2006-2009 Mercedes-Benz S-class (W221) S 420 CDI V8 (320 Hp) 7G-TRONIC

The Mercedes-Benz S 420 CDI, produced between October 2006 and April 2009, was a variant of the W221 generation S-Class. Positioned as a high-performance diesel offering within the S-Class range, it occupied a space between the more mainstream S 320 CDI and the flagship S 500 models. This model combined a powerful V8 diesel engine with Mercedes’ 7G-TRONIC automatic transmission, aiming to deliver both luxurious comfort and strong performance. The W221 S-Class itself represented a significant leap forward in technology and refinement for Mercedes-Benz, introducing features like PRE-SAFE anticipatory safety systems and advanced driver assistance technologies.

Powertrain & Engine Architecture

The heart of the S 420 CDI is the OM 629.911 4.0-liter V8 diesel engine. This engine is a longitudinally-mounted, turbocharged diesel powerplant featuring a common rail direct fuel injection system. The OM 629 engine family was known for its robust construction and relatively sophisticated technology for its time. The S 420 CDI utilized a bi-turbo setup – two turbochargers working in sequence – to minimize turbo lag and maximize power delivery across the rev range. An intercooler further enhances performance by cooling the compressed intake air. The engine boasts a compression ratio of 17:1, contributing to its efficiency and power output.

The valvetrain employs a Double Overhead Camshaft (DOHC) configuration with four valves per cylinder, optimizing gas flow for improved combustion. Engine oil capacity is 10.5 liters (11.1 US quarts / 9.24 UK quarts), and the coolant capacity is 9 liters (9.51 US quarts / 7.92 UK quarts). A particulate filter was standard equipment, helping to reduce emissions and meet Euro 4 standards. The 7G-TRONIC automatic transmission, a seven-speed unit, was specifically tuned to handle the high torque output of the diesel V8. This transmission offered smooth and responsive shifting, contributing to the overall driving experience.

Transmission Details: 7G-TRONIC

The 7G-TRONIC transmission is a torque converter automatic, known for its wide gear ratio spread and ability to adapt to driving conditions. It features a sophisticated electronic control system that optimizes shift points for performance or fuel economy. The transmission’s seven forward gears allow for closer ratios, keeping the engine within its optimal power band more frequently. Manual shift capability via steering wheel-mounted paddles was often available as an option.

Driving Characteristics

The S 420 CDI offered a unique driving experience within the S-Class lineup. Its 320 horsepower and substantial 730 Nm (538.42 lb-ft) of torque delivered strong acceleration, with a 0-60 mph time of approximately 6.3 seconds and a 0-100 km/h time of 6.6 seconds. While not as overtly sporty as the gasoline-powered S 500, the S 420 CDI provided ample power for effortless cruising and confident overtaking. The 7G-TRONIC transmission played a crucial role in maximizing the engine’s potential, providing smooth and timely gear changes. The torque curve was particularly impressive, offering strong pulling power from low engine speeds. Compared to the S 320 CDI (with its inline-six diesel), the S 420 CDI offered significantly more power and a more refined driving experience. However, it lacked the higher-revving character and ultimate performance of the S 500’s V8 gasoline engine.

Equipment & Trim Levels

The S 420 CDI was typically equipped with a high level of standard features, befitting its position in the S-Class range. Standard interior appointments included high-quality leather upholstery, wood trim, and power-adjustable seats with memory function. A multi-function steering wheel, automatic climate control, and a premium audio system were also standard. The S-Class was renowned for its advanced safety features, and the S 420 CDI included systems like ABS, ESP, and multiple airbags. Optional extras included features like a panoramic sunroof, upgraded audio systems (such as Harman Kardon or Bose), rear seat entertainment, and advanced driver assistance systems like Distronic adaptive cruise control and lane keeping assist. Trim levels were often differentiated by the choice of interior materials, exterior finishes, and the inclusion of specific option packages.

Chassis & Braking

The S 420 CDI utilized an independent multi-link suspension system at both the front and rear, providing a comfortable and controlled ride. Air suspension was standard, allowing for adjustable ride height and further enhancing ride quality. A transverse stabilizer bar was fitted to both axles to reduce body roll during cornering. The braking system consisted of ventilated discs on all four wheels, providing ample stopping power. ABS (Anti-lock Braking System) was standard, enhancing braking performance and stability. The steering system employed a hydraulic power steering rack and pinion setup, offering precise and responsive steering feel.

Market Reception & Comparison

The S 420 CDI was generally well-received by automotive critics, who praised its combination of performance, luxury, and fuel efficiency. The diesel V8 engine was lauded for its smooth operation and impressive torque output. Fuel economy figures were respectable for a vehicle of its size and performance, ranging from 9.4-9.6 l/100 km (25-24.5 US mpg) combined. Compared to the S 320 CDI, the S 420 CDI offered a significant performance upgrade, but at a higher price point. Reliability was generally good, although diesel particulate filter issues could occasionally arise. The S 420 CDI offered a compelling alternative to the gasoline-powered S-Class models for buyers who prioritized fuel economy and torque.

Technical Specifications

Legacy

The S 420 CDI, while a relatively short-lived model, has gained a following among enthusiasts who appreciate its unique combination of performance and efficiency. The OM 629 engine, when properly maintained, is known for its durability and longevity. On the used car market, S 420 CDI models are often sought after by those looking for a luxurious and capable diesel-powered sedan. However, potential buyers should be aware of the potential for diesel particulate filter issues and the need for regular maintenance. The S 420 CDI represents a significant chapter in the evolution of the S-Class, showcasing Mercedes-Benz’s commitment to diesel technology and its ability to deliver both luxury and performance.