High rpm boost falling away

[Huzqld]

High rpm boost falling away

Hi guys, 

Running a rb30det (neo). 270 cams, good inlet and exhaust manifold etc etc. gt35 with .82 rear. Aquamist  wmi.

Have always ran an internal gate and had boost control issues with it spiking but then falling away..

Changed to 50mm external gate and its better. But boost still falls away at about 5000rpm from 30 to 25psi ( trying to run 30psi) 

Gate is plumbed into exhaust and exhaust is 3inch, one muffler no cat.

The car made 545hp on that boost through an auto (Hub dyno)

Just looking for some ideas, does the gate need to be vented or am I possibly out of turbo/rear housing

[GTSBoy]

The gate shouldn't be flowing (much) if you're below your target boost.  So venting it is probably not the required solution.  If the gate is blowing open, then that's a whole 'nother thing.  What boost controller and MAC valve do you have on it?  You may need to apply more boost pressure to the other side of the diaphragm to control the opening from manifold pressure.

[Huzqld]

8 minutes ago, GTSBoy said:

The gate shouldn't be flowing (much) if you're below your target boost.  So venting it is probably not the required solution.  If the gate is blowing open, then that's a whole 'nother thing.  What boost controller and MAC valve do you have on it?  You may need to apply more boost pressure to the other side of the diaphragm to control the opening from manifold pressure.

I first had a mac 3 port on it and hooked it up as the turbosmart 2 port method(1) says.

We then fitted a 4port solenoid and hooked up as ts say, and it was a little better but not much, V88 is controlling boost 

Oh and 17psi spring in the gate.. 

Edited June 21, 2017 by Huzqld

[GTSBoy]

OK, so now you have to work out if it is simply exhaust back pressure limiting your total flow, or if you have a wastegate problem.  The exhaust backpressure is easily checked by running it without the restrictive parts of the exhaust on it.  The wastegate, not so easy to check.  There's all sorts of stuff about what duty cycle the MAC is getting up to and blah blah, but it's difficult to see how much it is actually opening and venting.

[Scott Black]

Turbosmart make a sensor for there gates you can buy to tell you how open they are

[Leroy Peterson]

A GT35 at 30psi making +400kw's would struggle with a 3" exhaust?

Usually people wanting power over 400 would got bigger than 3".