The S54 is the high performance equivalent to the M54, which was used in the E46 M3, Z3 M Coupé/Roadster, and the E85 Z4 M Roadster/E86 M Coupe.
The S54 is the high performance equivalent to the M54. The M54 was used in the E46 M3. The M54 was used in Z3 M Coupé/Roadster. The M54 was used in the E85 Z4 M Roadster/E86 M Coupe.