Eta is the seventh letter of the Greek alphabet. Eta is commonly used in engineering to denote "efficiency," as the efficiency of transmission of power generated in an engine to motive force at the drive shaft. In short, eta is the measure of efficiency. Departing from the 4-cylinder tradition established in the New Class and E21 model lines, the E28 5 Series and the E30 3 Series were primarily 6-cylinder cars. The decade of the 80's was a time for efficiency, especially fuel efficiency. In the 1980's BMW produced an efficient engine it called 'eta'. You can recognize these cars as the BMW 3 Series 325e and BMW 5 Series 528e models. These models shared engines, the 2.7-liter single overhead camshaft 6-cylinder M20 eta engine.