Help Understanding Exhaust Mods for 335i

JWadle · 08-09-2007, 01:24 PM

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?

jon@themshop · 08-09-2007, 02:37 PM

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.

MSimeon · 08-09-2007, 02:58 PM

Does the vishnu exhaust eliminate either of the cats?

jon@themshop · 08-09-2007, 03:00 PM

Quote:

Originally Posted by MSimeon

Does the vishnu exhaust eliminate either of the cats?

Yes it eliminates the secondary cats.

08-09-2007, 01:24 PM	#1
JWadle Since '75: 530i/733i/745Turbo/M5/M6/325iX/540i 7 Rep 191 Posts Drives: '07 335xi 6MT Sp Grph Join Date: Mar 2007 Location: Boulder, Colorado iTrader: (3)	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? __________________ '07 E90 335xi 6MT Sparkling Graphite Options: Premium~Sport~iDrive~Nav~HD Radio~iPod 6FL~Comfort Access~Rear Sunshade Mods: Short Shift Knob~M-Tech Wheel~CF Interior~BBS RGRs~Passport HW~Blacklines~Rear Fogs~Shadowline Trim~Euro Xenons~Lux 4~M-Tech Front & Sides~Blackout Grille~Rear Power Outlets~Berber Mats
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08-09-2007, 02:37 PM	#2
jon@themshop Captain 46 Rep 762 Posts Drives: BMW Join Date: Apr 2007 Location: Los Angeles/Eagle Rock, CA iTrader: (8)	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, 02:58 PM	#3
MSimeon First Lieutenant 30 Rep 372 Posts Drives: E92 335I BSM Join Date: Feb 2007 Location: San Bruno CA iTrader: (1)	Does the vishnu exhaust eliminate either of the cats? __________________ Ordered 4-25-07 Delivered 7-20-07 E92 335i black sapphire metallic, sport, premium, and Navi. JB4, K&N Drop in, Matte Black Window Trim. Linea Corse Dyna 19x8.5F 19x10R Previous Ride: 2004 Silverstone 350Z enthusiast. APS TT Built Motor at 12PSI. 467rwhp and 413 rwtq. Tuned at ZCARGARAGE. (Missed, but never forgotten)
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