Mercedes-Benz W 25 record car, 1936
The first long-distance duel between Auto Union and Daimler-Benz in 1934 for new land speed records, in which the Untertürkheim company also set a number of new records, set in motion an unimagined competitive dynamic over the next few years, which was noted with approval by the National Socialist rulers, who were always interested in activities that would attract publicity.
At Daimler-Benz, however, the topic was initially approached cautiously. The W 25 record-breaking car of late autumn 1934 was nothing more than an aerodynamically slightly modified Grand Prix racing car of the 750 kg Formula. In 1935, the record-breaking field was left entirely to the highly motivated competitor from Zwickau. Even in the spring of 1936 the new records achieved by Hans Stuck with the now much more powerful V16 mid-engined car were only followed from a distance. Only after prematurely ending the Grand Prix season, which had been such a bleak one due to inferior car material, did they strike back in the record runs.
The bar for new records was now so high that not much could be achieved with slightly modified Grand Prix cars. Although the chassis of the W 25 was still suitable as a basis, new ground had to be broken both with regard to the aerodynamic quality of the body and in terms of engine performance. The supercharged inline eight-cylinder engine of the ME 25 evolutionary stage, which had been used in the 750 kg Formula racing car until then, was simply maxed out in terms of performance with 494 hp/363 kW from 4.7 litres of displacement.
Fortunately, there was a promising alternative: the twelve-cylinder engine with the designation MD 25 DAB designed by designer Albert Heess from the beginning of 1935. The engine was originally intended to succeed the inline eight-cylinder and provide superior propulsion in a 750 kg Formula racing car. However, the very compact 5.6-litre V12, which had two Roots blowers, weighed almost 295 kg and was too heavy to comply with the 750 kg limit set by the Grand Prix regulations.
The main reason was the adherence to the traditional design of Mercedes-Benz racing engines with steel cylinders and welded-on sheet steel cooling jackets. The cylinders themselves were divided into two units of three per bank and welded to the cylinder heads to form a single unit. The dimensions for bore and stroke of 82 x 88 mm, as well as the entire design of the cylinder heads, corresponded to the M 25 AB, the first evolutionary stage of the inline eight-cylinder M 25 that was never used in races. The V12, equipped with two times two overhead camshafts and four valves per cylinder, had a cylinder bank angle of 60 degrees and an interesting design peculiarity: due to the use of forked connecting rods, the two cylinder banks were not slightly offset, as was actually customary, but parallel to each other. This trick saved Heess a few centimetres in overall length.
In terms of performance, the twelve-cylinder left nothing to be desired. During tests on the engine test stand and using one Roots blower with one hermetically sealed pressure carburettor per cylinder bank, the high-revving, only slightly long-stroke engine delivered a maximum output of 616 hp/453 kW at 5800 rpm - at least before the scheduled record runs - according to factory documents.
In order to achieve the targeted speeds in the region of 350 km/h, systematic aerodynamic shaping of the bodywork was required in addition to pure engine output. The experts at the Zeppelin airship construction company in Friedrichshafen, who were experienced in aerodynamics, were therefore consulted and it was realised that a fully faired body enclosing the wheels was absolutely necessary in order to achieve the best possible drag coefficient. The smooth body, low and tapering at the rear with a minimal cockpit opening, which was subsequently created in Untertürkheim, had a sensational drag coefficient of Cd = 0.235, but was beset with two problems that urgently needed to be solved. On the one hand, the low silhouette of the body did not allow enough space to completely cover the up to 22-inch rear wheels, and on the other hand, wind tunnel tests in Friedrichshafen revealed that the shape of the body produced massive front-axle lift.
Both shortcomings were eliminated with solid solutions. The part of the rear wheels that protruded beyond the body contour was covered with an additional fairing that complemented the car's lines passably well, while the lift, which with a measured value of 217 kg massively endangered driving stability at the highest speeds, was counteracted by attaching lead weights to the front axle.
The designers made some details of the body specification the subject of practical testing on the day of the runs. For example, there were different variants of the cockpit windscreen, and there was a choice between open and fully covered wheel arches, between wire wheels with and without an aluminium disc cover, and between rear wheels that ran open beyond the body silhouette or were fully covered.
The record-setting attempts were scheduled for 26 and 27 October 1936, where it had to be proved whether the significant effort that had been made in the meantime could be turned into results that would attract public attention. The scene was a newly constructed section of motorway between Frankfurt and Darmstadt, which had been closed in the direction of Frankfurt for the record-breaking runs. The driver was Rudolf Caracciola, who was at the peak of his career - not a daredevil, but an always level-headed driver with a great deal of sure instinct and a pronounced risk awareness.
The day started early and with unexpected problems. During the warm-up run at 7 a.m., the transmission popped out of 4th gear at high speed, leading to brief over-revving of the V12 engine, which no longer had full power from that point on. However, Caracciola and the team remained undeterred. Slowly, the Remagen native carefully approached higher and higher speeds over several runs, despite the potentially problematic gearbox. The optimal vehicle configuration was found in the fifth run, which started at 9 a.m. With fully covered rear wheel arches and the additional cover for the rear wheels, whose spokes were also covered and now measured 22 inches, Caracciola achieved two new international records in Class B (vehicles with engines of 5 to 8 litres displacement) despite less than optimal outside conditions - the wind was getting stronger. The new records were 364.372 km/h over one kilometre and 366.918 km/h over one mile, both with a flying start. The absolute top speed was 372.102 km/h, but according to the regulations only the average of the outbound and inbound run was taken as the record.
In the sixth and final run of the day, Caracciola delivered another highlight. Over 5 kilometres with a flying start, he virtually pulverised the existing record of Hans Stuck, who had achieved a figure of 312.419 km/h seven months earlier in an Auto Union. Caracciola pushed the new record to 340.523 km/h. After that, the runs were cancelled due to strong winds and were not resumed the following day.
Instead, the sequel followed barely two weeks later, on 11 November 1936, at the same location. In the meantime, the V12 engine was back to full power, and 4th gear, which tended to pop out, had been worked on as well. This time, the 5-mile, 10-kilometre and 10-mile records, each with a flying start, were targeted. Because of the longer distances, the motorway section to be driven on was extended to 38 km.
Again it was the fifth run of the day that brought new records. Over 5 miles Caracciola achieved 336.838 km/h, over 10 kilometres 331.889 km/h and over 10 miles 333.489 km/h. While the first two figures were new class records, the third was even a world record. The old records, set by Hans Stuck in an Auto Union in March of the same year, were 291.035 km/h, 288.612 km/h and 286.451 km/h. With the 12-cylinder record-breaking car, Caracciola had literally advanced into new dimensions of speed - and that was by no means the end of the story.