Mercedes 28/95 hp, 1921
As German and Austrian car manufacturers were prohibited from taking part in major international races – especially in France – in the first few years after the end of the First World War, Daimler-Motoren-Gesellschaft (DMG) initially turned its attention to the racing scene in Italy and the USA after the end of the war – countries where German makes were welcome at the start of prestigious racing events.
However, DMG did not have a vehicle at its disposal that corresponded to the Grand Prix formula with a displacement limit of 3 litres defined shortly before the start of the war, and a fundamental new development was unthinkable under the difficult conditions of the first post-war years. The Mercedes 28/95 PS thus came into focus as the basis for a vehicle suitable for motorsport that could be used in races outside the scope of the Grand Prix formula.
The six-cylinder model, which was added to the programme in 1914 shortly before the outbreak of war, initially served as the flagship of the passenger car range - and did so again with the start of post-war production in 1920. Like its predecessor the 37/95 hp, the new top model was also equipped with the characteristic pointed radiator and external exhaust pipes covered with metal hoses - both features that were to characterise the appearance of the most exquisite Mercedes and Mercedes-Benz models for decades to come.
But the special status of the 28/95 hp was not only visually apparent; the technical concept also qualified it as a very special vehicle. The 7.2-litre six-cylinder unit used as the drivetrain was the first series-produced Mercedes passenger car engine with an overhead camshaft and V-shaped overhead valves. The model for this sophisticated engine with a camshaft drive was the Daimler DF 80 aircraft engine, which had not only taken second place behind a Benz FX in the 1912 Kaiserpreis competition for the best German aircraft engine, but had also been used in the Grand Prix racing car of 1913.
Only 25 examples of this exclusive model were produced in 1914 and 1915, after which production came to a standstill due to the war. After the end of the war, a modified engine was installed that was better suited to rationalised series production. The cylinders were no longer individually turned from steel, but cast in pairs. However, the steel cooling water jackets, which were also welded on in pairs, were retained. To further improve the engine's smooth running and ensure that it remained oiltight, and not least to reduce wear, the previously open valves were concealed under a total of three valve covers made of light alloy, one for each pair of cylinders.
To revitalise the successful motorsport commitment, Paul Daimler and Max Sailer, the latter not only as a highly talented racing driver but also as an engineer in the service of DMG, decided to turn the still modern top model into a high-performance touring car capable of competing in motorsport.
With his wealth of experience, Sailer took the project into his own hands. The first and most important measure was to shorten the wheelbase of the chassis from the 3370 mm of the standard model to 3065 mm in order to improve the handling and agility of the car. In addition, the radiator was positioned further back and lower and the seat position was lowered in the interests of the lowest possible centre of gravity; the steering column of the racing touring car was correspondingly flatter.
Although the subject of four-wheel brakes was still a topic of intense debate in the early 1920s, particularly with regard to motorsport use, Sailer recognised that they would be essential for a 1800 kg car based on a production vehicle. The 28/95 hp was therefore also fitted with radially ribbed inside shoe brakes on the front axle.
Various statements were made about the power output of the six-cylinder engine, the design of which had now been slightly modified, but these are difficult to qualify due to the lack of reliable measurement data still available today. The specifications varied between 99 hp/73 kW and 110 hp/81 kW and were thus relatively close to the standard engine, which was said to have a maximum output of 90 hp/66 kW to 95 hp/70 kW. The rated engine speeds were between 1800 rpm and 2000 rpm, depending on the power rating - more was not possible for the long-stroke 7.2-litre six-cylinder engine, given that the crankshaft only had four bearings.
Sailer's conversion of the Mercedes top model into a competitive racing car was apparently so successful that this version even became part of the official passenger car range as early as 1921, under the designation 28/95 hp Sport.
DMG works driver Otto Salzer demonstrated the unimagined sporting qualities of the car as early as May 1921, when he set the absolute best time of 03:39.7 minutes in the renowned Zbraslav - Jíloviště (Königsaal - Jilowischt) hill climb, which was held near Prague for the fifth time, and at the same time set a new track record. Just four days later, on 29 May, the highlight of the Mercedes 28/95 hp's sporting career followed at the Targa Florio in Sicily. Max Sailer, who had driven the car to the starting point under its own power, as was customary at the time, managed to take second place overall in this extremely gruelling race lasting almost seven and a half hours, just two minutes behind the winner. He had to fend off tough competition from the assembled top Italian drivers and brands such as FIAT, Alfa Romeo and Itala.
As well as second place in the overall standings he achieved victory in the 6 to 7.2-litre production car class, thus winning the special Coppa Florio prize.
The career of the racing touring car was by no means over after this sensational success. Otto Salzer set the best time of the day at the Karlsbad - Marienbad - Karlsbad spa race in August 1921 and won three special prizes. Eight months later, on 2 April 1922, Max Sailer and his Mercedes 28/95 hp were back in Sicily. Meanwhile, the tried-and-tested six-cylinder engine had been fitted with a compressor in Untertürkheim, which boosted output to 140 hp/103 kW. Since autumn 1919, DMG had been testing the mechanical turbocharging of automobile engines, a technology that had its origins in aircraft engine development. By the end of 1921 it was ready to be used in the first racing engines.
The intake air compressor installed in the Type 28/95 hp was a vertically mounted Roots rotary piston blower on the left-hand side of the engine, which was driven directly by the flywheel ring gear and ran at three times the speed of the crankshaft. The compressed intake air was transported from the left- to the right-hand side of the engine via pressure lines running underneath the exhaust manifolds, and fed from there to two pressure-tight carburettors manufactured by DMG itself. By depressing the accelerator pedal the driver was able to activate the compressor, which could be engaged via a multiplate clutch.
Although Max Sailer, in the now significantly more powerful car, managed to improve his finishing time compared to the previous year by around fifteen minutes, this was only enough for 6th place in the overall classification. The race on the extremely demanding Madonie circuit had already been somewhat unfortunate for the Mercedes works team, which had started the race with six cars. Behind Sailer, Christian Werner, also driving a 28/95 hp racing touring car, but without compressor, took 8th place. Christian Lautenschlager and Otto Salzer, who had started in slightly modified Grand Prix cars from 1914, finished in 10th and 13th place respectively. Of the two completely redesigned 1.5-litre racing cars with twin-camshaft compressor engines, only the one driven by Paul Schleef managed the full race distance of 432 km to finish in twentieth place.
However, the enthusiasm of the locals knew no bounds: Conte Giulio Masetti won the race in a former Mercedes works car that he now owned - a seven-year-old 4.5-litre Grand Prix racing car, painted meanwhile in the Italian racing colour red, which had been used in the Grand Prix de l'A.C.F. of 1914 - taking victory in what was probably the most gruelling car race in the world at the time. Given the rather modest performance of their own vehicles, the Mercedes team greeted Masetti's victory with joy, but understandably with somewhat muted euphoria.
Although the Targa Florio appearances were to remain the most significant racing successes of the Mercedes 28/95 hp, further victories and placings were achieved with this model in the following years. The Dutch Mercedes representative Theo Wiemann, for example, won the International Kilometre Race near Scheveningen in 1922 and 1923 in a 28/95 hp. in 1922, it even put the Benz 200 hp driven by Franz Hörner in its place.