Basically the smaller engines have the single tip exhaust like the 30i for excample the larger engines with twin turbos have the double-sided exhaust tips like the 35i or 35is.
Complete details will you find here --->
Click to open the site
Or if you download the Z4 Brochure on this side --->
CLICK here to download
and here for the latest 2009 up for the e89 ---->
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See Exhaust in table below