Mercedes-Benz W 154 3-litre fully streamlined record car, 1939

Mercedes-Benz W 154 3-litre fully streamlined record car, 1939

After the tragic death of Bernd Rosemeyer at 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 standing start, the company also had an eye on those with a flying start.

While the current formula racing car, in this case the W 154, was used as the technical basis for the former and equipped with an aerodynamically appropriately shaped body with open wheels, the W 125 record-breaking car, which had been so successful the year before, was again used for the considerably higher speeds achieved with a flying start.

Instead of the supercharged MD 25 DAB V12 engine with 5.6 litres of displacement, however, the chassis of the car, which was fitted with a fully streamlined body, incorporated the 3-litre V12 with the designation M 154 used in the W 154, which also featured forced induction. This engine represented an ideal power unit for the record hunt in Class D.

The engine designers around Albert Heeß had been guided for the most part by the traditional recipes for racing engine construction at Daimler-Benz when designing the power unit, which corresponded to the displacement formula valid from 1938. 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. They 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.

In the area of mixture preparation, 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. Due to time constraints, 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 superchargers, 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 swept volume.

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 provided a power output of approx. 465 hp/342 kW at a nominal speed of 8000 rpm. This engine did not even benefit from the transition to a single two-stage supercharger, as was 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 could have been increased by a further 20 hp/15 kW or so.

The consistently aerodynamically designed body of the successful record-breaking car of January 1938 was characterised by a significantly lowered, vertically designed front end, which produced comparatively high downforce values and ensured excellent driving stability. Because of the ice cooling system still in use, there were two small oblong-round holes instead of a large cooling air inlet, which exclusively supplied intake air to the carburettors. The engineers did not touch the efficient, streamlined shape of the body. Minor modifications were only made under the outer skin, such as adapting the frame to the more compact and lighter 3-litre engine, fitting the matching gearbox including the rear axle, using front suspension springs with new characteristics, and installing front brakes. 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 at the rear axle.

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. Nonetheless, the people from Untertürkheim were not deterred and lined up two new W 154 record cars as early as 8 February 1939, the one with open wheels for new records with a standing start and the one with a fully streamlined body for those with a flying start.

The venue was no longer the Frankfurt - Darmstadt motorway as in previous years, 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.

Once the experienced driver from the Rhineland had set new records in Class D for both the kilometre and the mile with a standing start, he turned to the same distances with a flying start. The next day, 9 February 1939, he also set two new records in the W 154 with fully streamlined body: averaging the outbound and inbound legs, he covered the flying kilometre at 398.23 km/h and the flying mile at 399.56 km/h. The absolute maximum speed in one direction was 400.112 km/h.

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