what is this for??

[rfn_hodgson]

hello felles i have seen a few mk4 without this... its the black box on top of one of the coilpacks



what exactly is it for?/

[oceanic-green]

if I remember correctly it is called the vacuum reservoir. I got rid of mine when I did the N249 delete.  I think it was the N249 delete.

[Gti_Jamo]

As above, its stores vacuum to operate diverter valve if needed. Its removal gives a crisper dv noise through the induction. Its removal requires a bit of reworking of the vacuum piping and a resistor fitted but plenty guides around on n249 removal.

[Nino]

^^^ its only found on the AUM engines, as above it stores a vacume of air for the n249 valve that controls the DV. I'd recommend doing an n249 delete/bypass (see the how to section) by doing that your DV dumps atmospherically as apposed to "controlled" by a valve which often fails or dumps too early. You will also feel slightly better throttle response. 

[rfn_hodgson]

mine is the AUQ block and thats a pic of my engine and im waiting for some piping to turn up so i can do the n249 delete love little projects like this

[brewey]

^^^ its only found on the AUM engines

I have the reservoir on my ARZ.

[Nino]

My bad, just going by what I was previously told 
However the rest of my statement stands