ⓘ BMW M20. The BMW M20 is a SOHC straight-six petrol engine which was produced from 1977 to 1993. It was introduced eight years after the larger BMW M30 straight- ..

                                     

ⓘ BMW M20

The BMW M20 is a SOHC straight-six petrol engine which was produced from 1977 to 1993. It was introduced eight years after the larger BMW M30 straight-six engine, which remained in production alongside the M20.

The first cars to use the M20 were the E12 5 Series and the E21 3 Series. The initial M20 model had a displacement of 2.0 L 122 cu in, with later versions having displacements of up to 2.7 L 165 cu in.

The M20 began to be phased out following the introduction of the M50 engine in 1990. The final M20 engines were fitted to the E30 3 Series wagon estate and convertible model built in April 1993.

The M20 was the basis for the BMW M21 diesel engine. It is also loosely related to the BMW M70 V12 petrol engine.

                                     

1. History

By the 1970s, BMW felt the need for a six-cylinder engine smaller than the BMW M30, to use in the 3 Series and 5 Series. The resulting M20 had a displacement of 2.0 L 122 cu in, BMWs smallest straight-six engine of its day. Later versions had displacements up to 2.7 L 165 cu in and the M20 was used in the E12 5 Series, E21 3 Series, E28 5 Series, E30 3 Series and E34 5 Series.

Early versions of the M20 were sometimes referred to as the "M60", although the BMW M60 code has since been used for a V8 engine produced from 1992-1996.

                                     

2. Design

As per the M30, the M20 has an iron block, aluminium head and a SOHC valvetrain with 2 valves per cylinder. The major differences to the M30 are:

  • Slant angle of 20 degrees, compared with 30 degrees for the M30.
  • A timing belt rather than a timing chain
  • Bore spacing of 91 mm 3.6 in, rather than 100 mm 3.9 in
                                     

3.1. Models M20B20

The first models to use the M20 were the E12 520/6 and the E21 320/6, which used the 1.991 cc 121 cu in M20B20. This engine uses a bore of 80 mm 3.15 in and a stroke of 66 mm 2.60 in. The M20B20 initially used a Solex 4A1 four-barrel carburetor, had a compression ratio of 9.2:1 and a redline of 6.400 rpm.

The M20B20 first became fuel-injected in 1981, with Bosch K-Jetronic used. The compression ratio was raised to 9.9:1. In September 1982 coinciding with the release of the E30 3 Series, the fuel injection was updated to LE-Jetronic with a redline of 6.200 rpm. Other upgrades included a larger port known as "731" cylinder head, a lighter block and new manifolds. The M60 designation was finally dropped and replaced with M20.

In 1987, the M20B20 was again revised with the addition of Bosch Motronic engine management, a catalytic converter and a compression ratio of 8.8:1.

The M20B20 was not sold in North America.

Applications:

  • 1977–1982 E21 320/6 carburettor
  • 1982–1984 E30 320i L-Jetronic
  • 1987–1992 E30 320i Motronic
  • 1986–1987 E28 520i Motronic
  • 1981–1982 E28 520i K-Jetronic
  • 1976–1981 E12 520/6 carburettor
  • 1984–1987 E28 520i LE-Jetronic
  • 1984–1987 E30 320i LE-Jetronic
  • 1982–1984 E28 520i L-Jetronic
  • 1988–1990 E34 520i Motronic


                                     

3.2. Models M20B23

Six months after the M20 was launched with the carburetted M20B20, the larger, fuel-injected M20B23 was introduced for March 1978. The initial version of the M20B23 uses the same head known as "200" and block as the M20B20 but a longer 76.8 mm 3.02 in stroke crank. The bore is 80 mm 3.15 in. Fuel injection was K-Jetronic, the compression ratio is 9.5:1 and the redline is 6.400 rpm.

The 1982 version used LE-Jetronic, the 731 cylinder head, a compression ratio of 9.8:1 and the other upgrades as the per the M20B20; this resulted in a power output of 139 bhp 104 kW.

In 1984, the fuel-injection, exhaust and camshaft were upgraded to LE-Jetronic and power increased to 112 kW 150 bhp with a redline of 6.500 rpm.

The M20B23 was not sold in North America.

Applications:

  • 1977–1982 E21 323i K-Jetronic
  • 1982–1984 E30 323i L-Jetronic
  • 1984–1987 E30 323i LE-Jetronic
                                     

3.3. Models M20B25

In 1985, the M20B25 replaced the M20B23. The M20B25 has a capacity of 2.494 cc 152.2 cu in and initially produced 126 kW 171 PS; 169 bhp without a catalytic converter. It has an upgraded cylinder head known as "885", a bore of 84 mm 3.31 in, a stroke of 75 mm 2.95 in, a compression ratio of 9.4:1, a redline of 6.500 rpm and uses Bosch Motronic 1.1 engine management.

In 1987, a catalyzed model with Motronic 1.3 engine management was introduced. The compression ratio was reduced to 8.8:1 but thanks to the more sophisticated electronics power remained nearly as before, at 125 kW 170 PS; 168 bhp. The uncatalyzed engine was kept in production for Southern Europe and other markets where unleaded petrol was not regularly available.

Applications:

  • 1988–1991 Z1
  • 1989–1990 E34 525i
  • 1985–1993 E30 325i
                                     

3.4. Models M20B27

The M20B27 was designed for efficiency thus the e for the Greek letter eta in 325e and low-rev torque. This is an unusual design strategy for a BMW straight-six engine, which are usually designed for power at high RPM. Compared with the M20B25, the stroke is increased from 75 to 81 mm 2.95 to 3.19 in, resulting in a capacity of 2.693 cc 164.3 cu in. As per the M20B25, the bore is 84 mm 3.31 in. To reduce friction and improve efficiency, the M20B27 changes include using the 200 version of the head which has smaller ports, a different camshaft, four camshaft journals and softer valve springs. Due to these changes the rev limit on the M20B27 was reduced to 4.800 rpm. A compression ratio of 9.0:1 was used on the US 528e, and 11:1 on Euro 525e and 325e cars - this was reduced to 10.3:1 in 1985. The initial M20B27 engines produces 92 kW 123 bhp and 240 N⋅m 177 lb⋅ft at 3.250 rpm for models without a catalytic converter. Models with a catalytic converter produce 90 kW 121 bhp and 230 N⋅m 170 lb⋅ft.

Since many markets tax cars based on engine displacement, the etas larger displacement meant that it was not suitable for all markets. It was expressly developed with the American market in mind. BMWs corporate average fuel economy was at risk of not meeting requirements by 1984, primarily due to higher sales of their bigger, more expensive cars in the early 1980s. The first car to use the M20B27 was the US market 528e in 1982.

In late 1987, the fuel injection was upgraded to Motronic 1.3 on the US market plastic bumper 325e Super Eta, the cylinder head changed to the "885" version, the compression ratio was reduced to 8.5:1 and the redline increased to 5.200 rpm. Power output increased to 95 kW 127 bhp at 4.800 rpm.

In the early 1990s BMW South Africa used components from the Alpina C3 2.7 to produce an E30 specifically for Stannic Group N production car racing. The first iteration of this engine used in the E30 325iS produced 145 kW 194 bhp and the second revision, often referred to as "Evo2" or on the VIN plate as "HP2" produced 155 kW 208 bhp.

Applications:

  • 1989–1992 E30 325iS only available in South Africa
  • 1982–1988 E28 525e called 528e in North America
  • 1982–1987 E30 325e, 325e


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