M20B25 is a 2.5 L inline-six, 4-cycle naturally aspirated gasoline engine, built on the framework of the M20B20 2.0L motor. Being a part of the series of BMW M20 motors, it was built between 1985 and 1991 at the company’s plant in Munich.
This motor keeps a block of cast iron and head of aluminum with overhead camshaft and two valves in each of six cylinders. The sizes of cylinder bore and piston stroke are 84.0 mm (3.31 inches) and 75.0 mm (2.95 inches) correspondingly. Fuel injection is Bosch Motronic 1.1/1.3 Adaptive.
In 1985-1986, the first modifications of the motor featured Bosch Motronic 1.1 injection. The compression index was 9.7:1 while a redline was 6,500 rpm. The capacity was equal to 169 hp at 5,800 rpm and a torque of 226 Nm (23.0 kg-m; 166.8 ft-lb) at 4,000 rpm.
By 1987, the motor was modified with the addition of Motronic 1.3. The compression rate descended to 9.3:1. The motor’s output was 169 hp at 5,800 rpm. Moreover, the motor had a modification with catalytic converters. The compression index was 8.8:1 while the capacity was 168 hp at 5,800 rpm and a torque of 222 Nm (22.6 kg-m; 163.8 ft-lb) at 4,300 rpm.
M20B25 Engine Specs
M20B25 contains a cylinder block of cast iron. The size of bore is 84.0 mm (3.31 inches) and stroke is 75.0 mm (2.95 in). Pistons are combined with two compression rings and single oil ring.
Aluminum cylinder head #885 produces a great cooling performance. The motor keeps single overhead camshaft, controlled by timing belt. It produces intake and exhaust duration of 256 degrees.
Valves are operated by rocker arms. Width of the intake valves is 42.0 mm (1.65 inches) while width of the exhaust ones is 36.0 mm (1.42 inches) and the valve lift is 10.1 mm (0.397 inches). Besides, rocker arms serve to control of camshaft follower valve because one of its edges includes eccentric nut. In addition, they turn around rocker arm shafts.
|Manufacturer||Gunma Oizumi Plant|
|Also called||Subaru EZ30|
|Cylinder block alloy||Aluminum|
4 valves per cylinder
|Piston stroke, mm (inch)||80 (3.15)|
|Cylinder bore, mm (inch)||89.2 (3.51)|
|Displacement||2999 cc (183 cu in)|
|Power output||164 kW (220 HP) at 6,000 rpm
183 kW (245 HP) at 6,600 rpm
|Torque output||289 Nm (213 lb·ft) at 4,400 rpm
297 Nm (219 lb·ft) at 4,200 rpm
|HP per liter||73.3
|Weight, kg (lbs)||180 (397)|
|Fuel consumption, L/100 km (mpg)
|Oil consumption , L/1000 km
(qt. per miles)
|up to 1.0
(1 qt. per 600 miles)
|Recommended engine oil||0W-30
|Engine oil capacity, L (qt.)||5.7|
|Oil change interval, km (miles)||5,000-10,000
|Normal engine operating temperature, °C (F)||90 (194)|
|Engine lifespan, km (miles)
-No life span loss
BMW M20B25 Engine Modifications
1. M20B25 (from 1986 to 1988) is a catalyst engine with the compression index of 9.7. The output is 163 hp at 5,800 rpm and a torque of 215 Nm at 4,000 rpm.
2. M20B25 (from 1985 to 1987) resembles the previous type but without catalyst. It reaches 171 hp at 5,800 rpm and a torque of 226 Nm at 4,000 rpm.
3. M20B25 (from 1988 to 1991) is a restyled variation with short intake manifold. The compression rate is 8.8. The capacity is 170 hp at 5,800 rpm and a torque of 222 Nm at 4,300 rpm.
- 1985–1993 E30 325i
- 1989–1990 E34 525i
- 1988–1991 Z1
BMW M20B25 Engine Problems
1. Overheating. Cylinder head may overheat owing to dysfunction of cooling system. To avoid airlock emergence, it must be checked. Moreover, overheating may be a consequence of blocked radiator, water pump or thermostat.
2. Camshaft bed depreciation. After the car runs 125,000-155,000 miles or 200,000-250,000 kilometers, camshaft may start to clatter. The sole solution is to buy a fresh cylinder head.
3. Cylinder head fracture. The cause for that is constant overheating and usually concerns 4th of 5th cylinder. By acquiring a fresh cylinder head, the problem will be resolute.
It’s advisable to take care of timing belts and tensioner pulley. Timing belt failure may cause valve deformation. Besides, motor age and the stage of its depreciation may influence steady engine functioning. Honestly, this motor leaves much to be desired. That’s why, have a look at some other variants before acquiring it.
BMW M20 Engine Repair Video