BMW M20B25 Engine

1. Specifications
2. Overview, problems
3. Performance tuning

M20B25 Specs

BMW M20B25 engine reliability, problems and repair

BMW M20B25 straight 6-cylinder engine was developed on base of low displacement M20B20 motor in 1986. It belongs to BMW M20 family along with M20B23 and M20B27 engines. Diameter of cylinders was increased from 80 mm (3.15 in) to 84 mm (3.3 in) in new 2.5-liter engine block. 66 mm (2.6 in) stroke crankshaft were replaced with a new one (75 mm/2.95 in). The length of connecting rods is 135 mm. Height of cylinders block is still the same.

The same SOHC 12-valve cylinder head is used for this engine without hydro compensators. However combustion chambers was improved. Inlet valve diameter has been increased from 40 to 42 mm as well as diameter of exhaust valves which is 36 mm now. M20B25 camshaft specs : duration – 256 deg, valve lift – 10.1 mm. Cylinder head number – 885.

Valve adjustment is necessary every 6,000 – 13,000 miles (10,000 – 20,000 kilometers) mileage. Inlet and exhaust valve clearance is 0.25 (.01″) mm.

Replacement of the timing belt is recommended every 30,000 miles (50,000 kilometers) mileage. If the belt is broken, the valves are bended by the engine.

This engine was mainly used on BMW autos with 25i index.

Starting from 1990 M20B25 engine was replaced by new updated M50B25 engine.

BMW M20B25 engine modifications and differences

1. M20B25 (1986 – 1988) is the first version with catalyst. Specifications are as follows: compression ratio – 9.7, power – 163 HP at 5,800 rpm, 215 Nm of torque at 4,000 rpm.

2. M20B25 (1985 – 1987) is equal to first version of engine. The only difference is that is comes without catalyst. Specifications are as follows: power – 171 HP at 5,800 rpm, 226 Nm of torque at 4,000 rpm.

3. M20B25 (1988 – 1991) is upgraded version equipped by another short intake manifold. Specifications are as follows: compression ratio – 8.8, power – 170 HP at 5,800 rpm, 222 Nm of torque at 4,300 rpm.

BMW M20B25 engine problems and drawbacks

1. Overheat. M20 engine cylinder head is prone to overheating. The main reason of this malfunction is connected to problems with basic cooling system. It should be thoroughly examined in order to prevent airlock occurrence. Other possible reasons: clogged radiator, water pump, thermostat.

2. Camshaft pastels wear-out. This problem is very likely to occur in engines with 125,000-155,000 miles (200,000 – 250,000 kilometers) mileage. You will hear knocking of camshaft which will be the main evidence of malfunction. All you need is to replace the old cylinder head with a new one.

3. Crack of cylinder head. This problem is also very likely to occur in M20 engines due to permanent overheating. The crack occurs in 4^th of 5^th cylinder more often. All you need is to replace old cylinder head with a new one.

At the same time M20B25 engine owners should always monitor condition of their timing belts well as position of tensioner pulley. Otherwise it can lead to timing belt break down which will result in valve bending. At the same time age of motor as well as level of its wear-out can also be the main cause of various possible problems and malfunctions. In other words you should think twice before purchasing auto with such engine as it is not the best motor you can think of. Moreover we are not recommended buying M20B25 engine for swap. This is nothing more but ridiculous idea.

BMW M20B25 engine number location

The photo below shows the location of engine number.

BMW M20B25 engine tuning

M20B25 Stroker 2.7. N/A

The easiest way to increase 2.5-liter engine productivity is to turn it into 2.7-liter engine. You will need several additional spare parts to proceed with such upgrade. First of all you will have to buy crankshaft which can be found in diesel modifications of M21D24 or M20B27 engines. 130 mm M20B20 connecting rods will also be necessary for this purpose as well as pistons. Then you need to mill cylinder block up to 2.2 mm. Such upgrade will make it possible to add extra 10-15 HP.

M20B25 to M20B28: You need to install M51D25 crankshaft with 82.8 mm (3.26 in) stroke. Don’t forget to buy M20B20 camshaft, stock pistons and cylinder block which was already milled up to 1.1 mm.

You will also need sports camshaft in order to move on with your upgrade. Schrick 284/272 will be excellent choice for such situation along with strengthen springs and reinforced rocker. It will make it possible for you to obtain additional 20 HP. To finish the upgrade you will need head porting, light-weight flywheel, cold air intake, sports exhaust manifold, straight-through exhaust and ARP head studs. Don’t forget about 250 cc fuel injectors. Such improvements will make it easy to upgrade your engine up to 22 HP.

If you call for maximum power, you can use 290-duration camshaft along with Dbilas 6 individual throttle bodies (ITB).

M20B25 Turbo

If you want to implement successful turbo project on base of M20B25 as well as 2.7 (2.8) stroker, you need to complete the same actions as with M20B20 turbo engine. HERE you can get acquainted with the whole process.

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