W 154 3-litre record car with covered wheels, 1939

W 154 3-litre record car with covered wheels, 1939

After the tragic death of Bernd Rosemeyer during the record week in January 1938 and the subsequent termination of the record runs on the part of Auto Union, Daimler-Benz turned to the smaller Class D (vehicles with engines of 2 to 3 litres displacement) for the following year. Here, in addition to the records with a flying start, the company also had an eye on those with a standing start.

The technical basis was again the current formula racing car, in this case the W 154. Recruited for the special purpose was the chassis with serial number 11, which was characterised by its particularly lightweight construction. It was not designed for Grand Prix racing, but for use in hillclimbs. The supercharged 3-litre V12 with the designation M 154 used in the W 154 represented an ideal power unit for pursuing records in Class D.

The engine designers around Albert Heess had largely been guided in their engine development by the traditional recipes for racing engine construction at Daimler-Benz. So they had reverted to the highly resilient, but also heavyweight, design in which the cylinders were welded to the cylinder head to form a single unit and had welded-on cooling jackets made of sheet steel. The cylinder bank angle of the new power unit was an ideal 60 degrees in terms of vibration, with the six cylinders per bank divided into two units of three - another classic solution for Mercedes-Benz racing engines.

The gas cycle of the V12, which was only slightly undersquare with a bore/stroke ratio of 67 mm x 70 mm, was provided by two mercury-cooled intake and exhaust valves each with a disc diameter of 30 mm. These were actuated via rocker arms by two overhead camshafts per cylinder bank driven by a spur gear cascade. The combustion mixture was ignited by a spark plug positioned centrally in the cylinder head.

As far as mixture preparation was concerned, the initial hope had been to be able to use petrol direct injection in the near future, which had already found its way into production aircraft engines at Mercedes-Benz in 1937, but was not yet fully developed for use in automobile engines. For reasons of time, the successful M 125 inline eight-cylinder engine was finally chosen, as the performance-enhancing properties of a float carburettor over a pressure carburettor had been clearly recognised and exploited accordingly. In the new M 154, therefore, a twin-barrel float carburettor was installed upstream of the two compressors, together with an additional slide carburettor. The two upright Roots blowers themselves were significantly smaller than those of the predecessor engines due to the smaller displacement.

The sophisticated technology of the new 3-litre V12 resulted in a power-to-swept-volume ratio that took it into new dimensions. Whereas the previous inline eight-cylinder engines had a power-to-swept-volume ratio of around 100 hp/74 kW, the newly designed engine had a ratio of around 160 hp/118 kW. The power unit used in the new record-breaking car with serial number 10 provided a power volume of approx. 465 hp/342 kW at a rated speed of 8000 rpm. This engine did not even benefit from the transition to a single two-stage compressor, as had been successfully implemented on the formula racing cars in the 1939 Grand Prix season. By means of this far-reaching modification, the maximum power of the M 154/10 could have been increased by another 20 hp/15 kW or so.

After the experiences of the previous years, it was clear to all involved that a powerful engine alone was no guarantee for new speed records. Of decisive importance was a bodywork design that would adhere as closely as possible to the laws of aerodynamics. In the case of the W 154 record-breaking car, which was initially set to achieve new records with a standing start, the decision was made to opt for a solution that was more closely aligned with the design of a single-seater formula racing car. Instead of a fully integrated body, as the W 125 world record car of 1938 had shown, that of the W 154 consisted of a zeppelin-shaped centre section adjoined on both sides by full fairings, each tightly enclosing the suspension elements and wheels, resulting in a much more sculpted exterior appearance. Original tests with the normal single-seater body of the formula racing car, to which only fully enclosed wheels had been added, had not produced satisfactory results in testing.

A characteristic feature was the circular air inlet grille at the front of the actual car body, through which the intake air for the carburettors flowed. Due to the renewed use of the ice cooling system already proven in the W 125 record-breaking car, whose reservoir was now housed in the rear of the car, it was possible to dispense with a cooling air opening, which always had a negative influence on the aerodynamics.

Since record attempts with a standing start also required the lowest possible vehicle weight with optimal traction at the same time, the development team took a host of corresponding measures. For example, the brake system of the W 154 chassis was slimmed down: the front brakes disappeared completely and the remaining rear brake was reduced in size. The oil and water coolers and the so-called saddle tank located between the rear end of the engine and the cockpit were also omitted - components that were simply superfluous for the short distance of one kilometre or one mile to be covered.

To ensure that the enormous power of the 3-litre V12 engine was optimally transferred to the road at the start, a differential with 100% locking effect was installed on the rear axle. Unfortunately, however, it was found that it was not possible to use the same gear ratios for both the kilometre and the mile distances in order to achieve optimum times and thus speeds. However, since changing the gearbox or final drive on site would have taken too much time, a solution was found that was as simple as it was inexpensive. For the standing mile, they simply switched from 19-inch rear wheels to wheels with a diameter of 22 inches, whose larger rolling circumference eliminated the problem.

In 1939, the record runs had a different character than in previous years. With the withdrawal of Auto Union, a phase of maximum competitive intensity ended for Daimler-Benz as the remaining player, because the record week had become obsolete as an institutionalised duel between the two German car manufacturers. Nevertheless, the team in Untertürkheim were not to be deterred, and on 8 February 1939 they lined up the W 154 record-breaking car to set new records with a standing start.

The venue was no longer the Frankfurt - Darmstadt motorway as before, but an almost 10-kilometre section of the Leipzig - Berlin motorway between Dessau and Bitterfeld. Since the central reservation between the two lanes had also been concreted over with a view to the forthcoming record runs, this section of the road was - in principle - a 27-metre-wide track. For obvious reasons, Rudolf Caracciola, who always had an eye on the big picture, nevertheless preferred to use one of the actual lanes for each of his attempts.

Right away, the experienced man from the Rhineland succeeded in setting new records in Class D for both the kilometre and the mile with the new car. The records were 175.097 km/h and 204.578 km/h respectively. Barely a week later, on 14 February, they returned once more with the same vehicle to attack the record they had themselves just set. With success, because Caracciola set a new record with a speed of 177.427 km/h over the kilometre with a standing start.

No one suspected at the time that this would be the last speed record run by a Mercedes-Benz for almost forty years.

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