@coheed:
i grinded the inlet area of my backhousing down for half an inch to enable a gas presure compensation between both channels. I dont have pictures but see this photo to explain what i did.
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The effect was no improvement in knock but 400rpm loss in spool.
The main reason i did this is the fact that the4 GT3271 WG has only on of the two channels internally wastegated which is a bad construction in my opinion....
This leads to a rising boost pressure in the higher revs even with very soft WG-springs. I tried to improve this with my grinding to reduce the backpressure but it had almost no effect.
Finally i bought a new backhousing because my experiment failed