Guide: Mercedes-Benz C111-IID
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 / Gearbox
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
