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I spoke to PTF directly and they said hi-flow or catless is "recommended" for Stage tunes but not required. Lower restriction does improve top end HP by 5-10.
For your "cells" measurement…the numbers don't add…the highest number wins (loses). Having a 400CPI (cells per sq/inch) plus a 200 further back still nets 400CPI of restriction. The secondary 200CPIs don't impede flow much since the 400 already slowed things down.
Really the higher cell count does a better job at chopping up the pops so the exhaust isn't raspy. Our stock 400CPI cats are barely restrictive, so it's not a bad move to stick with OEM.
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'18 F80 Base 6MT | '19 F82 Exec DCT | '18 F82 Comp Exec DCT | '23 G80cx
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