Although this mighty engine hit the streets in the 1990s, the idea of building a twelve-cylinder that could be used in its flagships was nothing new to Mercedes. During the 1960s, the company’s engineers developed a prototype with 7.5 units intended for the “Big” 600 Pullman. It was considered to be offered alongside the 6.3-liter V8, but management eventually decided the eight-cylinder was more than adequate and the prototype was discarded.
Over the next decade, the idea was kept alive and experimented with a 90 degree V version. However, the engine was plagued by various technical problems, so the project was eventually canceled.
In the early 1980s, the engineers returned to the drawing board and developed a design for an innovative V6 that could be scaled up to twelve cylinders. It looked extremely promising this time, but once again Mercedes supervisors decided that its models would do well with the existing M117 V8.
That all changed in 1985 when they found out arch-rival BMW was preparing to launch the second iteration of the 7-series, a redesigned luxury sedan expected to be available with a new 296-hp, 5.0-liter V12. Concerned that sales of its popular W126 S-Class would suffer a major slump, Mercedes rallied the troops and accelerated the development of the new S-Class generation called the W140, which had been in work since 1981 and gave the green light to a new V12 capable of outperforming Bavaria’s upcoming M70 mill.
The scalable V6, tested several years earlier, was abandoned as engineers opted for a different approach. The architecture of the new engine was based on the proven design of the M104 in-line six-cylinder, which was basically the same recipe that BMW used to cook its twelve-cylinder.
While the M104 had a cast iron block, the new motor called the M120 received an advanced aluminum alloy unit lined with 17% silicon and 4% copper. This innovative combination led to considerable weight savings, increased performance potential and reliability.
A forged steel crankshaft with seven main shafts was built into it. The iron-clad, galvanized aluminum pistons, which were connected to the crank via forged steel connecting rods, could move back and forth in cylinder-free cylinders thanks to the high silicon content of the block.
Both the diameter of each cylinder and the spacing between each bore were the same as the M104’s six, which allowed the engineers to use its 24-valve aluminum cylinder head on the right side of the M120. For the other bank, they cast a mirrored version of the component and an integrated variable valve control for the intake cams.
The engine also received a multi-part oil pan from motorsport to prevent foam formation, a Bosch LH-Jetronic ECU coupled with knock sensors that allowed it to swallow lower octane gasoline without risking pre-detonation, and a gorgeous one-piece intake manifold.
Originally planned for the summer of 1989, the rollout of the W140 S-Class was delayed to the first half of 1991, also because the top people wanted to give the engineering team more time to perfect the V12.
When it finally finished, the 6.0-liter could develop up to 402 horsepower at 5,200 rpm, as well as a healthy 428-lb-ft (580 Nm) of torque at 3,800 rpm. Even more impressive is that already at 2,000 rpm, around 90% of the torque was transferred to the rear wheels, so that the 1,814 kg S-Class was able to sprint from 0 to 100 km / h in just over six seconds.
The M70 from BMW was no match, and the only production V12s that could stand a chance against the M120 in terms of performance came from Ferrari and Lamborghini. It was so good that it was chosen by Horacio Pagani for his Zonda, one of the few novel supercars from the 1990s that have made it to the present day.
After the introduction of the M120, several tuning houses like AMG, Brabus, RENNtech or Carlsson would enlarge the M120 and turn it into a monster, but the engine would achieve legendary status on the track thanks to the long-term partners of Mercedes and the future high-performance department AMG.
The year is 1996 and Germany’s leading touring car championship DTM (German Touring Car Championship) has just risen. Since Mercedes didn’t have a top racing series to compete in, Mercedes turned to the BPR Global GT, which was to become an FIA-approved championship in 1997.
The BPR was dominated by the McLaren F1, an iconic racing car powered by a BMW V12 loosely derived from the 7-series M70. Determined to step into the 1997 season and dethrone the McLaren-BMW partnership, the team of AMG and Mercedes engineers tasked with developing the new racing car procured a competition-specific F1 and built a thoroughly improved version of the M120 into it. This enabled them not only to reverse engineer the successful racing car, but also to test the new engine during the development of the chassis and body.
The engine, known internally as the GT 112, was able to spit out around 600 hp and proved to be extremely reliable. It helped the CLK GTR outperform BMW Motorsport’s McLaren F1 when AMG-Mercedes secured both the driver and team trophy in its first season.
In order to compete in the FIA GT championship, the manufacturers had to build at least 25 street versions of the racing cars. This resulted in the CLK GTR AMG Strassen (Street) version and with it the AMG-tuned M120 took to the streets. Drilled to 6.9 liters, it developed 604 hp at 6,800 rpm and 572 lb-ft (775 Nm) of torque at 5,250 rpm.
The souped-up engine was codenamed M297 and enlarged even further. In the 7.3 liter guise, it was built from 1997 to 2016 in a variety of performance versions with peak values of 789 hp and 664 lb-ft (900 Nm). Mercedes used the smaller, 518-hp M297 in their AMG-branded CL and S-Class models, while the most powerful iterations replaced the M120 in the Pagani Zonda, powering every single version of the supercar (there were many) that were built became from 2002.
The first series-produced V12 from Mercedes-Benz was not only the best twelve-cylinder in the company’s history, but also one of the best engines ever built. It started as the centerpiece of the epic W124 S-Class, then became a racing engine in the successful CLK GTR and ultimately helped transform the Zonda from a novelty into one of the most popular supercars of the 21st century.
You can find out more about this stellar engine in the fantastic video posted on YouTube by Alfa Guy 2
