BMW M20B25 engine

Technical Specifications

Reliability, problems and repair of the BMW M20B25 engine

In-line six-cylinder engine BMW M20B25 (popularly known as “Spider”) was developed on the basis of low-volume six M20B20 in 1986 and together with the base unit and motors M20B23, M20B27, is part of the BMW M20 family. This engine was used on BMW cars with the index 25i.

In the cylinder block of the 2-liter M20B20 it was decided to increase the cylinder diameter from 80 mm to 84 mm, replace the crankshaft with a stroke of 66 mm to a longer stroke of 75 mm. The length of connecting rods was increased from 130 mm to 135 mm. The height of the cylinder block remains the same – 206 mm. To fit all this, pistons with a compression height of 34.2 mm are used.

The cylinder head is still single-shaft 12-valve, but it is different. There are no hydrocompensators, so the adjustment of valves is required every 10-20 thousand kilometers. Valve clearances on the intake and exhaust 0.25 mm.

Valve diameter has increased, inlet valves are now 42 mm (was 40 mm), exhaust valves 36 mm (was 34 mm). M20B25 camshaft specs: phase 256, lift 10.1mm. Head number 885.

Timing belt drive, belt replacement is required about once every 50 thousand kilometers, in case of belt breakage, the engine bends valves. Fuel injection system.

Since 1990, the M20B25 engine was replaced by a new improved engine M50B25.

Modifications of the BMW M20B25 engine

• M20B25 (1985 – 1986) – the first version without the use of a catalytic converter. Power 171 hp at 5800 rpm, torque 226 Nm at 4000 rpm
• M20B25 (1986 – 1991) – analog of the above model, but with a catalyst. Compression ratio 9.7, power 170 hp at 5800 rpm, torque 215 Nm at 4000 rpm.
• M20B25 (1988 – 1991) – modified version with another short intake manifold. Compression ratio 8.8, power 170 hp at 5800 rpm, torque 222 Nm at 4300 rpm.
• M20B25 (1988 – 1991) – modified version with a different short intake manifold.

Problems and disadvantages of BMW M20B25 engines

1. Overheating. The M20 engines cylinder head is prone to overheating and leading, the causes are due to a faulty OEM cooling system. Check it for air plugs, the condition of the radiator, pump, thermostat operability.
2. Wear of the camshaft pastels. The problem is characteristic of power units with a mileage of more than 200-250 thousand kilometers (ie, for all), the failure will be indicated by the knock of the camshaft. This problem is treated by buying a new cylinder head.
3. Crack in the cylinder head. Disease of M20 motors arising mainly from overheating of the engine. Most often the crack is formed in the area of 4-5 cylinders and is treated by buying another cylinder head.

In addition, the owner of the M20B25 engine must carefully monitor the condition of the timing belt and the position of the tensioner, otherwise you are threatened with a breakage of the belt along with bending valves, sometimes with the destruction of the pistons. Among other things, given the age of the engine and their strong wear, to the known generic diseases are added and age-related … Therefore, when choosing a car with this motor you need to think carefully, it is clearly not the best choice for trouble-free operation. Buying M20B25 engine for swap is even worse, M50B25 is much more preferable.

Tuning the BMW M20B25 engine

M20B25 Stroker

The easiest way to increase the output of a 2.5-liter motor is to convert it into a 2.7-liter motor. For this procedure, you need to buy an M20B27 crankshaft with a stroke of 81 mm or from the diesel M21D24, with the same stroke but lighter. M20B20 connecting rods are 130 mm, pistons are stock, cylinder block is milled by 2.2 mm. Such modifications will add about 10-15 hp.

To get M20B28, you need to install a crankshaft M51D25, with a piston stroke of 82.8 mm. Buy M20B20 connecting rods, stock pistons, cylinder block is milled by 1.1 mm.

To move on, you need to buy sport camshafts such as Schrick 284/272, springs and reinforced rockers. This will give you about 20 more hp. If it’s not enough, then do porting of the cylinder-head with alignment of channels, put reinforced head bolts, light flywheel, cold intake, injectors (~250 cc), straight-through exhaust and we get ~220 hp.

To squeeze out what was not squeezed out, we buy even more evil shafts with phase for 290 with Schrick springs and 6-throttle intake Dbilas.

Turbo M20B25

For serious power gains, fiddling with the shafts won’t be enough…you need to blow, and blowing is better in the 2.7 liter stroker version. For these purposes any Chinese turbokit (with brains in a complete set) on the basis of supercharger GT28 or similar (it is possible original and expensive kit on the basis of Garrett GT28), the motor preliminary it is necessary to unclench, having replaced pistons on forged under degree ~8.5, or to face standard on ~1.5 mm, to put injectors 440 ss, an exhaust on 2.5-3″ pipe, to adjust and to blow up to 300-350 hp. But only the cost of finalization even with cheap Chinese components will be very solid, whether it is worth it or not is up to you.

MOTOR RATING: 3+