I have just purchased a 6 month old S4 Avant and will be tuning it. My experience so far is limited to my last car, another Audi that had an Oettinger tune done to a 3 ltr Tdi Quattro. This was a vast improvement. My research has led me to two companies at this point. Either ABT or APR. These are two different approaches to a similar end result. ABT seems to be a "simpler" approach if I can use that generalization. It comes with a two year warranty on the drivetrain but is more expensive. I don't mind the expense if I consider it warrants it. The APR seems to be logical as well, giving the supercharger a tweek for good measure. This system does throw up flags however and does not give any form of warranty. The guys who service my car install APR products. As a by the by Oettinger doesn't list the 3ltr Petrol engine in their products. I know there are many other tunes out there but one has to eventually distill things down. My question. Or questions. Has anybody had both at one time or another? Has anybody had problems on this drivetrain/motor with either? This is a much debated issue and I could keep searching but I just thought I'd throw this one up here as this is such a big forum. Thanks guys.