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Old 07-11-2012, 07:29 AM
Turbo_Bimmer Turbo_Bimmer is offline
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My 2¢; Personnally, I bought a 4.8is for the added power over a 4.4i. I'm not ready to loose power just for sound.

It has been explained here on this forum and I also I read on other performance forums how a X-pipe works.
It benefits of an exhaust pulse from the left bank croosing the X-pipe and 'pulling' ou sucking on the right bank to help the upcoming right pulse etc. As you can see, the timing of pulses crossing the X-pipe is very critical to have that suction effect. To achieve that 'good timing', the exhaust tubes length need to be the same lenght on both banks. Since our X5s do have a longer runner of tube on the left side, that pulse synchronisation is losted.
On our X5s, depending where the X-pipe is installed (more forward or rearward) the rpm at which the 'sucking' effect will happend will differ but won't be the whole rpm range.
Having pulses crossing the X-pipe from the left and right side at the same time will create a 'collision' or disrupted flow.

Regarding the H-pipe, there is just a minimal flow crossing at 90° in the middle section and this is the way the sound of the V8 is equalized. The H-pipe is not designed for a sucking effect like a X-pipe, so it rely on the middle section to tune the sound, not increase performance.

I know this is all theory, but exhaust design is a complex thing.
I am really surprised to hear many owners talking about the nice sound of an X-pipe and never one of them mentionned some lost of power. (exept one that switched to an H-pipe)
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