S 400 d

The S 400 d tops the diesel range of the W223 until the arrival of the S 450 d in 2023. Its engine is the OM 656 D29T R SCR, an inline six-cylinder with 2,925 cm3, an architecture Mercedes reintroduced in 2017 after years of V6s: aluminium block, cylinder liners coated with arc-sprayed steel to reduce friction, two-stage turbocharging with R2S twin turbochargers.

Output is 330 hp, with 700 Nm from 1,200 to 3,200 rpm: it is this torque plateau, more than the power, that defines the car. 0 to 100 km/h in 5.4 s, limited to 250 km/h, 6.7 to 7.9 l/100 km and 177 g/km certified, 9G-TRONIC gearbox and 4MATIC all-wheel drive. The after-treatment combines a particulate filter and double SCR catalyst with AdBlue injection.

A fundamental difference from the petrol versions of the same generation: the S 400 d does without the 48 V network and the starter-alternator fitted to the M256 and M176, which the 2023 S 450 d will inherit, credited with 367 hp, 750 Nm and 15 kW of electric assistance. No boost at start-up or energy recovery, but a simpler drivetrain with one less expensive component. Ride comfort relies on AIRMATIC air suspension. Certified consumption below 8 l/100 km for a saloon over five metres long means running costs are incomparable to those of a V8.

The fiscal rating here is much lower than that of the petrol versions, and this is the main argument for this engine. The CO2 penalty due at first French registration follows the scale for the year of first registration: at 177 g/km, the 2022 scale puts the car at 6,375 euros before application of the monthly reduction under article L. 421-7-2, in force since 1 March 2025. The tax on kerb weight, due above 1,800 kg at the 2022 scale at a rate of 10 euros per kilogram, is added, with the capping of article L. 421-74 only applying when the CO2 penalty reaches its ceiling.

In terms of use, an OM656 with particulate filter and double SCR is designed for long-distance driving: short urban journeys multiply incomplete regenerations, and it is the after-treatment system that ends up being costly. AdBlue is counted as a consumption. Finally, a Euro 6 diesel is classified Crit'Air 2 (French low-emission sticker) while an equivalent petrol is Crit'Air 1, which matters in low-emission zone cities.