its more than rust.. Stainless contains the heat much better than carbon steel (better power) and it won't break down from the thermal cycling like plain steel will (lasting longer)- I don't think any reputable mfg does carbon steel. Any of the pipes "should" work with any of the tunes. I run APR DP with a Unitronic tune though I had to add an o2 spacer for it not to throw codes. I think APR's design is the most "thermally safe" allowing higher tuning, but its probably the worst for actual emissions performance as it moves the cat away from the head more and won't "light off" as quickly. You can still get the Unitronic pipe I believe. It also requires the annual dance of removing the tune, getting emissions test, then reapplying it which is a bit annoying, but also means if you have testing you are best off with a tune you can do yourself at home - or you'll have to pay for it on and off every year. A DP with high flow cat is gonna be what, 900-ish? It is what it is unless you want to buy a high flow cat and weld up your own.... not impossible but for most of us that can, still not worth the bother. I think there's a few APR pipes out there still. I've had the best luck with Unitronic tunes so far (on both my S3 and R8) but have used other tunes also across the 12 or so vag cars I've had and I'm sure they are all fairly similar. I've only had actual issues that cost me money and hassle with APR. Typically any of the stage I tunes are similar and fine, but as you go up from there it starts to separate with more varied results. You'll also need a TCU tune. The Unitronic TCU tune is fine, but if I run full tilt and don't turn off traction control it won't shift properly. They weren't much if any help on that issue, so I just make sure I have TC off if I/m going to hoon about. (none of the vendors are perfect). Don't forget an intercooler upgrade too; you can get by without but won't be making all the power that the software will deliver. All-in-all, you'll be spending a lot more money to properly do stage II. The value prop over stage I is really not good, but I enjoy modding stuff so did that and a lot more besides just for the fun of it. You might be best just sticking to stage I.
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Originally Posted by Elvir
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