BMW M20

The M20 was an I6 piston engine of BMW.

The 12-valve, belt driven SOHC design was introduced in the 1976 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.

Powering the E21 and E30 3-Series, as well as E12, E28 and E34 5-Series cars, it was produced in 4 decades, with the last models of the E30 325i touring built in 1992. At that time, the newer M50 engines with 4 valves and DOHC were already used in the E36 and E34 for some years.

As BMW M21, it became a Diesel, also available with a turbocharger.

Models

Engine	Displacement	Power	Torque	Redline	Year
M20B20	2.0 L (1990 cc/121 in³)	92 kW (123 hp) @ 5800	165 N·m (121 ft·lbf) @ 4000		1981
		92 kW (123 hp) @ 5800	170 N·m (125 ft·lbf) @ 4000		1981
		95 kW (127 hp) @ 6000	174 N·m (128 ft·lbf) @ 4000	6200	1985
		95 kW (127 hp) @ 6000	164 N·m (120 ft·lbf) @ 4300	6200	1986
M20B23	2.3 L (2316 cc/141 in³)	102 kW (136 hp) @ 5300	205 N·m (151 ft·lbf) @ 4000		1982
		110 kW (147 hp) @ 6000	205 N·m (151 ft·lbf) @ 4000		1983
M20B25	2.5 L (2494 cc/152 in³)	126 kW (168 hp) @ 5800	226 N·m (166 ft·lbf) @ 4000	6200	1985
		120 kW (160 hp) @ 5800	215 N·m (158 ft·lbf) @ 4000	6200	1985
		125 kW (167 hp) @ 5800	222 N·m (163 ft·lbf) @ 4300	6200	1987
M20B27	2.7 L (2693 cc/164 in³)	92 kW (123 hp) @ 4250	240 N·m (177 ft·lbf) @ 3250		1983
		95 kW (127 hp) @ 4250	240 N·m (177 ft·lbf) @ 3250		1986
		90 kW (120 hp) @ 4250	230 N·m (169 ft·lbf) @ 3250		1985
		95 kW (127 hp) @ 4800	230 N·m (169 ft·lbf) @ 3200		1986

M20B20

The small 2 Liter engine, the entry-level version to the six cylinder ranges, was very smooth running, but lacked torque and required higher revs, resulting in higher fuel consumption. It was never imported to North America due to federal emissions regulations. It used the same cylinder head casting as the 2.3 liter m20 in the e12 and e21 2.3 liter motors and the pre 1987 2.7 liter eta motors. All of the 2 liter versions were carbureted until the e30 320i, which used Bosch Motronic version 1.1/1.3 to work with a catalytic convertor.

Applications:
** 1977–1981 E12 5-series
** 1977–1982 E21 3-Series
** 1982–1987 E28 5-Series
** 1983–1993 E30 3-Series
** 1988–1990 E34 5-series

M20B23

The 323i began as an E21 model and continued with better fuel injection and several internal revisions in the e30 323i. The e21 used the same cylinder head casting as the pre-1987 eta cars, only drilled to oil a seven bearing cam. The e30 323 used a specialized head casting with much larger ports. Though it had the same combustion chambers and valves as the e21 and eta heads, it had much more potential for high rpm power. The e30 323 head can be bolted on to an eta 2.7 liter block in conjunction with other modifications to the intake, exhaust and fuel injection systems to make very good power.Applications:
** 1977–1982 E21 323i
** 1983–1985 E30 323i

M20B25

The German version of the M20B25 produced 169 HP with a catalytic converter, and 171 without. It featured a 12-valve SOHC head with larger 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).

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

M20B27

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 with 81 mm (3.2 in) a longer stroke, for a total displacement of 2693 cc. The eta had only four effective camshaft bearings for reduced internal friction, with all seven cast into the head but only four corresponding journals on the cam. The heads were drilled to oil accordingly.

Output was between only 121 hp (up to 1987) and 127 hp (after 1987) but fuel efficiency was fairly good for the period and for that size engine. The low-revving high torque design was resonably well received, but the 325e designation caused it to be compared to the later 325i which had more power and a more exciting sound with its higher range of revs.

It is interesting to note that the eta and the b25 engine, despite a 47 hp disparity in power output, produce a similar amount of torque at about 170 ft·lbf at medium revs. In theory, this should have resulted in similar acceleration in top gear, but due to differences in gearing these were different. The 325/325e(s) used 2.79 and 2.93 rear axle ratios depending on the model year. The 325i sedan used a 3.64 ratio, and 3.91 in the touring and late ix variant. This equates to lower torque at the wheels on eta cars and results in substantially milder acceleration.

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 around 4700 rpm. This very low redline was the result of the four bearing cam, soft valve springs, intake manifold design and camshaft design. Considering the very low overlap and short duration the motor didnt make any real power up high anyway.

For the 1988 model year, the eta cars got a warming over from the factory. This entailed the previously mentioned Bosch Motronic Fuel Injection, a 325i head with the larger 325i valves, ports and revised combustion chambers, special pistons to fit the 2.7 liter stroke with the 2.5 liter head, a dual pipe exhaust system and a 5300 RPM rev limit. Though it only made slightly more power in stock form, it could be easily boosted by bolting on a 325i head with a more aggressive performance camshaft, a complete 325i intake manifold and throttle and plugging in the 325i engine control unit with a performance chip. Depending on which cam and chip is used, that motor can make over 200 horsepower.

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.

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

See also