In this series, we'd like to honor the engines that were made in vast quantities and/or remained in front-line service for decades (in addition to low-production Ass-Kickin' Engines), and the BMW "Little Six" definitely qualifies.
BMW's M30 "Big Six" went into plenty of 5-, 6-, and 7-series BMWs (not to mention the E9), starting in 1968, and did a great job of setting the standard for serious inline-six performance. However, the M20- which debuted in the E12 in 1977- made no apologies for its smaller displacement (though the biggest M20 displaced 2.7 liters versus the smallest M30's 2.5) and enabled legions of dentists, realtors, and small-time cocaine dealers to experience genuine Bavarian-style horsepower at the helms of their E21s, E28s, and E30s. From what we've seen at the 24 Hours Of LeMons, even a 300,000-mile M20 can take days of horrific abuse before giving up (in stark contrast to, say, the more fragile Honda B and D engines). Sadly, Cash For Clunkers no doubt sent many thousands of M20s to BMW Engine Heaven.