On the 35i X5, only the left side exhaust is open up to higher rpms. Your dealer said 4K rpms--I agree with them. Don't know about the 35d or the 50i.

That way you only have to keep one side clean unless you stand on it a lot.

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DeanLinAZ
2011 X5 35i, Sport Activity
Black Sapphire/Oyster
M Sport, Tech & Conven packs
Premium sound, Side camera opt