635CSi: Bavaria's Supersized XU-1

The desirable final version of BMW’s final E24 6-series which reached Australia in 1989 looked surprisingly similar to the original 1976 model. Under the skin, it was a very different car reflecting BMW’s huge progress in build quality, technology and engineering and buyer demands for a sportier edge.  

For a 1976 release, the BMW E24 6-series coupe enjoyed astonishing longevity until production ceased in April 1989 with its 1960s shark-nose styling barely changed. Although sales tapered off at the very end, the E24 posted best ever figures in the mid-1980s. The arrival of the M635CSi and new unleaded versions for emissions markets like Australia kick-started new interest.

Bigger than an LJ Torana GTR XU-1, the BMW 633CSi was surprisingly close in concept and profile including a hot inline six of similar size tightly wrapped by a centre-pillar four seater semi-coupe body. Like the Torana, it was much smaller than the “big BMW coupe” description suggested with a length of just 4755mm on a 2630mm wheelbase and a 1725mm width that was only just outside the Japanese 1700mm width limit. These dimensions are so close to a VK Commodore Group A not to matter except at around 1540kg, it was 200kg heavier, in anticipation of tough new crash requirements. Cutting this weight back to 1450kg on later cars changed the performance equation entirely even if full house luxury features soon put it back on.

The Torana connection is not entirely fanciful from a design perspective either. The E24 6-series marked a new design direction for BMW with its fuller and more aggressive profile appearing again just a year later on the first 7-series sedans, for a more serious attack on the top shelf Mercedes-Benz models. The styling was by Paul Bracq, a Frenchman with an enviable industry record after starting with Mercedes-Benz. His CV includes the 600 Der Grosser, the 1960s 220/300/250/280 coupes, the “pagoda roof” 230SL and the W108 250/280 sedans. In short, he created a new set of Mercedes-Benz design cues, and after a stint designing the French TGV trains, he faced a similar brief at BMW from 1970.

The next chapter in BMW styling was almost a blank page and one of the most exciting briefs in the industry. It also coincided with Bob Lutz’s appointment at BMW following his eight year career with General Motors in Germany. Lutz is credited with the Bracq-designed E21 3-series as an all-new model. The 6-series was the hero context for BMW’s new number-designated range that survives to this day.

Early 6-series examples were based on the E12 5-series, the first of a new range of models that reflected former Mercedes-Benz designer Paul Bracq’s new direction for BMW.  

It started with Bracq’s amazing Turbo gull-wing mid-engined concept car in 1972 that would later evolve into the limited build M1 in 1979. The Turbo established his new design context inside and out for the first E12 5-series that also arrived in 1972. The theme was extended in his 3-series (1975), 6-series (1976) and 7-series (1977) all of which would appear after his 1974 exit to Peugeot. This progression explains why the first 6-series under the skin owed more to the E12 5-series while later versions were tied to 7-series improvements.

GM designers posted to Germany recall quietly being invited to come on over and help Lutz fine-tune the new models and support Bracq’s tiny BMW team after hours. It was clear that the inspiration flowed in both directions as GM’s European divisions were soon producing some of the world’s best mass-market designs during this period.

Because the US market had taken an instant shine to Bracq’s new BMW designs even in a recession, Bracq’s addition of European discipline and class to the 1968-69 Mustang’s short-tail, long-bonnet, shark-nose three-box proportions was inspired. A closer look will reveal that E24 detailing was very similar to Holden’s similar transformation of the HB Viva into the LC Torana GTR.

Paul Bracq’s BMW Turbo concept car set the context for a rush of all-new BMW models that started in 1972. Along with BMW’s striking new “four cylinder” headquarters in Munich, it helped establish BMW as fresh and adventurous compared to its conservative arch rival Mercedes-Benz.

These included a wide grille broken up by body paint, accentuated centre grille and bonnet sections, slender bumpers, large horizontal tail lights, concealed centre pillar, sloping rear deck, shark-nose side profile and several body side crease lines to make the deep sides look sleek.

The 6-series had something else in common with the LC Torana GTR and the similar-sized local Charger. It was not a 2+2 coupe. Its big inline six took up plenty of space out front after competing with the full four seater cabin for its share of the relatively short wheelbase. As a result, any weight increases in the engine bay impacted on the car’s balance more than most sporty coupes.

Lutz, Bracq and BMW resolutely resisted the wedge and shovel nose that were both big trends at the time. A contemporary of the Lotus Esprit, Jaguar XJ-S, Porsche 924/928, Ferrari 308 GTB and Mazda RX-7, the 6-series in this context was semi-retro from the start. By charting its own course with constant improvement under the skin, it had an unusually long life for a coupe that was looking fairly boxy by the 1980s.

On its 1976 release, the 6-series coupe seemed less bespoke, more mainstream than its 3.0CS/CSi predecessor. This was intentional as it was the core hero coupe and styling context for BMW’s new sequential model number series that survives to this day.  

Model History

Released in the first half of 1976 in 630CS carburettor and 633CSi fuel injection versions, the first E24 models came with a four speed Getrag manual or three ZF auto. Even if the E12 5-series suspension was stiffened, the basic front struts and semi-trailing arm IRS rear left plenty of room for later improvement. The impressive dash was the next development of the 1972 Turbo concept car’s cockpit style interior.

Because so many 6-series models reached Australia as grey imports from a number of RHD markets during the years they were not officially offered in Australia, all models are of interest.

The first cars were built by Karmann before final assembly was moved to BMW later in 1977. As the 6-series attracted more enthusiast drivers than expected, it needed more grunt for Europe. In July 1978, the first 635CSi was launched with a single cam version of the early 3453cc engine on which previous 3.5-litre racing engines and the M1 powerplant were based. It gained a new Getrag 5-speed manual and stiffer anti-roll bars. It was the first with front and rear spoilers, the latter a thick black rubber-feel plastic item on the bootlid.

For 1979, a new 628CSi shared the 528i fuel injected engine and replaced the carburettor 630CS for better economy and comparable performance. The 633CSi was dropped in Europe. The 633CSi, which continued in other markets, gained a new generation DME (Digital Motor Electronics) engine management system for more precise fuel injection. ABS became an option. For the train spotters, the clock went from analogue to digital, one of the many changes made to the 6-series dash over the years.

This 1985-spec 635CSi is similar to the new unleaded 635CSi that returned to the Australian market in 1986 after deliveries stopped in 1981. Few local cars were sold with the wheels shown here as the Michelin TRX wheel and tyre combination was standard.

For 1980, the DME system was added to the 635CSi which now featured a more slender rear spoiler design. A unique US version of the 633CSi with several 635CSi details replaced the underpowered Federal-spec 630CSi.

The first major exterior changes occurred in 1981 after the rear bumpers were extended to the rear wheel arches and there was a new design alloy wheel. Special Michelin TRX rims and tyres with their unusual size were offered as an option.

The year 1982 was a big one for the 6-series with almost every area receiving attention, enough to qualify as a second generation. Upper and lower nose styling was now integrated. Fog lamps were flush-mounted in the front panel work of the 628CSi/633CSi although they still sat proud of the 635CSi’s larger front spoiler.

It marked the point where BMW addressed major engine reliability issues and suspension shortfalls in the 635CSi to the point where it could be considered for racing. Over 60kg was cut from the body, the process leaving it stronger, more crashworthy and lighter.

This pre-1984 635CSi series with its forward-mounted fog lights was not sold in Australia but this car features the TRX wheels and tyres fitted to 1986 Australian deliveries. They are now rarely seen as they lock owners into expensive tyre replacements with their unique metric diameter.

BMW’s new double-jointed front suspension lower control arms and re-calibrated rear trailing arm suspension angles with extra rear toe control links from the 7-series were transferred to the 6-series range. The package improved handling dramatically. ABS was standard on the 635CSi and optional on other models before it was standard from late 1984.

The big news was a new 635CSi engine that addressed reliability issues generated by the old one’s larger bores. Starting with the later 633CSi engine design, BMW combined its 86mm stroke with a new 92mm bore for a slightly smaller 3430cc capacity versus 93.4 mm bore/84mm stroke and 3453cc for the old one. Upgraded DME/Motronic fuel injection and a boost in compression ratio maintained outputs while improving tractability and fuel economy by a significant margin.

For 1983, ZF’s latest four speed automatic was offered as an option, the extra ratio cutting fuel consumption. The new transmission allowed BMW to link it with the engine’s DME for further refinements later. Just as the 6-series entered old age, a wild new M635CSi was previewed late in 1983 and generated renewed attention before it went on sale in 1984.

The M635CSi marked a 1984 return to the old 3453cc engine, this time a heavily modified version of the 24 valve twin-cam engine as fitted to the M1 mid-engined coupe. A wet sump replaced the dry sump while new heads and inlet tracts allowed the new engine to be inclined and thus clear the 6-series bonnet. A big boost in compression ratio combined with a new ML-Jetronic injection system and Motronic engine management boosted outputs considerably over the 635CSi while maintaining its fuel economy.

Stiffer springs, lower ride height, Bilstein gas shocks, chubbier anti-roll bars and four pot cross-drilled ATE front discs kept it tidy. A five speed close-ratio Getrag with the old dogleg shift pattern was the standard transmission. A beefier limited slip diff inside the larger 7-series housing dictated a reworked floor section beneath the rear seat.

A late model BMW M635CSi like this one was never sold in Australia but a right hand drive version introduced in 1985 allowed a small number to be privately imported. Its engine was a wet sump version of the one in the M1 next to it.

A choice of bigger Michelin TRX wheel and tyre packages prompted reworked wheelarches and there was a bigger, squarer front spoiler.

All models gained the latest two-stage reflector headlight design and the fog lights on the 635CSi were finally recessed into the front spoiler. The 635CSi auto option was offered with several programmable modes.

Two developments of interest to Australians included a RHD version of the M635CSi for 1985 and a special unleaded 635CSi for emissions markets. Although the power output was cut significantly over European versions, the emissions 635CSi offered more torque than the old US 633CSi emissions engines and leaded 2.8 models so it could be standardised across the 535i, 635CSi and 735i ranges in markets like Australia.

The 1985 specification was upgraded for 1986 with optional rear air-conditioning and M-Technic rear spoiler.

For 1987, the 635CSi engine was upgraded with the DME III from the next E32 7-series generation for a big boost in output (136kW to 155kW) and better fuel economy but not locally. An unleaded version of the M635CSi was also developed then sold in the US. Climate control was offered for the first time.

By mid-1987, the new ellipsoid headlight technology also from the latest E32 7-series was applied to the headlights and fog lights on the 6-series. Along with new impact bumpers standardised across all markets, these were the most obvious changes since 1976.

For 1988, there were no changes as the E24 entered its last year of production. The last car was built in April 1989 for a total of 86,216 between 1976 and 1989.
(References: http://www.e-24.ru/eng/, Glass’s Guide UK, BMW)

This M635CSi engine bay shows how snug it was for all engines in E24 6-series models and why weight changes were critical to chassis balance. No surprise then that tired engine mounts can allow the fan to cut into the radiator under heavy braking. (Image from: www.m535i.org)

Australian 6-series Deliveries

The first factory E24 6-series examples didn’t reach Australia until 1977, just ahead of the new 7-series. Although changes followed Europe, give or take several months depending on sales and shipments, the 3210cc engine had ADR 27a emissions controls for more conservative power figures.

Deliveries ended in 1981 with a handful of the second generation models arriving before it was withdrawn from the local BMW range. Because this coincided with a flatlining of interest in other markets, ongoing local demand was fed by a prolific range of cheap private imports that included the full European range of 6-series models until 1986.

Depending on where they came from, these imports can be riddled with structural rust that can be terminal. Key safety areas such as sills, rear floor, mounting points for rear suspension and steering were especially vulnerable.

Following BMW’s 1985 introduction of the unleaded 635CSi engine, this model was offered from May 1986 in the European 1985 body specification following the 1986 local introduction of low octane unleaded fuel. Few examples were the same as there was a choice of the wide-ratio ZF manual, the dogleg close-ratio Getrag manual and the programmable 4-speed ZF 4HP-22 automatic.

The standard wheel and tyre specification for most, if not all early, Australian deliveries was the Michelin TRX alloy wheel and tyre package. Because its oddball 165 TR390 rims and 220/55 VR 390 tyres were close to 15 ½ inches in rim diameter, it was common to replace them with 16 inch alloy rims and tyres to avoid the high tyre costs or to access later tyre technology. Later deliveries gave buyers the choice.

Until the final 1989 shipment, most Australian 635CSi deliveries were based on the 1986 specification until 1988, by which time several M-series options were offered including body and wheel packages.

At the very end, the impact bumper model with its many E32 7-series upgrades including the upgraded engine was offered in 1989, two years after its European introduction. Even if most were automatics, they were a desirable if expensive package.