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 balanced performance and refinement. It prioritizes mid-range torque and quiet operation, making it well-suited for long-distance driving. Emissions control was managed via a simple crankcase ventilation system, meeting early European standards (pre-Euro) through lean-burn tuning and precise carburettor calibration.
One documented characteristic is sensitivity to fuel quality and carburettor synchronization, particularly in high-mileage units. This issue, referenced in Mercedes-Benz Service Information Bulletin 07 50 01, stems from wear in throttle linkage bushings and float chamber degradation. In 1968, revised jetting and improved air cleaner design were introduced to enhance mixture stability under variable load conditions.
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 naturally aspirated inline-six provides smooth, linear power ideal for relaxed cruising but requires regular carburettor synchronization and throttle linkage inspection to maintain drivability. Use of high-quality petrol (RON 98) is recommended to prevent pre-ignition under sustained load. The timing chain is robust but should be inspected for stretch or guide wear during major services. Oil changes every 10,000 km using API SF/SG–rated 10W-40 or 20W-50 oil ensure valve train protection. The absence of emissions controls simplifies maintenance but requires adherence to period-correct tune-up procedures for optimal efficiency.
Oil Specs: Requires API SF/SG specification oils (Mercedes-Benz SIB 07 50 01). Modern low-SAPS oils not recommended due to zinc content requirements.
Emissions: No Euro standard applies; complies with pre-1970 German national emissions limits (KBA Type Approval #KBA/ENG/115939).
Power Ratings: Measured under DIN 70020 standards. Output assumes clean carburettors and correct ignition timing (Mercedes-Benz TIS Doc. A003020).
Mercedes-Benz Technical Information System (TIS): Docs A003010, A003015, SIB 07 50 01
KBA Type Approval Database (KBA/ENG/115939)
DIN 70020:1972 Road Vehicles — Engine Power Measurement
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.
Locate the engine code stamped vertically on the front-left cylinder block near the distributor (Mercedes-Benz TIS A003030). The 6th VIN digit indicates engine family ('5' for M115 series). Pre-1968 models have chrome valve covers with ribbed design; post-1968 units use matte black finishes. Critical differentiation from M114.939: M115.939 has larger bore and distinct intake manifold for twin carburettors. Service parts require production date verification—carburettor jets and ignition components for pre-1968 engines are not interchangeable with later revisions due to calibration changes (Mercedes-Benz SIB 07 50 01).
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.
Find answers to most commonly asked questions about MERCEDES-BENZ M115939.
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