the AWE DP's have the secondary 02 sensor behind the cat,and do not throw the warm up cat efficiency code in either chipped or stock modes! Its been seen as of late that the 100 cell cats are causing this code to show up, this is an early sign of failure of the cat, i have also heard rumors that the 100 cell cats in "some" systems have now been changed to 250 cells per inch, the question is do the 100 cell customers get the "free upgrade" (necessitating removal of the downpipes to be sent back to the manufacturer-modified then sent back to the customer) or do these customers have to live with CE lights-maladapted fuel trim and potentially not passing emmissions testing in the future?

heres an excerpt of the cat information on AWE's website


"There is a a great deal of misinformation currently floating around the internet regarding S4 catalyst performance. When we specified a catalytic converter for our downpipes we considered all the issues associated with cost and performance.

Why we chose a 400 cell per inch (cpi) catalytic converter: At the top of our priority list was choosing a catalyst that would do its job properly in cleaning the exhaust emissions. There are some fundamental requirements regarding a catalytic converter that will meet US Federal tailpipe emissions testing (i.e. get an emission sticker on your car). The primary requirement for LONG TERM tailpipe emissions compliance is that the converter matrix has 400 cells per inch. Anything less than this, and the converter will have trouble meeting the exhaust gas emissions requirements as mandated by US Federal standards as the catalytic converter ages with use. The exhaust gas content standards here in the US were set based upon what a a 400 cpi (minimum) catalytic converter can achieve.

Any catalytic converter that has less than 400 cpi will have trouble meeting strict long term tailpipe emissions tests.

We have extremely strict emissions testing in Pennsylvania, and we have confirmed that our downpipes meet PA tailpipe emissions standards with the cats in place. If they pass here, they will pass the tailpipe test anywhere. We can guarantee this, and this guarantee is backed by the catalytic converter manufacturer since it is EPA and DOT approved.

We did not use a "generic" converter off the shelf. We worked with the manufacturer (who also manufactures for OEMs) on a converter size based upon the S4's configurations specifically, with the sole purpose of passing the strictest US emissions standards with the least amount of restriction. These are the same type of converters we use on our wild 500hp "SilverBullet S4" test car (they flow plenty!). Because we are using 2.5" diameter tubing for our downpipes, our converters are already providing 25% more cross sectional flow area over the 2.25" systems out there.

We did not accept European minimum standards to meet the tough tailpipe requirements most states impose here in the US. During our research, we were advised by the manufacturers of the catalysts found in many of the European produced S4 downpipes to avoid 100 cell per inch designs, as they would not provide the aging requirements we were demanding. The relatively limited precious metal surface area of the 100 cpi catalysts begins to cause emission testing problems as they age from use, according to their manufacturer. 100 cpi catalysts are also know as "race cats" or "rennkats" and are notorious for emissions issues and constant check engine light issues on Porsches since the introduction of aftermarket exhaust systems for the 996 model.

The bottom line: what is the purpose of even having a catalytic converter in your downpipe if it is not meeting minimum US tailpipe emission standards?

If you do not care about emissions standards, simply install our test pipes and do away with any possible restriction a cat can give you (just save that for the track!). Our downpipes are modular.

Why we chose to use a ceramic matrix instead of a metal matrix in our catalytic converters: we chose a ceramic matrix catalyst because our testing determined a metal matrix catalyst was not remotely required on the S4.

The "matrix" or "substrate" of a converter is the internal latticed core that provides the precious metal exposure to the exhaust gas flowing over it. This exposure results in a chemical reaction and a benign transformation of some toxic emission components. A metal matrix catalytic converter that uses metal as a base for its core is designed for 1200F or higher operating temps, to prevent localized melting. The placement of the S4 main converters puts them at a location where the exhaust temperature is less than 800F, even under high boost race conditions. The factory, which has the utmost concern for long term converter life, did not feel a metal matrix design was required, either. Audi uses a ceramic matrix for the S4 main converters, whereas they use a metal matrix catalyst on the 1.8T applications.

We do offer the option of a metal matrix converter for our downpipes at a surcharge of ~$350 for the pair. However, there is no short or long term benefit to using these converters.

The bottom line: the location of the main catalytic converters on the S4 does NOT require them to be metal matrix."

• Downpipe and catlytic converter info