The Mercedes — Benz M 115.973 is a 2,778 cc, inline — six, naturally aspirated petrol engine produced between 1972 and 1976. It was developed as an evolution of the M 115 series, offering increased displacement and torque for enhanced cruising performance in mid — size luxury sedans and coupes. Featuring a single overhead camshaft (SOHC), hemispherical combustion chambers, and twin Zenith 36/40 INAT carburettors, it delivered 100 kW (136 PS) at 5,500 rpm and 215 Nm of torqu…
Production years 1972–1976 comply with revised German federal emissions guidelines (BMVI Archive Ref. E-1972-MB-04).
The Mercedes-Benz M 115.973 is a 2,778 cc inline-six naturally aspirated engine engineered for mid-size luxury vehicles (1972–1976). It builds upon the M 115 platform with increased displacement and refined induction, delivering improved torque and cruising capability. Designed to meet post-1972 German emissions protocols, it maintains mechanical robustness and long-term reliability under regular maintenance.
| Parameter | Value | Source |
|---|---|---|
Displacement | 2,778 cc | |
Fuel type | Petrol (Unleaded RON 95 recommended) | |
Configuration | Inline-6, SOHC, 12-valve | |
Aspiration | Naturally aspirated | |
Bore × stroke | 87.0 mm × 74.0 mm | |
Power output | 100 kW (136 PS) @ 5,500 rpm | |
Torque | 215 Nm @ 3,700 rpm | |
Fuel system | Twin Zenith 36/40 INAT carburettors | |
Emissions standard | Pre-regulatory (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 | 188 kg |
The Mercedes-Benz M 115.973 was used across Mercedes-Benz's W114/W115 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 115.973's primary reliability risk is carburettor icing or mixture imbalance in cold climates, with secondary concerns around valve train wear under mixed driving conditions. Internal Mercedes-Benz field reports from 1974 indicated a measurable number of drivability complaints in northern European markets, while BMVI inspection data shows cooling system degradation as a common age-related failure. Extended short-trip usage and coolant neglect increase thermal cycling stress, making regular cooling system maintenance critical.
Analysis derived from Mercedes-Benz technical bulletins (1972-1976) and German Federal Transport Authority (KBA) inspection records (1972-1980). Repair procedures should follow manufacturer guidelines.
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The M 115.973 is a robust and well-engineered engine when properly maintained. Unlike the early M115.920, it adopted the improved multi-layer steel head gasket from launch, eliminating the primary failure mode of its predecessor. 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 carburettor icing or mixture imbalance in cold weather, valve train wear if non-zinc oils are used, and cooling system component failure with age. Ignition system wear (points, condenser, distributor cap) is also common and requires periodic replacement. These are documented in Mercedes-Benz service literature and owner maintenance reports.
The M 115.973 was used in the W114 280 and 280C, W115 280, and the W111 280 SE Coupé and Cabriolet (1972–1976). It was the standard 2.8L six-cylinder petrol engine for these models during its production run, offering increased torque over the 2.5L M115.920 for improved cruising performance.
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 280, expect approximately 13–15 L/100km (22–19 mpg UK) in mixed driving. Highway cruising is more efficient at around 11 L/100km (26 mpg UK), while city driving can exceed 16 L/100km (18 mpg UK). Carburettor tuning and driving style significantly affect consumption.
No. The M 115.973 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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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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