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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 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 (90 kW) at 4250 rpm for all models produced through model year 1987 and 127 hp (95 kW) at 4800 rpm for the final year of production in 1988. Peak torque is 170 ft·lbf (230 N·m) at about 3250 rpm for all years of the eta. BMW built this engine with fuel economy and low-rev drivability as the top priority. In production it was coupled to a 2.93:1 differential for the E28 5 series and E30 3 series models starting model year 1986. The earlier 325e used a 2.79:1 ratio. 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 sparkling performance typically associated with the marque.
All eta engines used Bosch Motronic engine management systems that were calibrated for maximum fuel economy. The result were very conservative spark advance and fuel delivery curves. In addition, the Motronic had a built-in rev limiter that engaged at about 4750 rpm. While this was hardly sporting, due to the very low overlap and short duration of the cam and the long, narrow intake tubing, the motor would not have made much power over 5,000 rpm anyway. The low redline was an added safety margin to compensate for the four bearing cam and soft valve springs.
In cars produced from September 1987, the eta engine got a 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; a special eta-tuned 7-bearing cam; and a 5300 rpm rev limit. A new 6000 rpm tachometer was also used.
Applications:
1982–1987 E30 325/325e/325es
