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S4 (86-88 ) N/A's use exhaust backpressure to open the 5 and 6th ports.
3 inch pipes almost for sure arent going to have enough backpressure for your otherwise stock(?) S4, unless you find another way to open your 5 and 6th ports. You must be losing at least 20-25 hp top end because of this.
There are ways you can set up your ports with pretty much the same system the S5 cars do, using your own air pumps, but i cant remember where ive seen some nice articles on it, try a search or ask someone else?
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