Originally Posted by 1911A145
The stock exhaust is not too restrictive. Infact, if you remove the rear cat converters for those who and an N54, and replace the downpipes with catless ones, the stock exhaust is pretty good as is. Where other turbo cars may have a single 3 inch exhaust, the BMW has a double 2.-- midpipe that y-pipes into a 3 inch muffler. The catback can be upgraded, and also converted to all straight 3 inch, but unless you are running 5-- +HP there isnt any point to it IMHO.
Actually, in my googling yesterday, a 3" full exhaust is even restrictive at like 400hp - remember even evo/sti's go a 3" exhaust to run stock turbos and just a tune, 3.5" for bigger things
Also, are you sure the 135i has a 3" inlet? From everything i can gather at most it's a 2.75" with a crush
An as said - the 128/130 is a 2.5" with a crush and other inefficiencies so there's plenty of room for upgrades - we aren't talking about the 135i (remembering they still commonly replace their catbacks too for gains..)