BMW M20B25 Engine Specs

The BMW M20B25 (also referred to as M20 B25 or BMW 2.5 inline-six) is a 2.5-liter naturally aspirated inline-six petrol engine produced by BMW from 1985 to 1995. Part of the M20 engine family, the M20B25 was developed as a refined evolution of the earlier M20B20, delivering more displacement, power, and torque while maintaining the smooth, free-revving character that defined BMW's six-cylinder identity throughout the 1980s. Designed to meet the demands of both sporting drivers and everyday touring use, the M20B25 became one of the most celebrated engines of its era and the defining powerplant of the iconic E30 and E34 generations.

The M20B25 was engineered as BMW's answer to the growing demand for a capable yet accessible performance engine in the mid-size segment. A key step forward was the adoption of the Bosch Motronic combined fuel injection and ignition management system, which was considered advanced for its time and represented a significant improvement over the carburettors used in earlier variants. Motronic allowed precise fuel delivery and ignition timing across all operating conditions, contributing to the engine's notable combination of strong performance, acceptable fuel economy, and relatively clean emissions for the period. BMW's ambition was to build an engine that would feel equally at home in urban traffic and on long motorway cruises while producing the characteristic inline-six sound the brand had become synonymous with.

The M20B25 features an aluminum alloy cylinder block with Nikasil-coated cylinder bores, which provide excellent wear resistance and allow tight tolerances vital to long-term reliability. The aluminum alloy cylinder head uses a Single Overhead Camshaft (SOHC) layout driving 12 valves, two intake and one exhaust per cylinder. Drive is taken from the crankshaft via a timing chain rather than a belt, which requires no periodic replacement and contributes to the engine's ability to accumulate extremely high mileage with minimal intervention. The Motronic injection system meters fuel into each intake port individually, ensuring consistent mixture distribution and stable, smooth operation across the rev range.

The M20B25 has a bore and stroke of 84.0 mm x 75.0 mm, producing a displacement of 2,494 cc. The compression ratio is 9.0:1 and the engine requires a minimum of 98 RON fuel. Power output is 170 hp (125 kW) at 5,800 rpm, with a peak torque of 222 Nm arriving at 4,000 rpm - accessible enough to make the engine feel strong in everyday driving without requiring high revs. The firing order is 1-5-3-6-2-4 and the engine weighs approximately 145 kg. Oil capacity is 4.25 liters, with BMW recommending 5W-30 specification oil. Operating temperature is maintained between 80 and 90 degrees Celsius. With consistent maintenance, the M20B25 is widely regarded as capable of exceeding 400,000 km before a major overhaul is required.

The M20B25 was fitted to several of BMW's most celebrated models of the era, including the BMW E30 325i (1985-1991), E34 525i (1988-1995), E30 325i Touring (1987-1994), and the E30 325iX all-wheel-drive variant (1987-1991). These vehicles cemented the engine's reputation as deeply satisfying to drive: the broad torque curve delivers confident pull from low revs, while the engine remains smooth and refined all the way to the redline. Real-world fuel consumption ranges between 7.8 and 11.5 liters per 100 km depending on driving conditions - a figure that remains respectable even by modern standards for a naturally aspirated six-cylinder of this output.

Alternative names and designations for this engine include M20 B25, M20B25E, and BMW 2.5 SOHC inline-six, reflecting different regional documentation conventions and parts catalog references across BMW's global markets. Decades after its production ended, the M20B25 retains a devoted following among BMW enthusiasts worldwide. Its combination of mechanical simplicity, exceptional longevity, characterful inline-six acoustics, and the timeless driving experience it delivers in the E30 and E34 bodyshells have secured its place as one of the most iconic engines in BMW's history.