I noticed turbo flutter on partial throttle applications whilst on the track and i am not sure that the EFR turbo internal BOV is managing the boost levels well (21psi)
I asked DVS tuning and they said to check that i am actually getting a vacuum for the BOV, I didn't have a gauge so ordered one from eBay and it arrived last week.
After getting the car up to idle i checked the pipe to which i have connected the BOV to at the throttle body and found it was reading about 18-19 in-hg which is pretty good (photo above is when it was cold). I also took the opportunity to check for common issues like worn rings, head gasket leaks, vacuum leaks and valve issues and following this guide it all seemed to check out OK.
I have the upgraded EFR cover and spring and have heard of the diaphragm cracking but upon opening and inspecting i cannot see any cracking
Next would be replacing with the turbosmart bypass valve, but it i can save the money it would be good