S52 = the US spec motor (3.2 liter I6) in the 98-00 M Roadster.
S54 = the 3.2 M motor in the 01-02 M Roadster.
M52 = the 2.8 motor in the Z3 2.8 from 97-00.
M54 = the 3.0 motos in the Z3 3.0 from 01-02.
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