Milltek Hi-Flow cat vs OEM cat pic from weekend install.

RS4 SE · 05-01-2007, 02:23 AM

ESFO · 05-01-2007, 03:26 AM

When I get to work I want to check the inside angle of that bend. Anyone know the cell count on stock cats vs. Milltek?

ELEVENS · 05-01-2007, 03:45 AM

xr8edS4 · 05-01-2007, 04:08 AM

based on the email addy (.se)

RS4 SE · 05-01-2007, 04:52 AM

RS4 SE · 05-01-2007, 04:54 AM

Vijay · 05-01-2007, 05:33 AM

With the stock 2nd oxygen sensor in front of the main cat, it appears that the main cat is not involved whatsoever in ECU feedback regulation, only the pre-cats. Compare this to the Milltek where you have only one cat, and a pre and post cat sensor as one would expect.

S4gasbord · 05-01-2007, 05:55 AM

The second O2 sensor needs to read post-cat levels or the CEL will light up.

Vijay · 05-01-2007, 06:27 AM

This would seem to imply that in the stock configuration, only the first cat is involved in the ECU feedback mechanism, and that one could dump the second cat entirely from the stock exhaust without triggering a CEL?

S4gasbord · 05-01-2007, 07:44 AM

But your comment about dumping the second cat without triggering the CEL is correct. Having said that, why would you want to retain the front cat that is so close to the turbine outlet? Better to get rid of the first cat and move the second O2 sensor behind the second cat.

05-01-2007, 03:26 AM	#2
ESFO AudiWorld Super User Garage is empty, add now Join Date: Mar 2004 Posts: 7,737	Very interesting pic. When I get to work I want to check the inside angle of that bend. Anyone know the cell count on stock cats vs. Milltek? __________________ B6 S4 FAQ. B6 S4 Dyno Shootout. Hard to shift smoothly? Throttle doing strange things? Read this. Challenged by a Ford Sales Manager... Funny. B6 S4s really are the farsterst eVarR!!!.

05-01-2007, 03:45 AM	#3
ELEVENS AudiWorld Super User Garage is empty, add now Join Date: Jun 2006 Location: yoo ess of eh Posts: 3,977	Thanks. Hey, where are you located to get an 06' RS4? __________________ ______________________________ 2007 RS4 Daytona Gray Pearl Effect Silver Nappa Leather / Carbon Fiber Trim *Audi SuperSport Plus Suspension, GIAC software* No sunroof, rear airbags, resonator, or carbon (cleaned THRICE) Only took 3 passes to run 12.5 @110mph completely smog legal on pump gas 2010 Q5 Deep Sea Blue Pearl Effect Cinnamon Brown Leather / Brushed Aluminum Trim *[]S-Line w/ 20's, B&O, & hitch*

05-01-2007, 04:08 AM	#4
xr8edS4 Senior Member Garage is empty, add now Join Date: Aug 2001 Posts: 8,614	Sweden based on the email addy (.se) __________________ 07 RS4 Daytona Gray/Onyx 01 S4 Brilliant Black/Onyx... 6 spd Rowing Machines.

05-01-2007, 04:52 AM	#5
RS4 SE Junior Member Garage is empty, add now Join Date: Apr 2003 Posts: 344	Milltek is 200c and OEM is about 500c ? __________________ __________________________________________________ _______________________ 2006 RS4 Tourenwagen Mods: Chip Tuning (ChipCenter Sweden) Stasis Alcons, mono 6 370mm RS6 C6 356x28mm rear brake discs Milltek Downpipes Milltek Exhaust (Resonated) KW V3 Coilovers H&R rear sway bar Sabelt 6-point racing harness Braid Winrace 10x18" Dunlop Direzza 02G 265/35 18" Alcantara -08 owners' manual folder :)

05-01-2007, 04:54 AM	#6
RS4 SE Junior Member Garage is empty, add now Join Date: Apr 2003 Posts: 344	correct __________________ __________________________________________________ _______________________ 2006 RS4 Tourenwagen Mods: Chip Tuning (ChipCenter Sweden) Stasis Alcons, mono 6 370mm RS6 C6 356x28mm rear brake discs Milltek Downpipes Milltek Exhaust (Resonated) KW V3 Coilovers H&R rear sway bar Sabelt 6-point racing harness Braid Winrace 10x18" Dunlop Direzza 02G 265/35 18" Alcantara -08 owners' manual folder :)

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05-01-2007, 05:33 AM	#7
Vijay AudiWorld Super User Garage is empty, add now Join Date: Mar 2000 Posts: 5,485	Interesting position of the stock 2nd Oxygen sensor compared to the Milltek..... With the stock 2nd oxygen sensor in front of the main cat, it appears that the main cat is not involved whatsoever in ECU feedback regulation, only the pre-cats. Compare this to the Milltek where you have only one cat, and a pre and post cat sensor as one would expect. __________________ ********************************************** Current Stable: Audi Porsche * BMW* **********************************************

05-01-2007, 05:55 AM	#8
S4gasbord AudiWorld Senior Member Garage is empty, add now Join Date: Aug 2001 Posts: 1,099	This is typical of aftermarket downpipes. The second O2 sensor needs to read post-cat levels or the CEL will light up. __________________

05-01-2007, 06:27 AM	#9
Vijay AudiWorld Super User Garage is empty, add now Join Date: Mar 2000 Posts: 5,485	My comment was more about the fact that the stock downpipe DOESN'T have a post-2nd cat sensor... This would seem to imply that in the stock configuration, only the first cat is involved in the ECU feedback mechanism, and that one could dump the second cat entirely from the stock exhaust without triggering a CEL? __________________ ********************************************** Current Stable: Audi Porsche * BMW* **********************************************

05-01-2007, 07:44 AM	#10
S4gasbord AudiWorld Senior Member Garage is empty, add now Join Date: Aug 2001 Posts: 1,099	Umm, it usually is only the first cat involved in the feedback mechanism. But your comment about dumping the second cat without triggering the CEL is correct. Having said that, why would you want to retain the front cat that is so close to the turbine outlet? Better to get rid of the first cat and move the second O2 sensor behind the second cat. __________________

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