The M50 and M52 are both 24-valve DOHC straight-sixes, and the differences are easy to keep straight:
- VANOS: the early M50 (1990-92) has none; the M50TU (1993 on) added single intake VANOS; the M52 also runs single VANOS
- Block: the M50 is always cast iron; the M52 is aluminium/Nikasil except the US/Canada M52, which is iron
- Displacement: the M50 tops out at 2.5 (M50B25, the 325i); the M52 adds the 2.8 (M52B28, 328i) alongside a 2.5 (M52B25, 323i)
- Management: the M52 brought more modern management and, in the US from 1996, OBD-II
Tuners still prize the earlier M50 head for its flow — which is where the popular M50 manifold swap onto M52 cars comes from.