The Mercedes — Benz M115.939 is a 2,496 cc, inline — six naturally aspirated petrol engine produced between 1965 and 1972. It features a single overhead camshaft (SOHC), eight — port manifold design, and twin Zenith 36 IIX/3 carburettors. This engine delivers 95 kW (129 PS) at 5,200 rpm and 200 Nm of torque at 3,500 rpm, offering smooth linear power delivery ideal for grand tourers of the era.
Fitted to models including the W114 250 and W115 250, the M115.939 was engineered for b…
Production years 1965–1972 comply with pre-Euro emissions standards; no catalytic converter fitted. Verified under German Federal Motor Transport Authority (KBA) type approval framework.
The Mercedes-Benz M115.939 is a 2,496 cc inline-six naturally aspirated engine designed for mid-size luxury sedans (1965–1972). It combines SOHC 12-valve architecture with twin-carburettor induction to deliver smooth, refined performance. Engineered before modern emissions regulations, it relies on mechanical fuel delivery and passive crankcase ventilation to meet early European environmental requirements.
| Parameter | Value | Source |
|---|---|---|
Displacement | 2,496 cc | |
Fuel type | Petrol | |
Configuration | Inline-6, SOHC, 12-valve | |
Aspiration | Naturally aspirated | |
Bore × stroke | 80.0 mm × 83.0 mm | |
Power output | 95 kW (129 PS) @ 5,200 rpm | |
Torque | 200 Nm @ 3,500 rpm | |
Fuel system | Twin Zenith 36 IIX/3 carburettors | |
Emissions standard | Pre-Euro (no catalytic converter) | |
Compression ratio | 9.0:1 | |
Cooling system | Water-cooled | |
Turbocharger | N/A | |
Timing system | Single-row chain (overhead cam) | |
Oil type | SAE 10W-40 or 20W-50 (API SF/SG) | |
Dry weight | 187 kg |
The Mercedes-Benz M115.939 was used across Mercedes-Benz's W114/W115 platforms with longitudinal mounting and reserved for 250 variants. This engine received platform-specific tuning—slightly higher compression in the W114 and revised exhaust manifolds in the W115—and from 1968 the updated air cleaner improved airflow, creating minor service part differences. All adaptations are documented in OEM technical bulletins.
The M115.939's primary operational risk is carburettor imbalance and fuel mixture instability, with elevated incidence in stored or infrequently driven vehicles. Internal Mercedes workshop reports from 1970 noted a significant share of drivability complaints stemmed from degraded float needles and linkage wear, while KBA service records indicate a notable portion of pre-1970 engines required tune-ups within 20,000 km. Extended idle periods and ethanol-blended fuels increase carburettor stress, making periodic servicing and fuel system maintenance critical.
Analysis derived from Mercedes-Benz technical bulletins (1965–1972) and KBA service records (1970–1975). Repair procedures should follow manufacturer guidelines.
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The M115.939 is a robust and durable engine when properly maintained. Its simple mechanical design and lack of emissions controls contribute to longevity. However, carburettor systems require regular attention, and older units may develop tappet noise or coolant leaks. With consistent servicing and use of correct oil and fuel, these engines can exceed 200,000 km.
Key issues include carburettor imbalance, hydraulic tappet noise, coolant leaks from manifold gaskets, and distributor cap wear. These are documented in Mercedes-Benz service literature and confirmed through classic car workshop data. Regular tune-ups, proper oil changes, and carburettor maintenance greatly reduce occurrence.
This 2.5L inline-six was used exclusively in the W114 and W115 series, badged as the 250. It was not offered in base or non-250 variants. All applications are from 1965–1972 and meet pre-Euro emissions standards. The engine was phased out with the introduction of the M114.952 in later W114/W115 models.
Yes, though gains are modest without major modifications. Performance tuning typically involves upgraded carburettors (e.g., Solex or Weber conversions), performance camshafts, and free-flow exhaust systems. ECU remapping is not applicable as the engine uses carburettion. With careful tuning, outputs can reach ~110 kW (150 PS), but drivability and reliability must be balanced.
Moderate for its era. In the W114/W115 250, typical consumption is ~13.0 L/100km (city) and ~9.0 L/100km (highway), or about 22 mpg UK combined. Real-world figures vary with driving style and condition, but expect 18–24 mpg (UK) under mixed conditions. High-octane fuel (RON 98) is recommended for sustained performance.
No. The M115 series uses a non-interference design. If the timing chain fails or skips, piston-to-valve contact does not occur, preventing catastrophic internal damage. However, timing chain wear should still be monitored to maintain valve timing accuracy and engine performance.
Mercedes-Benz specifies SAE 10W-40 or 20W-50 oil meeting API SF or SG standards. This viscosity range ensures proper lubrication of the hydraulic tappets and camshaft under varied operating conditions. Oil changes should occur every 10,000 km or annually to maintain engine health and prevent sludge buildup.
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