Boost loss after 4600rpm?
 

Built motor, 8.8:1 compression
Powerlabs single turbo kit, Greddy GT35R .82
Tial MV-R (first release), new 10lb 44mm spring, manual controller
Equal length headers, 3" turbo-back, Tomei exhaust
UpRev GT MAF, Tuned on Osiris

I did have this same problem last year on a 6lb spring controlled to 10lbs. It would boost to 10lbs right away, then bleed off to around 7ish. We suspected a weak spring or bad diaphragm. I checked the diaphragm and it appeared fine, and now I have a new 10lb spring and still doing it. Using a new boost controller this time around also, so don't think that's it.

Built motor over the winter, got car tuned yesterday and having same problems with boost loss. Will boost to 20psi right away and hold steady until ~4600-5000rpm where it will drop and fluctuate 14-15psi for the rest of the RPM range. We did a leak test, everything held fine up to 18psi (didn't go any higher). Wastegate reference is the original Powerlabs location, right after intercooler. My next test is moving the reference to the manifold, and purchasing a elec boost controller. If that doesn't help then might look into a new wastegate. Looking for some more ideas or if anyone has ran into this problem before.

My EBC ramps up the wastegate duty cycle as time goes on after boost comes in. Your MBC won't do this.

Have you checked the line that goes to the top of the wastegate? If this is leaking, it won't push on the top of the diaphragm to keep it closed.

What kind of IC are you running? Where is your boost gauge referencing? (Should be at the mani post TB)

top port is open with the manual controller, just runs off bottom line right now.

Intercooler is whatever came with the original Powerlabs kit. The original reference is about 10 inches after the intercooler. Which source would you use?

Hmmm

I have two seperate sources for vac/boost. One is tapped into the plenum; off that I run the MAP sensor, and fuel pressure regulator.

My other reference point is right after the intercooler. I have that going to the bottom of the wastegates and through the boost controller solenoid then to the top of the wastegates like this photo.
https://cimg3.ibsrv.net/gimg/g35driv...6be6d586d2.png

So after a lot of driving this weekend I have noticed something I wanted to mention in hopes it sparks ideas. If I don't give 100% throttle, but stay at 75-80% I will see 18-20psi without dropping, but if I give full throttle I will see a drop in boost.

It could very well be your tune. Can you post a log? It would be nice to see what the wastegate duty cycle is at a part throttle pull as well as a wide open pull.

I don't have access to anything as its all locked.

I did, however, do some more testing. I installed a 16lb spring combo. Without controller it will run 16-17lbs consistent. With controller set at 20lbs, it will sometimes hold 20, sometimes run up to 22-23, and sometimes slowly bleed off to 17psi. I also noticed that before changing the WG springs, the controller was barely putting any pressure on the ball/spring, its literally 1/4 turn and its running completely open. And if you turned the adjustment screw back 1/2 turn the screw was almost all the way out of the body. I'm not overly familiar with a manual controller, but this doesn't seem right.

I'm going to be gone for a month for training starting the 21st, so when I get home I'm going to order a elec controller and see what happens. I don't feel confident in this manual controller.

I agree, go from there. See how an electronic controller does. I bet it's pulling hard when you stay in boost! 20 lbs is a lot!