If you change out the turbo to a slightly larger one, at what time would you need a aftermarket ECU or a piggyback fuel controller?
How much latitude does the stock S4 ECU have for mods?

I have a s4 with 3mm apex and a street port.

Thanks for your time!

Lloyd

 

Also, I did a search and couldn't find anything speaking to this question concerning the ability of the s4 turbo 2 ECU's ability to work with a turbo that is not stock.

Thanks again.

 

The S4 ECU stock is good for just that: Stock. The ECU doesn't like more boost and it will cut fuel (you can get around this). A bigger turbo will push more air and boost. You will quickly lean out. Have you looked into the Rtek?