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I've still got my precats in and I heard that they can be very restrictive to the turbos. What sort of brake can I get out of the car with KO4s, 440cc injectors and an RS4 fuel pump with the precats ungutted vs gutted? Thanks.

From what I remember on previous posts with K03's the difference is around 10-15 bhp I think. I was running 315 with Milltek and precats in, sure others without where up to 330ish.