I think most of the test pipes are 2.5, I made my own out of 2.5 and made the test pipe and down pipe all once peace, cut tubing and tac weilded it to fit then just took the whole peace out and ran some nice weilds, I took my old cat and cut that right after the o2 sensor and used that as my flange, just weilded some re-enforment weilds and it looks really clean, for the second o2 sensor i just made a 2 inch extension so the o2 sensor sits off the tubeing and drilled a small whole so it wouldnt get all the Exhaust, not only dose it look good but i dont get any codes..
'Eh, I don't know about 3" piping. How large is the turbo housing on an A4 (I'm new to Audis), ported or clipped, how many CFMs do they push? The td05h t-25 turbo comes stock with the 95-99 Eclipses, fairly small turbo but a 3" downpipe is right on the edge of giving the car fuel cut. Some ECU's handle it for some reason and others don't. Unless you'll be pushing a 50-trim turbo or more then a 2.5" dp will suffice for any car.
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1.8T down pipe. Does size really matter? - 
