about downpipes posted by kareeem266045 on December 07, 2001 at 12:25 PM
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The 3" DPs are too large and don't allow for gasses to excellerate and expand after the turbine. The best design is being done by Greg Dupree which are 2.5" and then expand to the 3" midpipes.
I really can't say what the exact hp increase is, but they are necessary if you want to increase the flow due to other mods.