The first generation 3-series, known as the E21, was released after the 1973 oil crisis, where in 3 years, it helped almost double BMW’s worldwide sales.
BMW 325i engines were not available until the 2nd generation, the E30. This engine, known as the M20B25, was made available in 1985 in Europe and reached North America in 1987. It was the engine on the 325i, 325is and 325ix, which was a 4WD variant.
BMW 325i engines of the M20 design had an 84mm bore and a 75mm stroke. It was also equipped with the Bosch Motronic 1.1/1.3 adaptive fuel injection and was compliant with the Euro emissions regulations of that period. With a rev limit of 6500 rpm, rated output for this engine was 168 horsepower.
The E36 generation 3-series, available from 1991 to 1998, featured a new generation of 6-cylinder engines known as the M50. The M50B25 used in the 325i models had the same 84 X 75 mm bore and stroke as the previous generation engines but this time produced 192 horsepower at 5900 rpm. From 1993, the M50TUB25 was introduced, which is a technologically upgraded (hence the TU) M50B25. The M50TUB25 included BMW’s VANOS variable cam timing system for the intake cam. While horsepower remained the same, the VANOS technology increased torque at the lower and medium rpm band.
In 1996, BMW replaced 325i car engines with the M52B28. This created the 328i line. An M52B25 engine variant was also introduced, which was a 2.5 liter badged as a 323i. This badge change confused a lot of people. The M52B25 had the same 84 X 75 mm bore and stroke combination, with the 2.5 liter engines being assigned a 10.5 compression ratio.
The fourth generation 3-series, the E46 was available from 1999-2004. While the 2.8 liter cars were called the 328, the 2.5 liter cars retained the confusing 323 name. The M54-series engines appeared in 2001 and at this point, BMW renamed the 2.5 liter-equipped car the 325, removing the confusing badging. The US-only BMW 325i engine of this period was the 2.5 liter M54B25, used from 200,1 which had 184 hp on tap and Double VANOS variable valve timing, which adjusted the timing of both intake and exhaust valves. As with the M52, the M54 had an 84 X 75 mm bore/stroke combination and 10.5 compression.
The present E90/91/92/93 cars are a complete reengineering of the E46, changing engines, transmission, the passenger compartment and suspension.
The E9x cars are powered by the N52-series engines, which features a crankcase made from magnesium and aluminum. This is the first time such a combination has been used for production cars, making the crankcase one of the lightest in the world in its engine class. It also features the 2nd generation Valvetronic and Double-VANOS systems, which resulted in a 12% increase in fuel efficiency over the previous M54 engines.
The BMW 325i engine for this generation has an output of 215 horsepower and a torque rating of 185 foot-pounds.