Ok, soooo here is the problem. Car will get to full boost (whatever its set to) then at exactly 6000rpms it will drop, quickly, and hold at 4psi.

Onto the specs...
The motor is a s5 13bt with a big street port and big exhaust port. The stock turbine is whats on right now with the stock wastegate. 3" dp into midpipe into stock catback. vspl FMIC ... etc.. etc.. its pretty built up, but those are the mods that you need to know ..

First we thought, maybe it was the spring to the wastegate. So we put a helper spring on it. Same results. We then thought maybe it was the turbo itself, so we took it off and inspected it. The turbo was fine, nothing more than a couple hairline cracks in the normal spots. Nothing that would cause anything like this. We pressure tested the wastegate to see if it was working properly, and sure enough, it was. We put it all back together and still, full boost until 6000 then drops to 4 and holds. We hooked up the boost controller, same results. We thought, ok ... maybe the exhaust if pushing the flapper open, even with the helper spring. So we unhooked the wastegate! What do you know, it goes up to about 14psi and drops to 4. We then thought, ok ... maybe, just maybe the exhaust is creating too much back pressure? so we unhooked the stock catback. SAME thing. Im at a loss here. I dont really care too much since I have all my parts to put together my big turbo setup, however I REALLY want to figure it out for personal knowledge. So if you have any ideas WHAT could be causing this, shoot them out... ill try anything.

Also running around with a wideband and my a/f's are perfect until the boost drops and then i get super rich ... go figure :-p

 

We are having pretty much the same problem at the moment. Difference being that our pressure drops to 0psi or under. Afr´s dip to low 9´s at this point.We were running .95bar with no problem, at wastegate spring pressure no problem yet. At .71bar it is back again. No boost or rev cuts on the ecu or boost controller.

John
88 GT-R

 

Your turbo is running out of breath , you need a larger one !!!

 

Would that be me, or sidewinderx7??? I doubt very much that mine is running out of breath!!

John
88 GT-R

 

I have a t76 sitting on my garage floor with all needed parts to install ... just waiting on my ecu to get here. I just want to figure out this problem for self knowledge, and I dont like letting the car beat me! Its a mystery I intend on figuring out! lol