One of the best, and certainly most cost-effective, ways to add power to your B6/B7 Audi S4 with the 4.2 V8 is the downpipes. The factory system contains two sets of catalytic converters on each downpipe, a pre-cat and a main cat. The pre-cat in particular robs the engine of power because it is very close to the engine and creates turbulance/disruption very early in the exhaust flow out of the engine. This thread on Audizine by Justincredible goes into greater detail as to why the downpipes are so restrictive and why its quite possibly the best “first” performance mod to do to your B6/B7 Audi S4. After some light reading of this thread, I was excited to go about this mod so I recently had my pre-cats removed/turned into piggie pipes by a local shop, Parts Score in Tempe, AZ. The mod definitely opened up a little extra power and increased the exhaust sound, so I wanted to share my experiences & thoughts and research on the mod here for others to enjoy. If you don’t want to read the whole thing – I’ll skip to the point…it’s a great mod and lives up to the hype!
There are a few options in terms of addressing the restrictive downpipes of the B6/B7 S4:
- Piggie pipes – taking the OEM downpipes and punching out/removing the catalytic converter material, allowing air to pass through more freely
- JHM Longtube Headers – these are supposed to be the best option performance wise, but also quite expensive at ~$3,000 and are made to order
- Aftermarket downpipes – Milltek, Fast Intentions and a custom solution by a fellow named TrexTurk all remove catalytic converters and come in larger pipe diameter for improved exhaust flow and sound, and range in cost from about $500-2000 depending on options and the source
Why I went with Piggie Pipes:
One downside to going with a fully catless system (such as JHM and other aftermarket headers) is that your exhaust will be both louder and potentially emit a bit of an odor, as well as a higher cost to purchase the parts. My A4 had a “test pipe” which eliminated the catalytic converter and essentially did the same thing as the downpipe solutions for the 4.2 V8 on the S4; the odor kind of bothered me on my A4, and I wanted an exhaust setup that was pretty tame (both sound and smell wise) for daily driver purposes, hence why I went with a Milltek resonated system to begin with (the quietest system for the B7 S4 I know of).
The other main advantage to using the OEM downpipes was that I could only remove the pre-cats (which rob the most power) while keeping the main cats, maintaining some sound & odor dampening as well as keeping my emissions somewhat reduced. Another advantage was cost – I had my downpipes removed, gutted, and reinstalled for less than $500, which is less than buying even the cheapest downpipes on the market. Lastly, one more perk of going for piggie pieps is the lack of a Check Engine Light as a simple O2 spacer setup could avoid a CEL warning in this case. The CEL for aftermarket setups can be removed via a tune, but I haven’t tuned my car yet and am I’m delaying a tune a little while longer to keep my warranty in tact. Parts Score, the shop that completed this work for me, found a trick using O2 spacers and a little “secret sauce” to avoid a CEL, and I’ve been running this way for 500+ miles with no issues. Other users report that a CEL occurs about 50% of the time with this method, but various options such as using two sets of 02 spacers or modifying the spacers can increase your likelihood of avoiding a CEL.
Sound & Performance Gains of Piggie Pipes:
The sound is slightly raspier and metallic – in a good way of course. I’d say the exhaust is about 10% louder at normal speeds and maybe 20% louder at wide open throttle. I need to get a video or sound clip, and will update this post when I do. I expected the removal of the pre-cats to make the exhaust louder than it did to be honest – its certainly a little louder, but not dramatic or even noticeable to most people. I didn’t have an opportunity to dyno the car before and after, but I can tell you that it feels faster and more responsive to the throttle, especially when really hammering it.
Overall, I’m really happy with this mod and it lived up to all of the reading & reviews that I found prior to going with it. The improvements in performance and sound at a nominal cost make it tough to beat, especially for a car like the B7 S4 which typically requires a lot of money to make modest improvements in power. I highly recommend this!