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      04-26-2015, 11:04 AM   #35
hassmaschine
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Drives: "NBO" 330i
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Right, it can be restrictive. But the stock cats are sized for the engne, so whatever flow restriction is present is minimal. It's no different than putting in an intake, the stock one is already pretty good so gains are minimal.

Headers make more power because they improve scavenging, have better merges, longer primaries, etc. If the only difference was the lack of cats, you could cut them off and make the same power as headers. While it will make a difference, headers still make more power.
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