Guide: Mercedes-Benz C111-IID - a Historical & Technical Appraisal

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BACKGROUND

By 1973, the growing importance of fuel-efficient engines was already clear: tough new emissions legislation was forcing manufacturers to clean up their act.

And then the energy crisis hit.

On October 17th 1973, the Arab OPEC members announced an embargo on oil sales to the USA, UK, Canada, Japan and the Netherlands.

The embargo was in response to the USA's support for Israel in the Yom Kippur War where Egypt and Syria, with the support of other Arab nations, had begun a military campaign aimed at regaining Arab territories lost to Israel during the 1967 Six Day War (when Egypt, Syria and Jordan had been the aggressors).

Within a short time, the price of oil had jumped more than 400%.

It continued to rise inexorably, even after the embargo was lifted on March 17th 1974 following the withdrawal of the last Israeli troops.

Mercedes had invested heavily in its new three-litre five-cylinder diesel engine and the firm considered this geopolitical turmoil a great opportunity for their oil burner.

Already renowned for fuel economy, Mercedes sought to disprove diesel’s reputation for noise and lack of dynamism.

To illustrate the power and durability of their ignition-compression engines, the firm came up with the idea of promoting diesel by setting speed records.

A series of short-distance diesel records were undertaken with a modified 240 D saloon.

Afterwards, the Mercedes engineers looked at those records that remained.

They knew the 240 D engine could attain a reliable 190bhp when turbocharged and intercooled. If such a motor was dropped into a C111-II, speeds of around 160mph were predicted; enough to set new records in every class.

The board approved a series of record attempts at the recently constructed 7.8-mile Nardo test track in Italy.

The first C111-II (chassis 31) was wheeled out from storage, equipped for record breaking and re-christened the C111-IID.

ENGINE / TRANSMISSION

The test department carefully prepared an OM 617 LA-derived three-litre five cylinder diesel engine as fitted to the W115 240 3.0D.

Although it only produced 80bhp in standard trim, the engine had plenty of tuning potential.

A Garrett AiResearch T-04B turbo was installed along with an air-to-air intercooler. No wastegate control was needed as the C111-IID didn’t require the kind of torque curve that would hasten its acceleration to cruising speed.

Because the records that Mercedes-Benz sought to achieve were in the under three-litre category, the engine’s 3005cc displacement was reduced by making the cylinder bore 0.1mm smaller.

Special pistons had lower crowns to reduce the compression ratio plus internal oil flow ducts to keep them cool. Thicker wrist pins and special connecting rods were also fitted along with a brand new camshaft.

Heavy-duty main bearings were installed as was a new low-profile sump with enlarged capacity.

Engine durability was increased by adopting sodium cooling for the inlet valves.

As the diesel unit was not as wide as the original four-rotor Wankel engine, there was plenty of space for the intercooler and two oil coolers in the engine bay.

The same five-speed transaxle was employed, albeit with a taller ratio on fifth. It comprised a ZF 5DS-25/1 gearbox with triple-plate clutch and limited-slip differential.

CHASSIS

The drivetrain was fitted to an otherwise sparsely modified C111-II.

Additional fuel tanks were installed in the nose to supplement those already in the sills.

Solid brake rotors were fitted to avoid the slight aerodynamic drag generated by ventilated discs.

Each wheel was equipped with an aero cover and special Michelin 215 VR 15 tyre. These tyres offered low rolling resistance and were capable of running at a steady 160mph with relatively high pressure.

Otherwise, the chassis and torsion bar suspension was unchanged (double wishbones at the front and a triple wishbone multi-link arrangement at the back).

BODYWORK

Several alterations were made to the GRP bodyshell to minimise drag and improve stability.

The nose was extended and now sloped gently downwards.

The retractable headlight mechanisms were removed and bigger radiator cooling vents were carved out from the upper surface. The vents were painted matt black to match those on the rear deck and sail panels.

Powerful new headlights were installed either side of the nose aperture.

The right-hand sail panel cooling scoop was re-routed to feed air to the intercooler.

Wing mirrors were removed and an antenna was mounted on the roof for car-to-pit communication.

1976 NARDO RECORDS

Between June 12th and 14th 1976, Mercedes engineers Hans Liebold, Guido Moch, Erich Waxenberger and Joachim Kaden took turns to drive 2.5 hour sessions for 60 consecutive hours around the Nardo test track.

At an average speed of 156.5mph, they set one record after another without any technical problems.

In total, 16 new world records were set, 13 for diesel vehicles and three absolute records (for over 5000 miles, 10,000km and 10,000 miles).

With a new set of ratios fitted after the event, Hans Liebold demonstrated the acceleration potential of the 190bhp turbo diesel engine and set a 0-62mph time of 6.8 seconds.

The success at Nardo spurred the development team to go even further with a scratch built diesel car in 1978 - the C111-IIID.

Text copyright: Supercar Nostalgia
Photo copyright: Mercedes-Benz - https://www.mercedes-benz.com

Guide: Mercedes-Benz C111-II - a Historical & Technical Appraisal

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BACKGROUND

The C111-I first appeared in public at the Frankfurt Motor Show in September 1969, but it wasn’t long before Mercedes-Benz unveiled an uprated version.

The appropriately named C111-II arrived at the Geneva Motor Show in March 1970.

It incorporated a series of modifications that yielded more power, extra torque below 4500rpm, better driver visibility and improved aerodynamics.

Like its predecessor, the C111-II attracted an enormous level of attention.

However, Mercedes were unwavering in their commitment not to produce it for public sale.

Instead, the firm would use the C111-II exclusively for research and development.

The R&D programme was two fold; establish whether Felix Wankel’s rotary engine had a future and experiment with bodyshells manufactured from glass-reinforced-plastic (GRP).

ENGINE / TRANSMISSION

Whereas the C111-I had a three-rotor Wankel engine mounted transversely in the chassis, the C111-II was equipped with a four-rotor unit.

Each of the piston chambers once again displaced 600cc, which meant overall displacement rose from 1800cc to 2400cc.

Like the three-rotor M950F unit, the new Type M950 KE409 engine ran a 9.3:1 compression ratio with mechanical Bosch fuel-injection.

Whereas some C111-Is had used dual ignition, which proved difficult to adjust, the C111-II was fitted exclusively with single ignition.

Peak power rose from 280bhp to 350bhp at an unchanged 7000rpm. The torque rating was also improved; the C111-II produced 289lb-ft between 4000rpm and 5500rpm compared to 217lb-ft at between 5000rpm and 6000rpm.

Transmission was once again via a five-speed ZF 5DS-25/1 gearbox with triple-plate clutch and limited-slip differential.

BODYWORK

Most of the bodywork changes were aimed at achieving a more aerodynamic profile.

The new car was a little longer and fractionally narrower than before.

Much of the additional length came at the front where the nose was extended and given a flatter upper profile.

Auxiliary driving lights were moved to each side of the nose aperture and the indicators were repositioned to each front flank. Previously, these supplementary lights had been located under clear plastic covers on the upper nose element.

To improve cooling, two enlarged vents between the pop-up headlights replaced the rectangular single bank on the C111-I.

To aid driver visibility, a bigger windscreen was installed. This did without the vaned scuttle trim of the 1969 car and ran a single wiper in place of the twin ‘Clap-Hand’ arrangement.

The engine cover was also modified with enlarged sail panel cavities. The engine cooling vents originally housed on the sail panels were relocated to the flat rear deck.

At the back, a tidier rear fascia incorporated circular tail lights in place of the stacked rectangular arrangement seen in 1969.

All told, these modifications resulted in a drag coefficient of 0.325 compared to 0.35 for the C111-I.

INTERIOR

Considerable changes were also made to the interior.

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The re-shaped instrument binnacle contained a rev counter and speedometer directly behind the steering wheel. Four supplementary gauges were located off to the right.

The stereo, clock and ventilation controls for the air-conditioning were moved to a vertical console that linked the dash and transmission tunnel.

Upholstery was a mostly leather.

Orange and black checked fabric was used the bucket seat centres and large sections of the door panels.

CHASSIS

The revised bodyshell was riveted and bonded onto an otherwise unchanged sheet steel monocoque chassis.

Torsion bar suspension was independent all round with double wishbones at the front and an advanced triple wishbone multi-link arrangement at the back.

Anti-squat and anti-dive controls were retained along with vacuum-assisted ventilated disc brakes.

Changes made to the C111-II included a lower ratio for the steering box, modified rear hub carriers with radius rods and ball joints, rubber bag tanks and bigger 15-inch ATS ‘Penta’ five-spoke wheels.

WEIGHT / PERFORMANCE

Weight rose from 1100kg to 1240kg, but the extra power afforded by the larger engine meant performance was improved.

Top speed was now 186mph (compared to 162mph) and two-tenths of a second was shaved off the 0-62mph time (now 4.8 seconds).

DEVELOPMENTS

Over the next few months, several further developments were made.

They included a system that sensed the fuel level in each tank and automatically directed the return flow from the fuel-injection to the tank with the lower fuel level.

One C111-II was equipped with an automatic transmission that used the torque converter and internal gears from a W100 600. It was housed in a special aluminium case sourced from Abarth. Two alternative shift methods were offered: one was fully automatic while the other used motorcycle-type actuation to select gears via a paddle lever.

An advanced anti-lock brake system was developed with Bosch.

MERCEDES ABANDONS THE ROTARY ENGINE

The four-rotor Type M950 KE409 was the most advanced of the Wankel engines created at Mercedes-Benz. Many of the engineering problems involved in the rotary-piston principle, especially in engine mechanics, had been solved.

However, the Wankel engine’s poor efficiency owing to the elongated, variable combustion chambers of the rotary-piston principle could not be overcome by technical modification.

Inherent design problems meant a Wankel engine would always require more fuel for the same power output as a conventional reciprocating-piston engine.

Furthermore, with high pollutant content and more stringent emissions legislation on the horizon, Mercedes-Benz chose to discontinue work on rotary engines in 1971.

PRODUCTION

Seven C111-IIs were built, one of which had a 300bhp M117 V8 engine and modified engine cover.

However, that was not quite the end of the story. The fuel crisis of 1973-1974 saw Mercedes-Benz turn their attention to diesel. As a way to improve the image of the compression-ignition engine, they used a diesel-powered C111-IID as a record setting test bed…

Text copyright: Supercar Nostalgia
Photo copyright: Mercedes-Benz - https://www.mercedes-benz.com

Guide: Mercedes-Benz C111-I - a Historical & Technical Appraisal

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BACKGROUND

When Mercedes-Benz unveiled their mid-engined C111-I at the Frankfurt Motor Show in September 1969, it garnered massive attention and interested parties began lining up to buy one.

The elegant W113 SL introduced in 1963 had proven enormously popular, but did not offer the same level of performance as its famous predecessor - the W198 Gullwing and Roadster.

By 1969, the W113 was nearing the end of its production life and it seemed natural to assume the startling new C111-I might take its place.

Unfortunately though, this was not to be the long-awaited successor to the iconic 1950s Gullwing.

Despite appearing almost production-ready, and with the high standard of fit and finish one would expect of a Mercedes, the C111 was created purely for research and development. Not even blank cheques from several wealthy individuals could convince the firm to change their mind.

Instead, the objective was to test new propulsion concepts and production methods.

Mercedes had first trialled a rotary-piston engine in late 1965 and wanted to establish once and for all whether the patented Felix Wankel design was the future of combustion engines.

They also used the C111 to experiment with bodyshells manufactured from glass-reinforced-plastic (GRP).

CHASSIS

The C111-I comprised a sheet steel monocoque chassis with a 2620mm wheelbase. To this, the GRP bodyshell was riveted and bonded in place.

Torsion bar suspension was independent all round; double wishbones at the front and an advanced triple wishbone multi-link arrangement at the back.

Anti-squat and anti-dive controls were installed along with vacuum-assisted ventilated disc brakes from the 6.3-litre W109 300 SEL.

Each sill contained a 60-litre fuel tank.

Fuchs 14-inch cast alloy ‘Mexican Hat’ wheels were switched to ATS ‘Penta’ five-spoke rims for the C111-I’s motor show debut. They were both originally shod with 195 VR 14 Dunlop Racing tyres.

ENGINE / TRANSMISSION

Mounted transversely in the chassis was a three-rotor Wankel engine designated Type M950F.

At the time, many manufacturers had an interest in Felix Wankel’s unconventional powerplant, but Mercedes were ready to push the rotary-piston format to its limit.

Compact and quiet compared to traditional reciprocating-piston engines, the motors installed in the C111-I and subsequent C111-II were at the zenith of Wankel technology.

Three rotary piston chambers of 600cc gave an overall displacement of 1800cc.

With compression set at 9.3:1 and mechanical Bosch fuel-injection, the Type M950F produced a peak output of 280bhp at 7000rpm. The torque rating was 217lb-ft between 5000rpm and 6000rpm.

As dual ignition proved difficult to adjust, single ignition was adopted.

The five-speed transaxle comprised a ZF 5DS-25/1 gearbox with triple-plate clutch and limited-slip differential.

BODYWORK

Six examples of the C111-I were built, often with slight detail variations from one example to the next.

The C111-I’s futuristic styling was created by chief designer, Bruno Sacco.

Although not the first mid-engine Mercedes-Benz concept, this new car was a complete departure from the SL-X produced in early 1966.

Sacco’s creation featured a sloping nose with retractable headlights located either side of a radiator cooling grille that was variously given a brushed alloy or matt black finish.

Supplementary lights were housed further down the nose underneath clear plastic covers.

The lower body was painted matt black to give the C111-I a slender profile.

Sacco incorporated Gullwing doors, each of which was equipped with a pair of pressurised hydraulic struts.

An unconventional clap hands wiper layout was used to clear the panoramic windscreen.

Side windows could not be lowered, but a vertical hinge allowed the rear quarter section to open outwards.

Some early C111-Is used a fastback rear windscreen, but this was soon ditched in favour of a small vertical screen with a flat rear deck and flying buttress sail panels either side.

Each sail panel housed a large bank of engine cooling vents that were variously finished in matt black or body colour. Some later examples had cut down vents that allowed the upper portion of the buttress to remain open.

A Kamm tail gave the C111-I a very short rear overhang. Aside from the rear face of the body-coloured engine cover (hinged at the trailing edge), the tail fascia was dominated by a full-width matt black grille. Either side of this were banks of three stacked light clusters.

The first few prototypes were painted metallic white, but all subsequent C111-Is were finished in ‘Weissherbst’ metallic orange. The GRP bodyshell meant the finished car was very light and stable.

Several of the C111-I’s design features were adopted by other manufacturers, most notably the retractable headlights mounted above clear panels for auxiliary lighting. This layout was used on both the Lamborghini Countach (designed by Bertone) and the Ferrari Berlinetta Boxer (Pininfarina).

The Ferrari also incorporated a full width nose-mounted grille similar to the C111-I plus its two-tone livery where the lower body sections were painted matt black.

INTERIOR

Inside, the C111-I displayed at Frankfurt certainly did not have the feel of a development test bed.

In typically Germanic fashion, it was very well laid out.

A boxy dash extended across to the passenger side of the cockpit.

A speedometer and rev counter were housed directly behind the steering wheel. Supplementary gauges were located off to the right and air-conditioning vents were littered all around.

A brand new steering wheel with four arced horizontal spokes and a basket-weave rim would become a Mercedes-Benz staple from 1971.

Upholstery-wise, tartan fabric was used for the seat and door panel centres with leather for the dash, sills, seat bolsters and upper door trims.

WEIGHT / PERFORMANCE

All told, the C111-I weighed in at 1100kg and the three-rotor Wankel engine gave effortless performance.

Top speed was 162mph and 0-62mph took five seconds.

LAUNCH

Demonstrations and test drives were given to select members of the press at Hockenheim between September 1st through 5th.

The C111-I then made its public debut at the Frankfurt Motor Show later that month.

An improved C111-II with four-rotor engine followed at Geneva in March 1970.

For comparison purposes, one C111-I was built with a 250bhp M117 V8 engine.

Text copyright: Supercar Nostalgia
Photo copyright: Mercedes-Benz - https://www.mercedes-benz.com