BMW M20B20 engine

Technical Specifications

Reliability, problems and repairs of the BMW M20B20 engine

In-line six-cylinder engine BMW M20B20 (“Spider”) appeared in 1977, as a replacement for the in-line fours and is the ancestor of the M20 family, which also includes the 2.3-liter M20B23, 2.5-liter M20B25 and M20B27 with a working volume of 2.7 liters. The M20B20 motor was used on cars with the index 20i.

The engine is based on a cast-iron cylinder block with a height of 206 mm and an intercylinder distance of 91 mm. Inside it is a short crankshaft with a stroke of 66 mm, connecting rods 130 mm, pistons 80 mm, and their height of 43.4 mm. All this goodness on 6 cylinders gives 2 liters of working volume.

On top of this block is an aluminum 12-valve cylinder head with a single camshaft (SOHC 12V). At first the head known as the 200 was used, but later it was replaced by the 731, which had a larger diameter intake. The diameter of the inlet valves is 40mm and the exhaust valves are 34mm.

There are no hydrocompensators, valve adjustment is required every 10-20 thousand kilometers, valve clearances are as follows: inlet 0.25 mm, outlet 0.3 mm.

In the timing belt is used, replacement of which is required every 50 thousand kilometers, if the belt is broken, the engine bends valves.
Power system on the first variations of the motor is carbureted, c 1981 carburetors gave way to mechanical and electronic injection.

In 1990, the 2-liter representative of the M20 series was replaced by a new engine – M50B20, which completely replaced the M20B20 in 1992.

Modifications of the BMW M20B20 engine

• M20B20 (1977 – 1981) was the first version of the engine with a carbureted fuel system. It had a 4-chamber carburetor Solex 4A1, compression ratio was 9.2, power 120 hp at 6000 rpm, torque 160 Nm at 4000 rpm.
• M20B20 (1981 – 1987) – version with electronic multi-point injection L-Jetronic. Compression ratio 9.8, power 125 hp at 5800 rpm, torque 170 Nm at 4000 rpm.
• M20B20 (1981 – 1987) – version with electronic multi-point injection L-Jetronic.
• M20B20 (1982 – 1984) – modification with mechanical multi-point injection K-Jetronic, compression ratio 9.9, power 126 hp at 5800 rpm, torque 170 Nm at 4000 rpm
• M20B20 (1985 – 1991) – LE-Jetronic electronic injection engine. This engine has a 731 head, its own intake and exhaust manifolds, compression ratio of 9.8, power of 129 hp at 6000 rpm, torque of 174 Nm at 4000 rpm.
• M20B20 (1987 – 1992) – final version with Motronic electronic injection and catalytic converter. Compression ratio reduced to 8.8, power 129 hp at 5800 rpm, torque 174 Nm at 4300 rpm.
• M20B20 (1987 – 1992) – final version with Motronic electronic injection and catalytic converter.

Problems and drawbacks of BMW M20B20 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, 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 M20B20 engine must carefully monitor the condition of the timing belt and the position of the tensioner, otherwise you are threatened by the 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 should think carefully, it is clearly not the best choice for trouble-free operation.

Tuning BMW M20B20 engine

M20B20 Stroker

Over the many years of the M20 series, many options have been built up to increase the displacement of the M20B20. The simplest and most effective option is a stroker to 2.3 L as the cylinder block height of both engines is the same. To convert M20B20 to M20B23, it is enough to buy a crankshaft, pistons, connecting rods and flywheel from M20B23, new cylinder head gasket, bolts, liners and rings. Building such an engine will give you about 150 hp.

An interesting option is to convert M20B20 to M20B25, but we will go further and increase the displacement from 2.0 L to 2.8 L. For these purposes we need to bore the cylinders to 84 mm, buy a crankshaft M52B28, connecting rods will remain stock, pistons M20B25 Kat (undercut by 3 mm), flywheel M20B20 Carb. Plus replace the radiator, the standard one will not cope. The end result is a more powerful M20B28.

M20B20 Turbo

Building a turbo M20 on the weakest motor of the series is a questionable decision, but it has the right to life. To realize the project it is necessary to buy a turbo kit based on Garrett T04E or Garrett GT30 turbo, with turbo manifold, block-off, intercooler, piping, with oil supply and oil drain, oil cooler, boost controller, westgate, downpipe and full 2.5″ exhaust. You also need to replace the pistons and connecting rods with forged pistons, compression ratio ~8.5, and buy a metal cylinder head gasket. Added to this are 440 cc injectors, Walbro 255 lph fuel pump, boost pressure sensor, oxygen sensor, oil pressure sensor, exhaust temperature sensor, and a Megasquirt ECU.
Building a turbo project like this on an M20B28 Stroker, the engine will make about 350-400 hp.

ENGINE RATING: 3+