what kind of difference would taking out my cat & replacing it with a decat pipe,what increase in power would i gain if any
Theres a lot of people saying you will gain a ton of power from decating ur car really it wont make you enough horse power for the ticket you could get. If you really want more power out of ur exhaust. Get a header system with a free flow exhaust. If you dont belive me then read this, http://www.uucmotorwerks.com/ its under tech tips.
I second it. Unless you're running race specs, I see no need whatsoever to decat a street car. If you do have an old exhaust system I would recommend getting a newer free-flow cat though. Converter technology has come a long way in the past few years, and you can have an emissions friendly exhaust without losing any power. Also seeing that you have a 323i, I'd look into removing the restrictions that BMW placed on that engine. I'd look at doing an M50 manifold conversion ( http://www.m50manifold.com ) which should get you a decent amount of power by removing intake restrictions. The stock manifold is undersized on purpose from the factory. I'd also look at your stock exhaust system, you'll probably get a decent amount of power out of it by even replacing it with a used M3 exhaust.
The car was desinged to be used with a cat, esp. BMW's. taking out the cat probably will not give a noticeable increase, it could actually decrease power.
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