As Ibanez says most CX 744's have RB211's while the ex-SIA aircraft have Pratt and Witneys.
The ex-SAA 744's are RB211 engined and will be in service when CX has finished having them converted to Freighters (I believe).
Currently the BCF's are B-HOU which is RB211 engined and B-HKH which is Pratt and Witney engined.