| I bring this up as I just encountered the problem today and have corrected it, and know that many folks are using similar components as I am. Recap: The turbochargers in my Z are the Z1 Stage2 design that I developed during my time working with them. This turbocharger uses a 62-trim T3 compressor wheel and a T04B housing mated to a stock cartridge with 360-degree thrust groups and carbon seals. Many of you will note that this is the same compressor wheel and compressor housing that is on the GT28RS/Sport700/Z1675RS turbochargers. The GT28R has a different compressor housing but it also relies on mechanical sealing. There are other turbos out there that also use this sort of sealing mechanism. When I initially assembled my Z and put it through the ringers at the track and dyno, the car was boosting up to some 31psi peak. Today, I revisited the dyno to put down some new numbers with the DXD Feramic Clutch and slightly cooler weather than the previous 666RWHP run @ ~31psi. I could not get the car to boost past ~26psi, and it would quickly fall off to ~20psi by redline (8000RPM). I performed a boost leak test prior to the dyno session and she was holding tight with a 7psi test pressure. It was noted that I had to trim some fuel from the previous racefuel tune to achieve a 12:1 A/F ratio, which further indicated to me that there was a decent boost leak somewhere. So this evening I put the leak tester back on and ran her up to 7psi - no leaks. On a whim, I decided to take the test pressure higher. *note* I have circumvented the PCV system in my powerplant so there is no concerns with blowing seals by overpressurizing the crankcase during a boost leak test, so I took her up to 15psi. To my amazement, there was a significant amount of air noise coming from the driver's side turbocharger area. I couldn't pinpoint it from atop the car so I jacked her up and had my wife pressurize the system while I slid underneath the car to inspect. I was shocked to find that the leak was coming from between the compressor housing and the backplate! It would leak down to ~10psi and then stop. I went ahead and pulled the turbo and removed the compressor housing to check the parts and found nothing out of the ordinary. In the design of this particular compressor housing and mating backplate, there is no seal used. The seal is achieved by the precision mating surfaces being pressed together by the mounting tabs and associated bolts. I can't say with any certainty that this particular problem is isolated to my set of turbos, but I have a feeling that it isn't and could present problems for others. Fortunately the fix is relatively simple: red RTV silicone will do the trick. I cleaned up the mating surfaces with some brake parts cleaner and applied a thin layer of red RTV to the backplate face and put the compressor housing into place, rotated it back and forth to smear the silicone around a bit, clocked the housing and installed the tabs and bolts. I let her sit for a little while and then pressure tested it on the bench yielding no leaks. Reinstalled the turbocharger and went out for a road test - she is back up to spec now and boosting to 2.1bar. (yes, I have racefuel in the tank from earlier) I could have chucked the backplate on the lathe and cut an oring seat and sourced an oring of appropriate size, but that would have been above any beyond what is necessary, not to mention, it would likely throw off the tolerance between the compressor wheel and housing to some degree. Regardless, applying a thin bead of some red RTV in this location will prevent this problem from occuring in your turbochargers and would be highly advised to do. The only thing I can think of is that the backplate or the housing slightly warped. The compressor housings on our turbos sit so close to the manifolds that there may be potential for warping, also compounded by the uneven compression applied by the bolts and tabs. Here are a few pics of the parts in question:
The images are of a GT28RS. I am touching the mating surfaces in this image:
 This is how they are assembled:
 I will be venturing back to the dyno as time permits - she did put down 635RWHP and 590RWTQ at 26psi though. :)
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PSA: Turbocharger assembly and a preventable problem >> - 
