X pipe should be same diameter as incoming and exiting pipe. The less the X angle the greater the scavenging but exhaust will seek the path of least resistance so angle and opening at the center of the X are less important than what fits best in the space you have to work with. Scavenging can contribute to performance but don't expect any improvement. Consider the investment for a better sound only. Try not to reinvent the wheel. Find someone that has made exhaust changes for a better sound on a similar model to yours and go that route. Will likely save you time and money.

Always best to weld the pipes to be sure you won't have exhaust leaks down the road. In your case I would initially use clamps so that if you don't like the sound or you end up with a drone you can more easily make corrections. Note that the accuracy of the sound of any recording is based on the quality of the recording equipment and the speakers when playing the recording.

Another option you might consider is cutouts. That way you can control the sound based on how far the flaps are open. Going that route will increase the performance and you can have a stock sound still available.

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