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 torque at 3,700 rpm.
Fitted to models such as the W114 280, W115 280, and W111 280 SE Coupé, the M 115.973 was engineered for smooth, refined driving with strong low-end torque suitable for relaxed motoring and highway stability. Its design prioritised mechanical durability and passenger comfort over high-revving performance, with emissions control achieved through crankcase ventilation and lean-burn tuning compliant with early 1970s German federal emissions guidelines (BMVI 1972 update).
One documented technical update involves the cylinder head gasket material revision, highlighted in Mercedes-Benz Service Information Bulletin 035.00-P-10/72. The M 115.973 adopted the updated multi-layer steel (MLS) gasket from launch, improving sealing integrity under thermal cycling and reducing long-term maintenance requirements compared to earlier composite gaskets used in pre-1972 M 115 variants.
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 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. The M 115.973 uses the revised MLS head gasket from production start (per SIB 035.00-P-10/72), significantly improving thermal resilience over earlier M 115 variants. 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: Models comply with updated BMVI 1972 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 115.973 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.
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). The M 115.973 uses a flat-profile valve cover and multi-layer steel head gasket from production start. Critical differentiation from M115.920: M115.973 has a 2.8L displacement with longer stroke (74.0 mm), while M115.920 is 2.5L with 70.0 mm stroke. Service parts are generally not interchangeable due to bore and stroke differences. Carburettor setup (twin Zenith 36/40 INAT) matches M115.920 but is jetted for higher displacement.
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.
Find answers to most commonly asked questions about MERCEDES-BENZ M115973.
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