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 BMW models to use the M20 were the E12 5 Series and the E21 3 Series. The initial M20 model had a displacement of 2.0L (122cuin), with later versions having displacements of up to 2.7L (165cuin).
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 touring 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.
History
By the 1970s, BMW saw a need for a new I6 engine to bridge the displacement gap between their smaller M10 I4 and the M30 I6 engines.:92 This new engine would solve the following business- and engineering-related issues:
The M30, despite being a turbine-smooth inline-6, was too large and too heavy to fit in the 3 Series engine bay.
The M10, despite being a small and light inline-4, became too buzzy and unrefined at larger displacements; 2.0 L is typically an upper limit for inline-4 engines in terms of maintaining refinement.
A 2.0 L variant of this new I6 would offer European buyers refinement and the turbine-like smoothness of an I6 while bypassing European displacement taxes.
The new engine could be evolved to accommodate the tightening emissions standards of the 1970s and 1980s.
BMW debuted the M20 engine at the 1977 IAA in two forms: a 2.0 L variant with Solex 4A1 carburetor making 90 kW and a 2.3 L variant with Bosch K-JetronicMPFI making 105 kW; both variants had a 80 mm bore. In 1982 BMW increased the bore to 84mm, ultimately leading to 2.5 L and 2.7 L variants. 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 M60 designation has since been re-used for a V8 engine produced from 1992 to 1996.
Design
The M20 shares the following design characteristics with the M10 and M30:
The engine block is a closed deck design that is made out of cast iron and features siamesed cylinder bores
The cylinder head is a cross-flow design made out of cast aluminum alloy
The valvetrain layout is SOHC with 2 valves per cylinder. Rocker arms with manually adjustable eccentric pushers actuate the valves
The M20 differs from the M10 and M30 in the following ways:
The camshaft is driven using timing belt instead of a timing chain to reduce noise and production costs
The bore spacing is reduced from 100mm (3.9in) to 91mm (3.6in) to reduce engine length and weight
The slant angle is reduced from 30 degrees to 20 degrees when mounting the engine in the engine bay, resulting in a more upright engine
M20 engine - exhaust side
M20 engine - intake side
M20 engine - top view of block and exhaust downpipe
Throughout the M20's production run, three different cylinder head casting variants were used:
1264200 (200): this variant was used for pre-1983 M20B20 and M20B23 engines, as well as in the pre-1988 M20B27 engine. This casting features very small intake ports with 40mm intake and 34 mm exhaust valves.
1277731 (731): this variant was used for post-1982 M20B20 and M20B23 engines. It keeps the same valve diameters as the 200 casting but enlarges the intake ports.
1705885 (885): this variant was used for the M20B25 and the 1988 M20B27 engines. This casting features the largest intake ports. The intake and exhaust valve diameters are both enlarged by 2 mm each over the 200 and 731 castings.
For markets that did not require a catalytic converter
Early high compression variant
95kW (129PS; 127hp) at 4,250 rpm
240N⋅m (177lb⋅ft) at 3,250 rpm
10.2:1
1985-1987
For markets that did not require a catalytic converter
Compression ratio reduced to mitigate knocking
95kW (129PS; 127hp) at 4,800 rpm
240N⋅m (177lb⋅ft) at 3,200 rpm
8.5:1
1988
"Super" Eta engine
Motronic 1.1
M20B20
Early M20 engine with K-Jetronic
The first models to use the M20 were the E12 520/6 and the E21 320/6, which used a 1,991cc (121cuin) version known as the M20B20VE or M60/2. This engine uses a bore of 80mm (3.15in) and a stroke of 66mm (2.60in). A Solex 4A1 four-barrel carburetor was used in the M20B20VE ("VE" is for vergaser- "carburettor" in German), and it has a compression ratio of 9.2:1 and a redline of 6,400rpm.
The M20 first became fuel-injected in 1981, with Bosch K-Jetronic used in a 92kW (123hp) version called the M20B20KE. The compression ratio was raised to 9.9:1.
In September 1982 (coinciding with the release of the E30 3Series), the fuel injection was updated to LE-Jetronic with a redline of 6,200rpm. Other upgrades included a larger port (known as "731") cylinder head, a lighter block and new manifolds.[citation needed] The "M60" designation was dropped and this version was known as the M20B20LE.
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 the United States, but was available in the E30 (320i) in Canada.
In March 1978 (six months after the M20 was launched), a fuel-injected and larger displacement version known as the M20B23KE (or M60/5) was introduced. This version uses the same head (known as "200") and block as the 2.0litre version but a longer 76.8mm (3.02in) stroke crank. The bore is 80mm (3.15in) and it has a capacity of 2,316cc (2.3L). Fuel injection was K-Jetronic, the compression ratio is 9.5:1, the power output is 105kW (143PS; 141hp) and the redline is 6,400rpm.
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 2.0litre version. This version is called the M20B23LE and has a power output of 102kW (137hp).
In September 1983, the M20B23LE's fuel-injection, exhaust and camshaft were upgraded and power increased to 110kW (148hp) with a redline of 6,500rpm. The 102kW (137hp) version continued to be available in certain markets with strict emissions regulations (such as Switzerland) until replaced by the 325i.
The M20B23 versions were not sold in North America.
In 1985, the M20B25 replaced the M20B23. The M20B25 has a capacity of 2,494cc (152.2cuin) and initially produced 126kW (171PS; 169hp) (without a catalytic converter). It has an upgraded cylinder head (known as "885"), a bore of 84mm (3.31in), a stroke of 75mm (2.95in), a compression ratio of 9.4:1, a redline of 6,500rpm 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 125kW (170PS; 168hp). The uncatalyzed engine was kept in production for Southern Europe and other markets where unleaded petrol was not regularly available.
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 81mm (2.95 to 3.19in), resulting in a capacity of 2,693cc (164.3cuin). Since many markets tax cars based on engine displacement, the eta's larger displacement meant that it was not suitable for all markets. It was expressly developed with the American market in mind. As per the M20B25, the bore is 84mm (3.31in). 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 4,800rpm. The initial version, called the M20B27ME, produces 92kW (123hp) and 240N⋅m (177lb⋅ft) at 3,250rpm for models without a catalytic converter. Models with a catalytic converter produce 90kW (121hp) and 230N⋅m (170lb⋅ft).
In the United States, BMW's 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. The compression ratio of the U.S. M20B27ME version was 9.0:1, compared with for cars sold elsewhere 11.0:1.
In 1985, the M20B27ME.E version was introduced, increasing power output to 95kW (127hp) despite a lower compression ratio of 10.3:1.
For the 1988 model year, the digital motor electronics was upgraded to Motronic 1.1 on the US market plastic bumper 325e and 528e 'Super Eta', the cylinder head changed to the "885" version, the compression ratio was reduced to 8.5:1 and the redline increased to 5,200rpm. Power output increased to 95kW (127hp) at 4,800rpm.
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 145kW (194hp) and the second revision, often referred to as "Evo2" or on the VIN plate as "HP2" produced 155kW (208hp).
↑Hans-Rüdiger Etzold: So wird’s gemacht (volume 68): BMW 5er Reihe 9/72 bis 7/81 (TYP E12), BMW 5er Reihe 7/81 bis 8/87 (TYP E28), Delius Klasing, Bielefeld, 1990, ISBN 978-3-7688-0666-4
↑Heylen, Vic (1983-04-28). "BMW: Met de ETA op de zuinige tor" [BMW: With Eta on an economy run]. De AutoGids (in Dutch). Vol.4, no.94. Brussels, Belgium: Uitgeverij Auto-Magazine. p.130.
M20 engine - exhaust side
M20 engine - intake side
M20 engine - top view of block and exhaust downpipe
