Using the N55 mid-section on the N54 135i models gives a sweet audio upgrade. The N55 exhaust does not have catalytic converters in the mid-section - two less pieces in the exhaust to dampen the sound. There may be a slight power and throttle improvement with the N55 midpipe but the real reason for this upgrade is to get a little more volume from the exhaust. This midpipe has a resonator.
The N55 midpipe is a direct replacement for the N54 midpipe on 2008-2010 135i as well as 2011 1M models. It uses the factory gaskets and hardware.
Notice of Emissions Non-Compliance. The N54 has catalytic converters in the original midpipe. Even though using the N55 midpipe will not cause a check engine light or fault code, removing the stock cats may be a violation of emissions laws. Install at your own risk. Using it is prohibited in any motor vehicle that is subject to emissions inspection or control. It is not legal for sale or use in California on any pollution-controlled motor vehicle.
This is the Genuine BMW N55 midpipe so it can also be used as a replacement piece on 2011-2013 135i N55 cars.