Infobox Automobile
name=BMW M20 engine
successor=BMW M50
engine=2.0 L (1990 cc/121 in³)
2.3 L (2316 cc/141 in³)
2.5 L (2494 cc/152 in³)
2.7 L (2693 cc/164 in³)
similar=Mercedes-Benz M110
The M20 is an straight-6 piston engine of BMW.

The 12-valve, belt driven SOHC design was introduced in the 1977 BMW 520/6 and 320/6. With displacements ranging from 2.0 to 2.7 liters, it was the "little brother" to the larger BMW M30 engine. It was intended to replace the larger displacement 4-cylinder motors and was born out of BMW's conviction that a small six had more development potential than a large four (i.e. 2 liters+)

Powering the E21 and E30 3-Series, as well as E12, E28 and E34 5 Series cars, it was produced for nearly two decades, with the last examples powering the E30 "325i touring" built until sometime in 1993. By that time, the newer twin-cam M50 engines with 4 valves per cylinder had already been used in the E36 and E34 for a couple of years.

Three different head castings were used over the engine's production run. The earliest was #1264200 aka the "200". These were used in all e21 320/6 and 323i and e12 520/6 engines and later in the e28 and e30 eta engines (eta = "Power With Economy"). The next version was #1277731 aka the "731". This head was the same as the 200 but featured larger intake ports. The final version was #1705885 or "885" introduced in the 325i. Ports were further enlarged, valves were larger and the combustion chamber was redesigned to improve flow and thermodynamic efficiency.

As the BMW M21, it became a diesel engine that was also available with a turbocharger.


The M20B20 is a 2.0 L M20 engine. Entering production in March 1977 in the E12 520 and later in the E21 320/6, the engine had a bore of 80mm and a stroke of 66mm. Fueling was handled by a Solex four-barrel carburettor. These early versions all used the "200" head. When the E30 debuted in 1982, the 2.0 was fitted with Bosch L-Jetronic fuel injection and the revised "731" cylinder head featuring larger intake ports. In 1987, the 2.0 was again revised with the addition of Bosch Motronic engine management and a catalytic converter. Later models also received larger valves. The 2.0 has the distinction of having the longest production run in the M20 series with the last ones rolling off the line in early 1993.

* 1979–1983 E21 320/6
* 1982–1984 E28 520
* 1982–1988 E28 520i
* 1983–1992 E30 320i
* 1988–1990 E34 520i


This motor was only used in 3-series models. The 323i began as an E21 model in 1977 and was phased out as the E30 models came into production. It initially used the same head and block as the carbureted 2.0 liter but used a longer 76.8mm stroke crank. It also featured Bosch K-Jetronic Injection. So equipped the E21 323i produced 143 hp. The E30 version featured the revised "731" head with larger intake ports and used L-Jetronic instead of K-Jet. Early E30 323s made 139 hp but later versions got updated cams and power rose to 150 hp. The 2.3 liter motor was superseded by the more powerful 2.5 in 1985.

* 1977–1982 E21 323i
* 1982–1985 E30 323i


The German version of the M20B25 produced 170 hp with a catalytic converter and 171 without. It featured "885" head with larger ports and valves, 84 mm (3.3 in) bore, and 75 mm (3 in) stroke. Most significantly it had redesigned pistons and combustion chambers for better power, more resistance to detonation and higher thermodynamic efficiency. It was equipped with Bosch Motronic 1.1/1.3 Adaptive fuel injection. With a minor modification that adjusts the idle speed of a cold engine, the 325i fulfills "Euro 2" emission requirements in Germany, resulting in a lower tax compared to "Euro 1" (as of 2005).

* 1986–1994 E30 3 Series
* 1989–1990 E34 5 Series
* 1988–1991 E30 Z1


The 2.7 L M20B27 was designed for efficiency (thus the "e" for the Greek letter "eta" in "325e"). It had an 84 mm (3.3 in) bore and a longer 81 mm (3.2 in) stroke for a total displacement of 2693 cc. The eta engine used the same "200" head casting as the carbureted 2.0 liter and K-jetronic 2.3 as featured in the E21 but only utilizes four camshaft bearings for reduced internal friction. The heads still have seven journals cast into them though and can be drilled to oil a seven bearing cam.

Output was 121 hp at 4250 RPM for all models produced through model year 1987 and 127 hp at 4800 RPM for the final year of production in 1988. Peak torque is 170 ft·lbf at about 3250 RPM for all years of the eta. This was the first engine BMW built with fuel economy as the top priority. In production it was coupled to either a 2.93:1 differential for the E28 5 series and most E30 3 series models or a 2.79:1 for early E30 3 series cars with manual transmissions. The tall final drive gearing was required to keep the engine in its best operating range at various road speeds. These features did achieve exceptional efficiency, but at the expense of the performance typically associated with the marque.

All eta engines used Bosch Motronic engine management systems that were calibrated for maximum fuel economy. The result was a very conservative spark advance curve and fuel delivery curve. In addition, the Motronic had a built-in rev limiter that engaged at about 4750 rpm. This very low redline was the result of the four bearing cam, soft valve springs, intake manifold and camshaft design. Due to the very low overlap and short duration of the cam and the intake manifold tuning, the motor does not make any power over about 5,000 RPM.

Starting in cars produced from September 1987 for the 1988 model year, the eta cars got a more substantial refresh from the factory. This included the more recent Bosch Motronic version 1.1/1.3 with adaptive idle control and revised circuitry, the "885" head casting with the larger valves, ports and revised combustion chambers, new pistons to fit the 2.7 L stroke with the 2.5 L head, a special one year only intake manifold with even narrower runners that opened up at the flanges to match the larger ports of the new head, a dual exhaust system and a 5300 RPM rev limit. A new 6000rpm tachometer was also used. Though it only made slightly more power in stock form, it could be easily boosted by installing a 325i cam and springs, a complete 325i intake manifold and throttle and plugging in the 325i engine control unit. Depending on which cam, chip and intake is used, the combination can make over 180 horsepower with the stock compression ratio.

In aftermarket modifications that are inspired by custom versions of Alpina and AC Schnitzer, the long-stroke eta engine block is often combined with a 325i head to a so-called "327i" that combines the high low-rev torque of the eta with the top-range power of the 325i.

* 1982–1987 E30 325/325e/325es
* 1982–1987 E28 525e (or 528e in the US)


ee also

External links

* [ BMW E30 Timing Belt replacement procedure] Most important maintenance item on BMW M20 motor

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