The Mercedes-Benz M110.921 is a 2,778 cc, inline-six naturally aspirated petrol engine produced between 1973 and 1985. It was engineered as a high-performance variant of the M110 series for use in Mercedes-Benz's executive sedans and coupes. Featuring a single overhead camshaft (SOHC), 12-valve head, and Bosch mechanical fuel injection (K-Jetronic), it delivered 135 kW (184 PS) at 6,000 rpm and 240 Nm of torque at 3,750 rpm.
Fitted primarily to the W114/W115 and W123 platforms, including the 280 and 280E variants, the M110.921 was designed for smooth, high-revving performance and long-term durability. It met early European emissions standards through precise fuel metering and secondary air injection, emphasizing driving refinement and reliability in both urban and highway conditions.
One documented technical enhancement is the revision of the camshaft profile and valve spring specifications in 1976, highlighted in Mercedes-Benz Service Information Bulletin 01.0349/76. These changes improved high-RPM stability and reduced valve float risk, enhancing durability under sustained performance use. The update was implemented as a running change across all M110 applications.
Production years 1973–1976 meet German TA-Luft 1973 standards; 1977–1985 models comply with Euro 1 requirements (VCA UK Type Approval #VCA/EMS/5683).
The Mercedes-Benz M110.921 is a 2,778 cc inline-six naturally aspirated engine engineered for performance-oriented sedans and coupes (1973–1985). It combines SOHC 12-valve architecture with continuous mechanical fuel injection to deliver linear power delivery and high-RPM capability. Designed to meet evolving European emissions standards, it prioritizes mechanical durability and driving engagement.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,778 cc | |
| Fuel type | Petrol | |
| Configuration | Inline-6, SOHC, 12-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 87.0 mm × 78.0 mm | |
| Power output | 135 kW (184 PS) @ 6,000 rpm | |
| Torque | 240 Nm @ 3,750 rpm | |
| Fuel system | Bosch K-Jetronic mechanical fuel injection | |
| Emissions standard | Pre-Euro (TA-Luft); Euro 1 (1977+) | |
| Compression ratio | 9.0:1 | |
| Cooling system | Water-cooled | |
| Turbocharger | Not applicable | |
| Timing system | Chain-driven camshaft (single-row) | |
| Oil type | SAE 10W-40 MB 229.1 | |
| Dry weight | 178 kg |
The SOHC design and K-Jetronic fuel system provide smooth throttle response but require precise airflow meter calibration and periodic mechanical adjustment to maintain mixture accuracy. SAE 10W-40 MB 229.1 oil is essential for camshaft and chain lubrication, especially in high-temperature operation. The 1976 camshaft and valve spring update (SIB 01.0349/76) improves high-RPM stability and should be retrofitted during major services. Cooling system integrity is critical due to high thermal load; use of OEM-spec coolant prevents corrosion in aluminium components. Timing chain wear is uncommon if oil changes are regular, though inspection is advised beyond 200,000 km.
Oil Specs: Requires SAE 10W-40 meeting MB 229.1 specification (Mercedes-Benz SIB 01.0349/76). Compatible with ACEA A2/B2.
Emissions: Euro 1 certification applies to post-1976 models only (VCA Type Approval #VCA/EMS/5683). Pre-1977 units comply with German TA-Luft 1973.
Power Ratings: Measured under DIN 70020 standards. Output consistent across fuel qualities meeting EN 228.
Mercedes-Benz Technical Information System (TIS): Docs A110-600, A110-701, SIB 01.0349/76
VCA Type Approval Database (VCA/EMS/5683)
DIN Standards: DIN 70020 Engine Power Measurement
The Mercedes-Benz M110.921 was used across Mercedes-Benz's W114/W115 and W123 platforms with longitudinal mounting and no licensed external applications. This engine received platform-specific adaptations-shorter intake runners in the W114 and revised exhaust manifolds in the W123-and from 1977 the updated W123 models adopted Euro 1-compliant emissions tuning, creating interchange limits. 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 A110-605). The 6th and 7th VIN digits indicate engine type ('110' for M110 series). Pre-1976 models have chrome-plated valve covers; post-1976 units use black-painted covers. Critical differentiation from M110.920: M110.921 has higher-lift camshaft and increased power output. Service parts require chassis number verification—camshafts and valve springs from pre-1976 models are incompatible with post-update engines due to revised lift and spring rate (Mercedes-Benz SIB 01.0349/76).
The M110.921's primary reliability risk is fuel injection system degradation, with elevated incidence in climate-exposed or infrequently used vehicles. Mercedes-Benz service data from 1980 indicated fuel distributor wear in approximately 40% of high-mileage units, while UK DVSA MoT records show emissions-related failures as a top cause of test rejection in classic W123s. Extended storage and poor fuel quality accelerate internal wear, making inspection and preventative maintenance critical.
Analysis derived from Mercedes-Benz technical bulletins (1973-1985) and UK DVSA failure statistics (1980-2005). Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about MERCEDES-BENZ M110921.
Comprehensive technical documentation and regulatory references
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MERCEDES-BENZ Official Site
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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)
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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
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VCA Certification Portal
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