The BMW M50 B25 (256S1) is a 2,494 cc, inline-six naturally aspirated petrol engine introduced in 1990 as part of BMW's new generation M50 engine family. It features a cast-iron block, aluminium head, dual overhead camshafts (DOHC), and Bosch Motronic 1.7 electronic fuel injection. In standard tune it delivers 141 kW (192 PS), with torque output of 250 Nm, offering smooth and refined performance typical of BMW's inline-six heritage.
Fitted across BMW's E34 platform, the M50 B25 (256S1) was engineered for longitudinal mounting and rear-wheel drive layouts. Emissions compliance was achieved through catalytic converter systems, meeting Euro 2 standards from 1992. The engine is renowned for its balance of performance, efficiency, and mechanical sophistication, supporting decades of enthusiast ownership and restoration.
One documented concern is valve cover and rear main seal oil leaks due to aging rubber gaskets. This issue, referenced in BMW Service Information Bulletin 11 05 92, is linked to material degradation over time. From 1992, BMW revised the gasket materials and introduced improved sealing designs to enhance long-term reliability.
Models from 1992–1996 meet Euro 2 emissions standards (VCA UK Type Approval #VCA/EMS/7903).
The BMW M50 B25 (256S1) is a 2,494 cc inline-six naturally aspirated petrol engine engineered for executive sedans (1990–1996). It combines DOHC valvetrain with electronic fuel injection to deliver high-revving performance and driver engagement. Designed to meet evolving emissions standards, it balances mechanical sophistication with serviceability.
| Parameter | Value | Source |
|---|---|---|
| Displacement | 2,494 cc | |
| Fuel type | Petrol | |
| Configuration | Inline-6, DOHC, 24-valve | |
| Aspiration | Naturally aspirated | |
| Bore × stroke | 84.0 mm × 75.0 mm | |
| Power output | 141 kW (192 PS) @ 5,900 rpm | |
| Torque | 250 Nm @ 4,700 rpm | |
| Fuel system | Bosch Motronic 1.7 electronic fuel injection | |
| Emissions standard | Euro 2 (1992–1996) | |
| Compression ratio | 10.5:1 | |
| Cooling system | Water-cooled with mechanical thermostat | |
| Turbocharger | None | |
| Timing system | Chain (single-row, front-mounted) | |
| Oil type | SAE 10W-40 mineral or 5W-40 synthetic | |
| Dry weight | 180 kg |
The M50 B25's DOHC design and Motronic 1.7 fuel injection offer excellent drivability and reduced emissions compared to its M30 predecessor. The timing chain system is robust and less prone to wear than earlier designs, but oil changes every 10,000 km are recommended to ensure longevity. Catalytic converters require unleaded fuel. The engine performs best on 95 RON fuel. Modern ethanol-blended fuels can degrade old rubber components; use ethanol-compatible hoses for long-term reliability. The revised valve cover gasket (post-1992) significantly reduces the risk of oil leaks.
Oil Specs: No BMW Longlife specification; use high-quality mineral or synthetic 10W-40 meeting API SL/CF standards (BMW SIB 11 05 92).
Emissions: Euro 2 certification applies to 1992–1996 models (VCA Type Approval #VCA/EMS/7903). Pre-1992 models are non-catalyst.
Power Ratings: Measured under DIN 70020 standards. Output is consistent across production years (BMW TIS Doc. A27005).
BMW Technical Information System (TIS): Docs A27000, A27001, A27002, SIB 11 05 92
VCA Type Approval Database (VCA/EMS/7903)
DIN Deutsches Institut für Normung: DIN 70020 Power Measurement Standard
The BMW M50 B25 (256S1) was used across BMW's E34 platform with longitudinal mounting and RWD layout. This engine received platform-specific adaptations—revised intake manifolds in the E34 LCI and dual-exhaust setup in performance variants—and from 1992 the facelifted E34 models adopted catalytic converters to meet Euro 2, creating interchange limits. No licensed or shared-production partnerships existed for this engine. All adaptations are documented in OEM technical bulletins.
Locate the engine code stamped on the front timing cover near the oil filter housing (BMW TIS A27010). The 7th VIN digit indicates engine family ('M' for M-series). All M50 B25 (256S1) units use Bosch Motronic 1.7 with a combined air flow meter and temperature sensor. E34 models (1990–1996) have a distinctive valve cover with "M50" casting. Critical differentiation: This variant is a DOHC 24-valve engine, unlike the SOHC M30 engines. Service parts are generally interchangeable, but catalytic converter-equipped models (post-1992) require compatible exhaust manifolds.
The M50 B25 (256S1)'s primary concern is valve cover and rear main seal oil leaks due to aging rubber gaskets. UK DVLA and MOT records indicate that poorly maintained examples often fail on emissions or idle quality, while well-serviced engines regularly exceed 200,000 km. Cold-start performance and idle stability are most affected by neglected fuel and ignition systems.
Analysis derived from BMW technical bulletins (1990s), UK DVSA MOT failure statistics (2018-2024), and restoration guides. Repair procedures should follow manufacturer guidelines.
Find answers to most commonly asked questions about BMW M50B25.
Comprehensive technical documentation and regulatory references
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BMW 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)
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
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