when it comes to exhaust flow on a turbo car, emissions regulations make manufacturers restrict the flow by adding catalytic converters and mufflers to meet the pollution and noise pollution standards set by authorities. The Madness high flow catless downpipes eliminate the main catalytic converters, with a 3 inch v-band inlet you can re use your factory vband clamps and they come with 2 bungs on each downpipe to re install the pre cat and post O2 sensors ( in conjunction with our EVOTECH software we are able to disable the secondary O2 to keep the check engine light from coming on, so no need for a O2 conditioner or spliced in resistor ). these downpipes will work with the stock or any aftermarket exhaust because of their stock like downpipe to mid pipe flanges. Our 08 135i is equipped with this excellent product adding rwhp and rwft-lbs through-out the power band in the neighborhood of 20rwhp and 22 rwft-lbs on stock boost levels, making throttle response and spool time virtually instant. When tuned at higher at boost levels the Madness catless downpipes are a must for your 135i/335i/535i/Z4 35i. These downpipes are for racing and off-road use only.