The BMW 4 Series Convertible (G23) 420d, produced from 2020 through January 2024, represents a compelling entry point into the premium mid-size convertible market. Positioned as a diesel-powered variant within the G23 generation, it combines a 2.0-liter turbocharged engine with mild hybrid technology and an 8-speed Steptronic automatic transmission. This configuration aimed to deliver a balance of performance, fuel efficiency, and refinement, appealing to buyers seeking a stylish and comfortable open-top experience. The 420d sat below the more powerful 430i and M440i models in the 4 Series Convertible lineup, offering a more accessible price point without sacrificing the core BMW driving experience.
Technical Specifications
| Brand | BMW |
| Model | 4 Series |
| Generation | 4 Series Convertible (G23) |
| Type (Engine) | 420d (190 Hp) Mild Hybrid Steptronic |
| Start of production | 2020 |
| End of production | January, 2024 |
| Powertrain Architecture | MHEV (Mild Hybrid Electric Vehicle) |
| Body type | Cabriolet |
| Seats | 4 |
| Doors | 2 |
| Combined fuel consumption (WLTP) | 4.9-5.5 l/100 km (48 – 42.8 US mpg) |
| CO2 emissions (WLTP) | 127-143 g/km |
| Fuel consumption (economy) – urban (NEDC) | 4.7-5 l/100 km (50 – 47 US mpg) |
| Fuel consumption (economy) – extra urban (NEDC) | 3.9-4.3 l/100 km (60.3 – 54.7 US mpg) |
| Fuel consumption (economy) – combined (NEDC) | 4.2-4.5 l/100 km (56 – 52.3 US mpg) |
| CO2 emissions (NEDC) | 111-119 g/km |
| Fuel Type | Diesel |
| Acceleration 0 – 100 km/h | 7.6 sec |
| Acceleration 0 – 62 mph | 7.6 sec |
| Acceleration 0 – 60 mph | 7.2 sec |
| Maximum speed | 236 km/h (146.64 mph) |
| Emission standard | Euro 6d |
| Weight-to-power ratio | 9.3 kg/Hp |
| Weight-to-torque ratio | 4.4 kg/Nm |
| Gross battery capacity | 0.4 kWh |
| Battery voltage | 48 V |
| Battery technology | Lithium-ion (Li-Ion) |
| Electric motor power | 11 Hp |
| Electric motor Torque | 25 Nm (18.44 lb.-ft.) |
| Power | 190 Hp @ 4000 rpm |
| Torque | 400 Nm @ 1750-2500 rpm (295.02 lb.-ft. @ 1750-2500 rpm) |
| Engine displacement | 1995 cm3 (121.74 cu. in.) |
| Number of cylinders | 4 |
| Cylinder Bore | 84 mm (3.31 in.) |
| Piston Stroke | 90 mm (3.54 in.) |
| Compression ratio | 16.5:1 |
| Fuel injection system | Diesel Commonrail |
| Engine oil capacity | 5.5 l (5.81 US qt | 4.84 UK qt) |
| Coolant capacity | 7 l (7.4 US qt | 6.16 UK qt) |
| Kerb Weight | 1775 kg (3913.21 lbs.) |
| Max. weight | 2270 kg (5004.49 lbs.) |
| Max load | 495 kg (1091.29 lbs.) |
| Trunk (boot) space – minimum | 300 l (10.59 cu. ft.) |
| Trunk (boot) space – maximum | 385 l (13.6 cu. ft.) |
| Fuel tank capacity | 59 l (15.59 US gal | 12.98 UK gal) |
| Minimum turning circle | 11.4 m (37.4 ft.) |
| Number of gears | 8 |
| Gearbox type | automatic transmission Steptronic |
Powertrain & Engine Architecture
At the heart of the 420d Convertible lies the BMW B47D20B 2.0-liter inline-four diesel engine. This engine is a member of the modular BMW engine family, known for its efficiency and refinement. The B47 utilizes a common-rail direct injection system, delivering fuel at extremely high pressure for optimized combustion. A twin-scroll turbocharger with intercooling further enhances performance, providing strong torque across a wide rev range. The engine’s 16.5:1 compression ratio is typical for modern diesel engines, contributing to thermal efficiency. The mild hybrid system, incorporating a 48V lithium-ion battery and an 11 horsepower electric motor, provides a power boost during acceleration and enables smoother start-stop operation, reducing fuel consumption. The electric motor is integrated into the drivetrain, assisting the engine when needed. The Steptronic 8-speed automatic transmission is a crucial component, offering smooth and quick gear changes. It’s programmed to adapt to driving style and conditions, optimizing both performance and fuel economy.
Driving Characteristics
The 420d Convertible, with its 190 horsepower and 295 lb-ft of torque, delivers a respectable level of performance. Acceleration from 0 to 60 mph takes approximately 7.2 seconds, and it achieves a top speed of 146.64 mph. While not as overtly sporty as the higher-powered 4 Series variants, the 420d provides ample power for comfortable cruising and spirited driving. The automatic transmission’s gear ratios are tuned for efficiency, with taller gears for highway driving and shorter gears for quicker acceleration. Compared to a manual transmission version (not widely available in this configuration), the Steptronic automatic offers greater convenience and ease of use, particularly in urban environments. However, some driving enthusiasts might find the automatic slightly less engaging than a manual. The mild hybrid system contributes to a seamless driving experience, with the electric motor filling in torque gaps and providing a subtle boost during acceleration.
Equipment & Trim Levels
The 420d Convertible typically came well-equipped, reflecting its premium positioning. Standard features included BMW’s iDrive infotainment system with a central touchscreen display, leather upholstery, automatic climate control, and a premium sound system. Safety features such as automatic emergency braking, lane departure warning, and parking sensors were also standard. Optional extras included a head-up display, adaptive cruise control, heated seats, and a wind deflector to reduce turbulence in the cabin when the roof is down. Interior trim options allowed for personalization, with a range of wood, aluminum, and carbon fiber accents available. The convertible top itself was a multi-layer soft top, offering good insulation and noise reduction when closed.
Chassis & Braking
The G23 4 Series Convertible utilizes a sophisticated chassis design. The front suspension features coil springs and a transverse stabilizer bar, while the rear suspension employs an independent multi-link setup, also with a stabilizer bar. This configuration provides a good balance of ride comfort and handling precision. The 420d typically came equipped with ventilated disc brakes on all four wheels, providing ample stopping power. ABS (Anti-lock Braking System) and electronic stability control are standard safety features. The steering system is an electric power-assisted rack and pinion system, offering precise and responsive steering feel. The weight distribution is carefully balanced to enhance handling characteristics.
Market Reception & Comparison
The 420d Convertible was generally well-received by automotive critics. It was praised for its refined engine, comfortable ride, and stylish design. Fuel economy was a significant selling point, with the diesel engine offering considerably better mileage than the gasoline-powered alternatives. Compared to the 430i, the 420d offered a more affordable entry point into the 4 Series Convertible range, but with a noticeable trade-off in outright performance. The 420d’s mild hybrid system was also seen as a positive feature, contributing to improved efficiency and reduced emissions. Reliability reports have been generally positive, with the B47 engine proving to be a robust and dependable unit.
Legacy
The BMW 4 Series Convertible (G23) 420d represents a significant step forward in diesel-powered convertible technology. The combination of a refined engine, mild hybrid assistance, and a comfortable chassis has made it a popular choice for buyers seeking a stylish and efficient open-top experience. On the used car market, the 420d Convertible holds its value reasonably well, thanks to its reputation for reliability and fuel economy. The B47 engine is known for its longevity, and with proper maintenance, these vehicles are expected to provide many years of reliable service. The G23 generation 4 Series Convertible, and the 420d variant in particular, has cemented BMW’s position as a leader in the premium convertible segment.
