Sure, I can sum up what I have learned about these cars…great idea.
The factory S6 design is, in my opinion, flawed. Someone got a great idea to stuff a v10 into the S6… but then some other team decided they needed to cram 4 catalytic converters in the middle of the factory headers. Restrictive cat placement caused performance issues and extreme heat soak, and created a maintenance nightmare when you have to drop the engine to get to the upper o2 sensors.
Heat soak causes premature part failure and inconsistency in performance. As problems started to arise in these cars, dealerships really didnt want to start replacing everything under warranty to try and figure things out. So, Audi just updated the ECU tuning to be less sensitive to the issues. So, the cars looked like they were running OK (no check engine light,) but in reality they were far from good. My car ran mid-14s at the 1/4 mile when we first got it, but had no check engine light. But when we scanned it, we could see the misfires. Just, the ECU didnt complain.
So, then what happened is the cars got driven deeper and deeper into a hole without things being fixed. Even if a few expensive things were fixed, the car still wouldnt run right because you have to fix EVERYTHING and fix the factory design flaws to make the cars reliable. Not only does the average S6 owner have no idea of all the things that have to be fixed, but even if they did, no one wants to spend the huge $$$ to fix it all.
So then, the market got flooded with a bunch of v10 S6s that dont run right. There are times when they may run OK, but will never have long term reliability in stock form. Plus, the stock car requires well functioning o2s and cats to run right, and both require an engine drop to replace (or gut the cats.)
As the newest v10 S6s are now a decade old, pretty much any one of these v10 S6s you see on the used market, are likely going to need pretty much all of the following (unless they have already been replaced)
Intake Manifold
Carbon clean
Fuel Injectors
Various seals
o2 sensors
coil packs
and likely catalytic replacement (which means either you gut them or go with aftermarket)
On my car, we also needed a new radiator, thermostat, spark plugs and battery, among other stuff. All are critical to the car functioning properly.
OK, so after you replace all that stuff, now you have to deal with the factory header/cat/o2 design problem. In my opinion, there are 2 valid ways to deal with this:
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Keep the stock headers, gut the cats, and replace all o2 sensors with new ones. If you dont want to relocate the o2 sensors, then you will have to drop the engine again in the future to replace some of them. Considering the o2 sensor readings are vital to the performance of the car, I would have them relocated downstream. But note, this MAY mean you need a custom ECU tune to adjust for their relocation.
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JHM headers, high flow cat, relocate the o2 to be accessible, JHM matching exhaust, and custom ECU and TCU tune to make everything work. Even though this is much more costly, my opinion is that this is the way to do it RIGHT, and the only option I would consider.
On my car, when we were troubleshooting the misfires, we pretty much replaced everything mentioned above, and every other related part that could contribute. It wasnt until all the parts were replaced that the ECU gave us a clearer picture of what had to be done to finally get it running perfectly.
For whatever reason, my car had to have a custom tune to read from only certain number of o2 sensors, not all original 8. My guess is that this had something to do with the much more efficient 5 into 1 header design, and relocation of the o2 sensors. Apparently, this was not required on the other cars that had similar mods, but on mine it was. So, I would not recommended these mods without being prepared to also do a custom JHM tune. CountVohn really saved the day, and was probably the only person on the earth who could have figured it all out and got it running right. So, a HUGE thanks to him!!!
Also, when the car finally started running amazing and making tons of power, now the transmission had no idea how to handle it…LOL. It would have huge pauses in between gear shifts, as there was way more torque then it was expecting. A custom JHM TCU tune fixed all that, and now it shifts like lightning.
To give you an idea of the torque, my car runs 275 width tires front and rear. If I leave the ESP traction control enabled, the car cuts all power under WOT going from the 1 to 2 shift. That is because it spins the 275 enough to kick in the ESP. This is in the middle of summer with lots of grip. And, if you dont disable ESP then the car doesnt allow you to really drive it fast and spirited in general. Its more subtle, but I can feel it kicking in around turns when driving spirited.
With ESP disabled, you can power-steer the car around turns and it just rips. WIth all the extra power, you can really throw it around. It is a really fun and powerful POINT N SHOOT rocket now that it is all done…But, the effort and $$$ to get there was significant.
So, to sum it all up, if your car is not running right:
Replace all the items I listed above with new. (I know you dont think they ALL need to be replaced, but they do)
Make sure the battery, radiator, thermostat, throttle bodies, HPFP are all functioning 100%, or replace.
Make sure the battery is AGM. The lead acid batteries gave me issues.
Fix the factory header design flaws with one of the two methods I outlined earlier.
After all mechanical items are addressed, get a JHM ECU and TCU tune.
