Downpipe/DECAT - Negative Fuel Trims

Guys,

Working on a 4.0 TFSI V8 Biturbo, and installed downpipes without cats. Currently I have negative fuel trims which means I have less air coming into the engine, but from my understanding the engine should breath more since I have less back pressure? Obviously first step would be to adjust the fuelling. Would I still do that over the MAF scaling since this engine operates over the MAF? Still having an stock intake. Asking because Andre rescaled always the MAF as first approach, though every body says you only touch that if you change the housing or anything that would affect the physical airflow going through the MAF housing.

By changing the cats you might have also changed the cells in fuel maps where the engine gets to since it srtarted to breathe easier and quicker. For instance, you hit a cell now with 14.7 instead of 13.8 as it used to be so ECU is removing some fuel to get to new target afr. It should work out eventually... Modern ECUes have fuel trim ranges as wild as +- 25 percent or even more, so your computer should be able to handle the change of cats without having to rescale MAF. However if fuel trims will not come to reasonable values in the near future I would double check the hardware making sure there is no some flow restrictions introduced by the change...