RMcQ

Once again I spent some time as the solo member of the Mid-Atlantic Mod Crew anti-social committee.

The idea with modifying stock downpipes is to gut the pre-cats for improved flow and performance. This is a low cost DIY mod if you are willing to invest the labor. Getting the pipes out of the car involves 4 bolts on the turbos, 2 o2 sensors, one hanger bracket bolt, and the exhaust clamps beyond the main cats. It doesn't sound like much, but it is a good bit of labor to get it all out.

When the downpipe (DP) is out of the car this is what you have (note: the first 3 pics are not mine)

Borrowed pic showing components of DP:'
<img src="http://pictureposter.audiworld.com/AudiWorldPics/2000/S4_precat.jpg">

When you look down into the stock DP, this is what you see. The mesh is of course the catalytic material. Think this affects flow much being only a couple inches from the turbine wheel?
<img src="http://pictureposter.audiworld.com/78292/img_0664.jpg">

The "harvesting" strategy involves basically bashing the material out. I used a combination of cold chisels, pry bars, long drills, and long neck pliars. This stuff puts up a good fight. Eye and breathing protection mandatory.
<img src="http://pictureposter.audiworld.com/78292/img_0658.jpg">

So once I got them gutted I had to clean them up a bit. I used a wire wheel and a flapper wheel. They looked so nice afterward (as if I am a machine shop or something. ha!)

DP with ick:
<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/Dp_with_ick.jpg">

Tool 1:
<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/wire_wheel.jpg">

Tool 2:
<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/flapper.jpg">

<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/flapper2.jpg">

Ready for reinstallation:
<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/DP_clean.jpg">

So I got them back on the hovercraft (car is still on jack stands awaiting return of front half shafts from Raxles.) and was able to start it but not drive it. Full report will have to wait. I was hoping that opening them up will bring a *little* more life to my Milltek exhaust, but couldn't tell at idle or with small throttle blips.

Rather than relocate the secondary O2 sensor behind the main cat I decided to try the cheap and easy method recently developed by the S4 guys.

You can trick the secondary O2 sensor into thinking everything is OK by using spark plug anti-foulers. They are available at Pep-Boys or whatever. They are "Help" brand part #42009. Total cost to me for 2 packages was $10.49.

<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/anti_foulers.JPG">

You have to stack two to make it work. The first one has to be drilled out to 19/32" which is the size of the O2 sensor body:

<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/drilling2.jpg">

The O2 sensor goes into the drilled anti-foulger
<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/1_antifouler.jpg">

Then the combo goes into a second unmodified anti-fouler and this of course gets screwed back into the original location on the DP.
<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/stacked_antifoulers.jpg">

I have not driven it yet, but on start and idle it passed readiness quickly.
<img src="http://www.mcqfamily.com/allroad/DPs_antifouler/o2_readiness.jpg">

BSK

to buy brand new cats somewhere instead of just replacing a pipe that had been made.
i ordered a downpipe because of that.

RMcQ

test. In Maryland all they do is electronically check OBD-II readiness. I can't imagine how they could possibly "bust" me. There is no way (short of reading Audiworld!?!) that anyone could tell that there is nothing in the pre-cats.

Perhaps in the future they will include a cold start sniff test. That is the only case where the car would not funtion as good as OEM.

absolutcq20v

Schweet, can't wait to take another test drive...without the wobble wobble.

BSK

thats easy. i dont have emissions in my area so i should be good.

RMcQ

And a few more HP!

tazerproof

It was a cat-related CEL....the first time they told me to come back..the second time it passed no problem...

absolutcq20v

Pass anything but a gas station! :P