There are always problems that have to be worked out with an engine swap. Exhaust location, electrical connections, computer related issues, etc. There are likely some components used in the drive line or suspension that are not sufficient to handle the extra weight and torque.

I agree with crystalworks. If you want a V8 buy one. That said, it is better to stay with the devil you know than devil you don't know. If you want to keep what you have and want it to be faster or sound better there are many proven ways you can go to do that will still be reliable and a painless install.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas