very very different....M635CSi uses S38 like M1 or M5 (280+bhp) while the 635CSi uses the m30, around 218bhp. i think M6 is what they call it in the US, uses S38 but slightly detuned at 250+ bhp
like i told u before there is one in S'wak but he aint selling it its his baby! apparently he spends more time with his M635csi rather than his own wife..