BMW M20B20 Engine

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

M20B20 Specs

BMW M20B20 engine reliability, problems and repair

6-cylinder straight BMW M20B20 engine was presented in 1977. It was introduced in order to replace its predecessor straight 4-cylinder engine and was the first motor in M20 series. The main difference between M20B20 and M20B25 as well as M20B23 and M20B27 is in engine capacity.  For example, M20B25 is 2.5 L motor while M20B23 is 2.3-liter engine and M20B27 is 2.7-liter motor respectively.

The engine is based on a block of cast iron cylinders with aluminum 12-valve cylinder head and one camshaft (SOHC 12V). Diameter of inlet valve is 40 mm while exhaust valve is 34 mm in diameter. At first engines used head which was known as 200. Later it was replaced by 731. It was different from its predecessor due to expanded diameter of inlet channels. 

Valves adjustment is necessary every 10,000 miles (15,000 km) of mileage. Valve clearance is as follows: inlet – 0.25 mm (.01″), exhaust – 0.33 (.013″) mm. The length of M20B20 connecting rod is 130 mm.

The timing belt requires replacement every 30,000 miles (50,000 km) of mileage. If the belt is broken, the valves are bended by the engine.

Carburetors were used as the main power supply system. In 1981 they were replaced by mechanical and electronic injection.

M20B20 was used in BMW autos with 20i index.

In 1990 new M50B20 was introduced in order to replace 2-liter M20 motor.

BMW M20B20 engine modifications

1. M20B20 (1977 – 1981) is the first version of engine with carburetor fuel supply. The specifications are as follows: compression ratio – 9.2, power – 120 HP at 6,000 rpm, 160 Nm of torque at 4,000 rpm.

2. M20B20 (1981 – 1987) is a version with L-Jetronic electronic multi-point fuel injection. The specifications are as follows: compression ratio – 9.8, power – 125 HP at 5,800 rpm, 170 Nm of torque at 4,000 rpm.

3. M20B20 (1982 – 1984) version comes with K-Jetronic mechanical multi-point fuel injection. The specifications are as follows: compression ratio – 9.9, power – 126 HP at 5,800 rpm, 170 Nm of torque at 4,000 rpm.

4. M20B20 (1985 – 1991) motor version has LE-Jetronic electronic direct injection system. The specifications are as follows: compression ratio – 9.8, power – 129 HP at 6,000 rpm, 174 Nm of torque at 4,000 rpm.

5. M20B20 (1987 – 1992) engine version comes with Motronic 1.3 electronic direct injection system. The specifications are as follows: compression ratio – 8.8, power – 129 HP at 5,800 rpm, 174 Nm of torque at 4,300 rpm.

BMW M20B20 engine problems and drawbacks

As well as its 2.5-liter brother, M20B20 can also have several drawbacks, malfunctions and minuses. HERE you will have a chance to learn about all of them. At the same time this motor has proved to be rather weak when it comes to BMW 5-Series. This is why M50B25 will be the best bet for your E34.

BMW M20B20 engine tuning

M20B20 Stroker

M20 series have been well-known for many years already. This is why there appeared lots of ways which make it possible to increase its capacity. The most common way is to use stroker due to the fact that the height of cylinder block is the same. If you want to upgrade your M20B20 and turn it into M20B23, all you need is to buy crankshaft, pistons, connecting rods and flywheel which are used in M20B23. You will have a chance to build engine with at least 150 HP.

At the same time you can also upgrade your M20B20 into M20B25. We decided to go even further and increase capacity from 2.0 liters to 2.8 L. First of all we need to bore cylinders up to 84 mm (3.3 in). We also need to buy M52B28 crankshaft, M20B25 Kat pistons as well as M20B20 Carb flywheel. Keep in mind that you should also replace the radiator as the old one will not be able to handle with such capacity.

If you want to gain maximum power, follow instructions which are given HERE: cams, porting, inlet, exhaust…

At the same time you need to have a clear understanding that after you boring cylinders up to 84mm, the walls of cylinders will be rather thin. This will reduce the lifespan of your engine. You should not expect your motor operate after 6,000 – 13,000 miles (10,000 – 20,000 kilometers) of mileage.

M20B20 Turbo

Despite the fact that the idea to build M20 turbo using rather weak engine may seem not to be so brilliant, it still has a right to exist. You will need additional turbo kit that is based on Garrett T04E or Garrett GT30 turbocharger for implementation of this project. It should come with blow-off, intercooler, turbo manifold, piping, oil return line, oil radiator, oil feed line, boost controller, fuel pressure regulator, downpipe and 2.5” full exhaust.

At the same time you also need to replace pistons and connecting rods with forged ones which have at least 8.5 compression ratios. Don’t forget to buy metal cylinder head gasket and injectors with 440 cc capacity, Walbro 255 lph fuel pump, boost pressure sensor, oxygen sensor, oil pressure, exhaust gas temperature sensor, ECU Megasquirt.

After implementation of this turbo project on base of M20B28 Stroker you will be able to increase engine capacity up to 350-400 HP.

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