By William Pearce
For the 1938 European Grand Prix season, the Association Internationale des Automobile Clubs Reconnus (AIACR) issued a rule change that limited the displacement of supercharged engines to 3.0 L (183 cu in) and normally aspirated engines to 4.5 L (275 cu in). Rather than modifying its existing W125 racer with its supercharged 345.6 cu in (5.66 L) straight-eight engine, Mercedes-Benz built an entirely new car for the 1938 season. Designated W154, the car was designed by Rudolf Uhlenhaut, Max Sailer, and Max Wagner.
The Mercedes-Benz W154 was an open-wheel, front-engine Grand Prix race car. The W154’s chassis was essentially the same as that used on its predecessor, the W125. The W154 had a 107.4 in (2,728 mm) wheelbase, a 58.0 in (1,473 mm) track for the front wheels, and a 55.6 in (1,412 mm) track for rear wheels. The car’s frame was made of tubular steel and was covered with aluminum body panels contoured to improve aerodynamics.
Powering the W154 was a V-12 engine known as the M154. The M154 engine was designed by Albert Heess and was inspired by the 570 hp (425 kW), 340 cu in (5.58 L) DAB V-12 engine that was intended for the W125 during the 1936 Grand Prix season. The DAB engine made the W125 too heavy for the Grand Prix class, and the engine was replaced by the lighter M125 straight-eight. On 28 January 1938, a tuned DAB engine developing some 736 hp (549 kW) was installed in a special, streamlined W125 chassis. The car and engine combination was known as the Rekordwagen (record car). Driven by Mercedes-Benz driver Rudolf Caracciola, the W125 Rekordwagen set a new Class B (5.001–8.000 L / 305–488 cu in) speed record of 268.863 mph (432.692 km/h) over 1 km (.6 mi) and 268.657 mph (432.361 km/h) over 1 mile (1.6 km).
The two cylinder banks of the M154 V-12 engine were set at 60 degrees. Each bank was comprised of two three-cylinder blocks made of steel. The engine used side-by-side connecting rods and a one-piece crankshaft. Mounted to the front of the M154 engine were two Roots-type superchargers. Air entered a carburetor attached to the superchargers at the very front of the engine. The air/fuel mixture was then compressed by the superchargers operating in parallel and flowed through an intake manifold positioned in the Vee of the engine. The superchargers were driven at 1.5 times engine speed and delivered around 20 psi (1.38 bar) of boost.
Each cylinder had two intake and two exhaust valves that were driven by dual overhead camshafts. The M154 engine had a bore of 2.64 in (67 mm), a stroke of 2.76 in (70 mm), and a displacement of 180.7 cu in (2.96 L). The engine’s compression varied from 5.95 to 1 and 6.60 to 1 depending on the desired reliability. The different compression ratios resulted in the engine’s output varying from 433 hp to 474 hp (323 kW to 353 kW) at 8,000 rpm. It was also noted that the superchargers used an additional 160 hp (119 kW) at 8,000 rpm.
The V-12 engine was angled in the W154’s frame so that the car’s drive shaft extended back along the left side of the driver and to the rear differential. This configuration allowed the driver to be seated next to the driveline and some 4 in (102 mm) lower in the car, which lowered the racer’s center of gravity and improved its aerodynamics and handling. To compensate for the smaller and less-powerful engine compared to the W125, the W154’s gearbox had closer ratio gears with a fifth gear added to maintain top speed. The M154 weighed around 2,161 lb (980 kg) and had a top speed of over 193 mph (310 km/h).
The W154 did very well during the 1938 Grand Prix season, sweeping the top three spots, with two other cars tied for fifth. The car gave Mercedes-Benz driver Rudolf Caracciola his third European Championship title. Some engine and aerodynamic modifications to the W154 were planned for the 1939 season, but before the season got underway, Mercedes-Benz decided to use the W154 to make attempts on the Class D (2.001–3.000 L / 122–183 cu in) standing start speed record.
Chassis number 11 of the 15 W154s built was modified by enclosing the wheels and suspension in aerodynamic fairings. The sides of the cockpit were also enclosed by panels; the one on the right side was easily removed for entry into the cockpit. Further streamlining improvements were made to the rest of the body, and unneeded equipment was removed to make the car as light as possible. Overall, 68 lb (31 kg) were shed, reducing the car’s weight to 2,092 lb (949 kg). Since the record runs were brief, the radiator was removed, and an ice tank was installed above the rear axle. Hot coolant from the engine flowed into the tank and melted the ice, and the now-chilled coolant flowed back to the engine. With the radiator removed, an inlet in the nose of the car fed air directly to the engine’s carburetor. The streamlined W154 record car’s engine developed 468 hp (349 kW) at 7,800 rpm.
On 8 February 1939, Caracciola climbed into the streamlined W154 car as it sat on a special section of the Autobahn south of Dessau, Germany. Called the Dessauer Rennstrecke (Dessau Racetrack), this 6.2 mi (10 km) stretch of the Autobahn was specially made for record attempts and was 82 ft (25 m) wide with the median paved over. From a stop, Caracciola and the W154 rocketed down the Autobahn, covering 1 km (.6 mi) in 20.56 seconds and 1 mile (1.6 km) in 28.32 seconds—both times were new Class D records. Unfortunately, the top speed achieved was not recorded, but the times averaged to 108.800 mph (175.097 km/h) over 1 km (.6 mi) and 127.119 mph (204.578 km/h) over 1 mile (1.6 km).
The next day, Caracciola drove the W125 Recordwagen streamliner with a M154 engine installed. The car fell into Class D with the smaller engine and was used to set new records for the flying km and mile. Caracciola traveled 1 km (.6 mi) in 9.04 seconds at 247.449 mph (398.230 km/h) and 1 mile (1.6 km) in 14.50 seconds at 248.276 mph (399.561 km/h).
Not entirely satisfied with the standing start record in the special W154 car, Caracciola set another record on 14 February when he covered 1 km (.6 mi) in 20.29 seconds, averaging 110.248 mph (177.427 km/h). This was the last speed record set in Germany before World War II. The Mercedes-Benz T80 was to make an attempt on the world speed record in 1940, but the war derailed those plans.
The W154’s engine was modified for the 1939 Grand Prix season. The new engine was known as the M163 and used two-stage supercharging. Two superchargers were still at the front of the engine, but now they operated in series, with one feeding the other. The superchargers rotated at 1.25 times crankshaft speed. The first supercharger (stage) provided 12 psi (.83 bar) of boost, which was increased to 19 psi (1.31 bar) after the second supercharger (stage). While similar boost was achieved with the earlier supercharger set up, the two-stage system only consumed 84 hp (63 kW) at 7,500 rpm, about half of the earlier system. This allowed the M163 engine to produce 480 hp (358 kW) at 7,500 rpm. While that was only 6 hp (5 kW) more than the M154 engine, the 500 rpm decrease made the M163 engine much more reliable than its predecessor. Because of the M163 engine, the 1939 cars are often referred to as W163s, but they were still W154s. World War II prevented an official winner of the 1939 Grand Prix season to be declared. However, all the races had been run, and Mercedes-Benz cars occupied the top four spots.
W154 chassis number 11, the one used for the record run, was returned to Grand Prix racer configuration. In 1951, the car was raced by Juan Manuel Fangio in two Grand Prix races in Argentina. The car was preserved and is owned by Mercedes-Benz.
– The Mercedes-Benz Racing Cars by Karl Ludvigsen (1971)
– Classic Racing Engines by Karl Ludvigsen (2001)