In April 1971 a new SL rolled out onto the highway, the Mercedes-Benz 350 SL. For the first time in the history of the model series an eight-cylinder power plant did duty under the long bonnet.
From all sides it made the impression of a strong, self-confident, imposing open-top vehicle. Its fathers also gave it an equally well designed, removable coupé roof for the road. Besides elegance and quality the body radiated safety, since the crash behaviour of the two-seater was far ahead of its time.
The decision to manufacture the R 107 series (“R” as in Roadster instead of “W” as in Wagen = car) was taken by the Board of Management after intensive debates on 18 June 1968. At dispute was whether there should be a Targa-roof version, i.e., one with a removable roof panel, instead of the fabric-topped variant, because owing to higher safety standards alarming news was to be heard from the USA regarding the licensing of open-top cars. That a decision finally was made in favour of an open-top two-seater with a fabric roof and an additional removable hardtop can be attributed to Hans Scherenberg, the head of Development, who fought tooth and nail for it: “The SL gave me great pleasure, but also caused me great trouble. This was no easy decision for us,” he summed up the decisive meeting.
The coupé question still was unanswered, however. It was not decided that day. Discussion centred around whether one should additionally, and soon, make a four-seater sports coupé based on the R 107 series, or wait for the coming S-Class (W 116) to build it on that basis. But then a production model would not have arrived until much later, in the mid-1970s.
Karl Wilfert, then the head of Body Design in Sindelfingen, developed – pretty much on his own authority – a coupé based on the R 107 and presented it one day to the Board of Management as a “rough draft”. Rejected at first, Wilfert managed to push through his idea of a sports coupé with the tenacity which was so characteristic of him.
And so six months after its premiere the SL was followed in October 1971 by a comfortable four-seater Sports Coupé, the 350 SLC, whose unconventional lines also found it many friends around the world in the course of the years. Internally the series was designated C 107. Up to the windscreen its appearance matches that of the open-top variant; behind the windscreen the overall height and length grows. A flat roof spans the four-seater passenger compartment in a gentle curve, going over into a large and very steep rear window that arches in two directions. The boot lid is slightly convex in shape, unlike the SL’s.
In the side view the length of the Coupé is documented, firstly, by the 360 millimetre longer wheelbase (2820 millimetres versus 2460), secondly by the line of the side windows. Without awkward B-pillars they are completely retractable, as is usual in a Mercedes-Benz Coupé. The SLC’s coefficient of drag is better than that of the SL so that the Coupé attains the same performance despite an added weight of some 50 kilograms.
A particularly noteworthy fact is that it fully lived up to its classification as a “Sports Coupé”, gaining wins for Mercedes-Benz in many rallies and long-distance races.
Safety as agenda
Béla Barényi’s safety concept with front and rear crumple zones and a rigid passenger cell found expression in the 107 series in a further developed form. The backbone of the R 107 series is not simply a shortened and reinforced saloon floor assembly, as in the predecessor, but an independent frame-floor unit with a closed transmission tunnel and box-shaped cross and longitudinal members which featured differing sheet metal thicknesses and a resultant carefully defined crumple pattern.
The SL definitely had to be an open-top car, and that being the case the only protection in a possible roll-over would be provided by the A-pillar plus windscreen. They were thoroughly redesigned and had 50 percent more strength to show than in the previously built version. In addition, to enhance its strength the windscreen was bonded into the frame. This resulted in a remarkable power of resistance in the roof-drop test with the result that the open-top car could be licensed for the USA even without a Targa bar. To complete the logic the rear window of the hardtop also was bonded into its frame.
Even in the interior there were pioneering changes to report. The hard dashboard made way for an ingenious sheet-steel design that yields on impact both in the top section and the knee area and is foam-padded. The switches and levers were recessed. Another new feature: the four-spoke steering wheel based on the latest findings of accident researchers. The proven impact absorber was still in place, but the steering-wheel rim, spokes, padded boss and hub were covered with polyurethane foam. As further safety feature the fuel tank was no longer installed in the rear end but above the rear axle, protected against collision. From March 1980 the anti-lock braking system ABS was offered; from January 1982 also airbag and belt tensioner.
A bestseller right off the bat
But it was not the safety aspects that motivated customers around the world to quickly reach for the new SL. It was the promise of an open-top automobile – the only one offered in the USA over a period of several years – that was a successful piece of work in every respect. Its distinctive front end with the dominant SL face, the wide-band headlamps and grooved indicator covers had a powerful aura; the lines of the low silhouette were harmonious – soft top open or closed, or with hardtop. And the very slight inward curve of the boot lid, along with the concave hardtop, were reminiscent of “Pagoda” days. The wide-band tail lights with their ribbed surface not only were largely insensitive to soiling, but additionally gave the rear end a touch of vigour.
Extremely conducive to comfort and ease of operation was the easily and speedily operated soft top, a refined version of the “Pagoda” top. It took just 30 seconds to open or close it. Folded, it disappeared underneath a cover that was meanwhile customary in the SL series.
A number of details underscored the car’s safety aspirations. The seats were available from the start with head restraints, and seat belts also were included. Physical well-being and driver-fitness safety were served by the heating system with its very spontaneous response, supported by new air ducting at the doors. Newly developed wind-deflecting mouldings on the A-pillars, which also served to channel off mud-laden water in the rain, and dirt-repelling covers on the exterior mirrors enabled good visibility. They kept the side windows clean even in inclement weather. The windscreen wipers arranged closely to each other in the centre of the car swept a respectable 70 percent of the windscreen area, were always optimally positioned in the flow of air and did not lift off even at higher speeds.
Engines with catalytic converter
During its 18-year “lifetime”, which was not planned to last that long, but in the end was indeed successful, this SL got a whole series of six- and eight-cylinder engines. Its model designations accordingly are quite varied.
The eight-cylinder models were led by the 350 SL (1971 to 1980), whose 3.5-litre engine (M 116) already was known from the W 108, W 109 and W 111 series. The 147 kW which it delivered at 5800 rpm helped the SL, which did weigh 1600 kilograms after all, to clock nine seconds for 0 to 100 km/h and reach a top speed of 210 km/h. The 350 SLC had identical performance figures.
From autumn 1971 onwards the 450 SL also was produced. Its engine (M 117) developed an output of 165 kW at 5000 rpm. Top speed was 215 km/h, and it needed 8.8 seconds to go from 0 to 100 km/h. In 1972 the corresponding Coupé version, the 450 SLC followed, with identical engine and identical performance. Prior to March 1973, both were destined exclusively for export to North America; after that they were included in the general sales range.
In July 1974 the SL model range was extended: as a consequence of the oil crisis of 1973, the SL and SLC now were available as models 280 SL and 280 SLC with the 2.8-litre M 110 engine. It developed 136 kW at 6000 rpm and had proven its reliability in the two years before in the “Stroke Eight” series W 114/115 and in the W 116-series S-Class. Both models had identical performance: the top speed was 205 km/h; sprinting from 0 to 100 km/h in 10.1 seconds was possible.
So three SL engines now were available – nowadays nothing unusual, but in those days something new in the history of this model category. Only the attentive observer could distinguish between the three variants: The 280 SL could be recognised by its narrower tyres in comparison to the 350 SL and the 450 SL. In addition, the 450 SL featured an inconspicuous front spoiler which was attached to the rear lower end of the front apron and distinctly increased the radiator’s air throughput.
Between November 1975 and February 1976 the fuel injection systems of all three engines were changed for better compliance with the emission standards, which meanwhile also had become stiffer in most European countries. The electronically controlled Bosch D-Jetronic was abandoned for the newly developed mechanically controlled Bosch K-Jetronic. The changeover entailed minor losses in performance in all three cases: in the 280 SL to 130 kW at 6000 rpm, in the 350 SL to 143 kW at 5500 rpm, and in the 450 SL to 160 kW at 5000 rpm.
At the same time the compression ratios of the 2.8 and 3.5-litre engines were slightly reduced. The 3.5 and 4.5-litre engines additionally got a contactless transistorised ignition and hydraulic valve play compensation to facilitate maintenance.
The compression ratio of the 2.8-litre unit was raised to the old figure again in April 1978. With a few supporting measures the engine then regained its earlier power potential of 136 kW, but now already at 5800 rpm.
In September 1977 Mercedes-Benz launched the 450 SLC 5.0 with a V8 engine (M 117) enlarged to a displacement of five litres. A hidden innovation was the first-time application of hypereutectic cylinder contact surface machining, which made it unnecessary to insert cylinder liners. The engine delivered 177 kW at 5000 rpm, good for zero to 100 km/h acceleration in 8.5 seconds and a top speed of 225 km/h. The vehicle’s bonnet and boot lid were made of aluminium, and it had light-alloy wheels as standard. On the outside the 450 SLC 5.0 was recognisable by, among other things, a narrow spoiler on the rear end.
Revising the series
At the Geneva Motor Show in March 1980 the SL and SLC presented themselves in updated form. The interior appointments including steering wheel were matched to those of the 126-series S-Class, and the engineering was brought up to the same level. The previous three-speed automatic transmission with torque converter was replaced with a four-speed variant. Models 280 SL and 280 SLC were given a five-speed manual transmission as basic equipment. In addition, the hardtop now was included in the standard specifications of the open-top variant. But above all the light-alloy eight-cylinder engines of the 126-series S-Class, slightly modified, made their arrival in the 107 series. The six-cylinder engine of the 280 SLC remained unchanged.
The new 500 SL, equipped with the 5.0-litre V8 (M 117) familiar from the 450 SLC 5.0, replaced the 450 SL and made an output of 177 kW at 5000 rpm available, to give the new top-of-the-range model a 0 to 100 km/h acceleration of 7.8 seconds and a top speed of 225 km/h.
Models 350 SL and 350 SLC were sent off into retirement after nine years of production. Their successors were the 380 SL and 380 SLC, whose 3.8-litre light-alloy engine (M 116), with 160 kW at 5500 rpm, originated after the pattern of the five-litre unit, by enlarging the bore of the long-serving 3.5-litre V8 with grey cast iron cylinder block. Both models attained top speeds of 215 km/h and needed nine seconds to go from 0 to 100 km/h.
From the outside the new models were almost indistinguishable from the previous models, except for the model plate. All three SL models now had a light-alloy bonnet and the discreet front spoiler familiar from the 450 SLC 5.0; the 500 SL also got a light-alloy boot lid with black plastic rear spoiler, already familiar from the five-litre Coupé.
In autumn 1981 both V8 engines were thoroughly redesigned in the context of the “Mercedes-Benz Energy Concept” to reduce their consumption and pollutant emissions. Along with an increase in compression ratio the measures comprised camshafts with changed valve timing, air-bathed injection valves, and an electronic idling speed control. Owing to the altered camshaft tuning the maximum torque could be shifted to a lower engine speed range and, in the case of the 3.8-litre engine, even increased. This power plant underwent particularly far-reaching changes: to get a more favourable volume-to-surface ratio the bore was reduced and the stroke increased. The modified 3.8-litre V8 thus had a slightly larger displacement. In both eight-cylinders, in exchange, so to speak, for the improved economy, minor losses in power had to be accepted, output dropping to 150 kW at 5250 rpm in the 380 SL and to 170 kW at 4750 rpm in the 500 SL. As in the 126 series the final drive ratio was adjusted to the changed engine characteristics and made higher, from 3.27 to 2.47 in the 380 SL and from 2.72 to 2.24 in the 500 SL.
For the SLC Coupés these changes came too late, however: at the Frankfurt International Motor Show in September 1981, along with the “Mercedes-Benz Energy Concept” the 380 SEC and 500 SEC models of the C 126 series were presented, spelling retirement for the SLC models, which had been built for exactly ten years.
But even after ten years of production no thought was being given to a replacement for the SL models. Four years after the Energy Concept was presented, they even came in for extensive refinements, and so in September 1985, again at the Frankfurt show, a completely revised SL model range was introduced. The emphasis was on a restructured engine range. A discreet facelift, primarily recognisable from the front spoiler and wheels with aluminium rims (diameter: 38.10 centimetres), was also part of the package. The front axle was done over and the brakes enlarged with fixed callipers. To prevent the cars from pulling to one side during braking, the steering offset was reduced.
Comprehensive model refinement package
All engines were available in two versions: with a catalytic converter and slightly less output, and as so-called catalyst retrofit version without catalytic converter. The catalyst retrofit versions could be fitted later on with a catalytic converter, for example when the widespread supply of unleaded petrol was ensured, and had their ignition system, electronics and cable harness prepared for this.
A newly designed 3.0-litre six-cylinder engine (M 103) which had made its first appearance in the 300 E of the mid-range series W 124 nine months earlier replaced the tried and tested 2.8-litre engine, as it had already done previously in the respective S-Class Saloon. As a result the 280 SL was discontinued, and after a 22-year interruption there was again a sports car with the magic model designation 300 SL. It delivered an output of 138 kW at 5700 rpm without catalytic converter (top speed: 203 km/h; 0 to 100 km/h in 9.6 seconds) and 132 kW with catalytic converter (200 km/h; 9.9 seconds).
A new edition to the range was the 420 SL with the 4.2-litre V8 engine (M 116), which at 5200 rpm delivered 160 kW without and 150 kW with catalytic converter. It was created by adopting the bore of the original 3.8-litre engine and combining it with the stroke of the “post-Mercedes-Benz Energy Concept” 3.8-litre engine, and now replaced that unit in the SL, the S-Class Saloon and the SEC Coupé. The 420 SL accelerated from 0 to 100 km/h in 8.5 seconds (with catalytic converter: 9 seconds) and attained a top speed of 213 km/h (205 km/h).
The 5.0-litre engine (M 117) also was modified; with catalytic converter operation in mind it now had an electronic ignition system and the electronically/mechanically controlled Bosch KE-Jetronic injection system and developed an output of 180 kW at 4750 rpm. With catalytic converter the output was 164 kW at 4700 rpm. These values helped the 500 SL attain a top speed of 225 km/h (with catalytic converter: 215 km/h) and accelerate from 0 to 100 km/h in 7.3 seconds (7.8 seconds).
The most spectacular new development in the engine range was a 5.6-litre eight-cylinder (M 117), which was created by increasing the stroke of the 5.0-litre V8 and gave the SL an output of 170 kW at 4750 rpm. The 560 SL was reserved to the export markets USA, Australia and Japan. Fitted with an emission control system in the US version it had a top speed of 223 km/h and sprinted from 0 to 100 km/h in 7.7 seconds.
Production of the R 107 series ended in August 1989, more than 18 years after the production start-up of the 350 SL. With that this SL series set an internal record that probably never will be broken: in the entire history of the company no other passenger car series has ever been produced over such a long period. All told, in Sindelfingen 237,287 open-top cars were built, a number which impressively demonstrates the great popularity of the 107 series. Of the Coupé a total of 62,888 were built from 1971 to 1981.
The R 107 series in the press
In a first test of the Mercedes-Benz 350 SL auto motor und sport, Germany, No. 9, 1971, wrote: “Good suspension comfort, definitely up to saloon standard, proves to be an essential feature of the 350 SL: at low and high speeds it absorbs big bumps well and takes small bumps in a way that they never are a disturbance even on very poor roads.”
In 1986 Road & Track, USA, No. 11, 1986, compared the Mercedes-Benz 560 SL, which was in the last era of its production, with the Cadillac Allanté and summed up: “Legendary quality is Mercedes’ primary stock in trade. But brilliant performance and outstanding ABS braking have freshened it this year. Against these attributes, Cadillac brings better handling and greater luxury to bear while failing to match Mercedes’ performance and quality.”
Source: Daimler AG