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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > Help Understanding Exhaust Mods for 335i



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      08-09-2007, 12:24 PM   #1
JWadle
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Help Understanding Exhaust Mods for 335i

I thought I understood some basics about exhaust systems, but once I began researching exhaust upgrades for my 335xi I realized I don't understand the issues all that well.

From what I have read, catback systems can provide better sound but limited power increases. It seems like the big power boost comes from downpipes or full systems that include downpipes. Is this true for NA and turbo engines alike?

I'm also confused about the primary/secondary cats on the 335i. I've seen someone refer to the secondary cats as "dummy cats". Which cats are replaced with catback systems and/or downpipes? What about the "racing cats" that are available with systems like SuperSprint?

Perhaps most important, what exhaust mods can be done and still pass emissions? I know that passing emissions tests and being "legal" are not necessarily the same.

In summary, I guess what I'm asking is this: What exhaust mods can be made to a 335i that will give maximum power boost and still pass emissions tests?

Can anyone enlighten me on these questions?
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      08-09-2007, 01:37 PM   #2
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The seconday cats are not dummy cats. They are there to reduce the NOx (oxides of nitrogen) from the result of high combustion temperatures.

It's hard to say which catalytic converter is referred to when manufacturers say "cat-back exhaust" They could be talking about the primary or secondary cats. Borla and Dinan will have their exhausts after the secondary cats. Systems like Britalman and AA Tuning have their systems eliminate the secondary cats and is bolted/clamped after the primary cats. Supersprint you can order them in sections.

The aftermarket downpipes usually eliminate the primary cats.

Removing the secondary cats generally will increase power but on most states it is illegal to do so even though the DME doesn't monitor them.

Your next smog won't be till 2011. Is your BMW a lease? If so I wouldn't worry about it.

Since you have a Xi model only exhausts for sedan applications will fit. If you want the best chance in passing smog then I recommend Borla, Dinan, or Supersprint X-pipe with rear mufflers.
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      08-09-2007, 01:58 PM   #3
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Does the vishnu exhaust eliminate either of the cats?
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      08-09-2007, 02:00 PM   #4
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Quote:
Originally Posted by MSimeon View Post
Does the vishnu exhaust eliminate either of the cats?
Yes it eliminates the secondary cats.
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