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 model…
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 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.
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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.
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The M 115.920 is a robust and well-engineered engine when properly maintained. Pre-1972 models had a tendency for head gasket seepage under high load, but the 1972 update with an MLS gasket resolved most issues. Regular oil and coolant changes, proper carburettor maintenance, and use of zinc-containing oil for the flat-tappet camshaft are essential for longevity. Well-cared-for examples can exceed 200,000 km.
The primary issues are cylinder head gasket failure on pre-1972 units, carburettor icing or mixture imbalance in cold weather, and valve train wear if non-zinc oils are used. Cooling system component failure (hoses, thermostat, pump) is also common with age. These are documented in Mercedes-Benz service literature and owner maintenance reports.
The M 115.920 was used in the W114 250 and 250C, W115 250, and the final-year W111 250 Coupé and Cabriolet (1970–1971). It was the standard 2.5L six-cylinder petrol engine for these models during its production run, offering a balance of performance and refinement.
Yes, but within limits. Performance tuning typically involves upgrading to twin SU carburettors, performance camshafts, or exhaust manifolds. Some owners retrofit modern fuel injection. However, the engine's 9.0:1 compression and SOHC design cap significant power gains. Most modifications focus on improving throttle response and drivability rather than high output.
Fuel economy is modest by modern standards. In a W114 250, expect approximately 12–14 L/100km (20–17 mpg UK) in mixed driving. Highway cruising is more efficient at around 10 L/100km (28 mpg UK), while city driving can exceed 15 L/100km (19 mpg UK). Carburettor tuning and driving style significantly affect consumption.
No. The M 115.920 is a non-interference engine. If the timing chain fails, the pistons will not contact the valves, preventing catastrophic internal damage. However, a broken chain will still disable the engine and require significant repair. The chain is durable but should be inspected periodically, especially on high-mileage engines.
Mercedes-Benz specified SAE 20W-50 mineral oil meeting API SD/CC standards. Modern equivalent oils with zinc (ZDDP) additives are recommended to protect the flat-tappet camshaft. Oil should be changed every 6,000 km or annually to ensure optimal valve train lubrication and engine longevity.
Comprehensive technical documentation and regulatory references
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Regulatory Stability
EU regulations are referenced using CELEX identifiers for long-term stability.
EUR-Lex
EU emissions and type-approval regulations (e.g., CELEX:32007R0715, CELEX:32017R1151).
GOV.UK: Vehicle Approval & V5C
UK vehicle approval processes, import rules, and MoT guidance.
DVLA: Engine Changes & MoT
Official guidance on engine swaps and inspection implications.
Vehicle Certification Agency (VCA)
UK type-approval authority for automotive products.
Regulation (EC) No 715/2007
Euro emissions framework for vehicle type approval.
Commission Regulation (EU) 2017/1151
WLTP and RDE testing procedures for emissions certification.
GOV.UK: Vehicle Approval
UK compliance and certification requirements for imported and modified vehicles.
VCA Certification Portal
Type-approval guidance and documentation.
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