The Mercedes-Benz M 115.920 is a 2,497 cc, inline-six, naturally aspirated petrol engine produced between 1970 and 1975. It was engineered as a refined and durable powerplant for mid-range luxury sedans and coupes, featuring a single overhead camshaft (SOHC), hemispherical combustion chambers, and twin Zenith carburettors. It delivered 90 kW (122 PS) at 5,500 rpm and 180 Nm of torque at 3,700 rpm, providing smooth acceleration and cruising capability.
Fitted to models such as the W114 250, W115 250, and W111 250 Coupé, the M 115.920 was designed for balanced performance and driver comfort in urban and highway conditions. Its engineering prioritised mechanical refinement and longevity over outright power, with emissions control achieved through early crankcase ventilation systems and lean-burn tuning, allowing compliance with pre-1975 German emissions guidelines (DIN 70020).
One documented service update involves the cylinder head gasket material revision, highlighted in Mercedes-Benz Service Information Bulletin 035.00-P-10/72. Early units experienced occasional head gasket seepage under prolonged high-load operation due to thermal expansion variances. In 1972, Mercedes introduced an updated multi-layer steel (MLS) gasket with improved sealing geometry, reducing long-term maintenance requirements on later production runs.
Production years 1970–1971 comply with DIN 70020 emissions standards; 1972–1975 models meet revised German federal guidelines (BMVI Archive Ref. E-1972-MB-04).
The Mercedes-Benz M 115.920 is a 2,497 cc inline-six naturally aspirated engine engineered for mid-size luxury vehicles (1970–1975). It combines SOHC valvetrain architecture with twin two-barrel carburettor induction to deliver smooth power delivery and mechanical durability. Designed to meet early German emissions protocols, it balances drivability with long-term reliability under regular maintenance.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,497 cc | |
| Fuel type | Petrol (Unleaded RON 95 recommended) | |
| Configuration | Inline-6, SOHC, 12-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 87.0 mm × 70.0 mm | |
| Power output | 90 kW (122 PS) @ 5,500 rpm | |
| Torque | 180 Nm @ 3,700 rpm | |
| Fuel system | Twin Zenith 36/40 INAT carburettors | |
| Emissions standard | Pre-regulatory (DIN 70020, BMVI 1972 update) | |
| Compression ratio | 9.0:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Not applicable | |
| Timing system | Chain-driven camshaft (single-row) | |
| Oil type | SAE 20W-50 (API SD/CC) | |
| Dry weight | 185 kg |
The twin-carburettor setup provides linear throttle response ideal for relaxed driving but requires periodic synchronization and float-level checks to maintain idle stability and fuel efficiency. SAE 20W-50 mineral oil is essential for bearing and camshaft protection under high-temperature operation. Extended idling or short trips accelerate valve train wear due to insufficient oil circulation. The SOHC design is robust but sensitive to coolant loss; overheating may lead to head warpage, especially on pre-1972 units with composite gaskets. Post-1972 models benefit from the updated MLS head gasket per SIB 035.00-P-10/72, significantly improving thermal resilience. Carburettor icing in cold climates can occur; use of intake manifold heat riser is critical for winter reliability.
Oil Specs: Requires SAE 20W-50 API SD/CC specification (Mercedes-Benz Owner's Manual 114/115). Modern equivalents acceptable with zinc additives.
Emissions: Pre-1972 models comply with DIN 70020. 1972–1975 units meet updated BMVI guidelines (BMVI Archive Ref. E-1972-MB-04).
Power Ratings: Measured under DIN 70020 standards. Output assumes optimal carburettor tuning and fresh ignition components (Mercedes-Benz TIS A115-891).
Mercedes-Benz Technical Information System (TIS): Docs A115-882, A115-891, SIB 035.00-P-10/72
DIN 70020 Engine Power Measurement Standard
SAE International: J272 Engine Oil Viscosity Classification
The Mercedes-Benz M 115.920 was used across Mercedes-Benz's W114/W115/W111 platforms with longitudinal mounting and no licensed external applications. This engine received platform-specific adaptations-tuned exhaust manifolds for sedan versus coupe applications-and from 1972 the updated W114/115 series adopted the revised MLS head gasket, creating minor service part incompatibilities. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped horizontally on the front-facing side of the cylinder block, near the exhaust manifold (Mercedes-Benz TIS A115-882). The 7th digit of the VIN identifies the engine family ('B' for M115 series). Pre-1972 models have a composite head gasket and ribbed valve cover; post-1972 units use a multi-layer steel gasket and flat-profile cover. Critical differentiation from M114: M115.920 has a 2.5L displacement with twin Zenith carburettors, while M114 variants are 2.3L with a single Solex unit. Service parts require production date verification—head gaskets before 07/1972 are not compatible with later models due to sealing surface revisions (Mercedes-Benz SIB 035.00-P-10/72).
The M 115.920's primary reliability risk is cylinder head gasket integrity on pre-1972 units, with elevated incidence under sustained highway loads. Internal Mercedes-Benz field reports from 1973 indicated a measurable number of early engines requiring gasket service before 100,000 km, while BMVI inspection data shows carburettor-related drivability issues in cold climates. Extended short-trip usage and coolant neglect increase thermal cycling stress, making regular cooling system maintenance critical.
Analysis derived from Mercedes-Benz technical bulletins (1970-1975) and German Federal Transport Authority (KBA) inspection records (1972-1980). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ M115920.
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