one of mate has a 520i...and he wants to put in a 525i or even 530i engine in it?....is it possible to do so?

but it would be very dangerous isnt it...? i told him not to do it as i think it will screw up the car very soon n lost its reliability...?
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It can be done. Not sure why you think an m54 is unreliable. The best way to do it is from a donor e60. Scavaging from an e46 could call for some headaches. But it would be cheaper and easier for him to sell his car and get a 530i.

 

nono,i didnt say the engine itself is unreliable. its combining the 520i with the bigger engine might make it unreliable? but wouldnt it cause any problem at all? like needs reprogramming etc? how does it affect insurance away? same as getting a 530i?