Low boost issue - hybrid turbo
 

Trying to get my head around a boost issue i'm having with my current setup and having little success dialling it in.

The turbo is a hybrid consisting of an FP Green front housing and billet wheel mated to a 7cm ported td05 housing. The front housing has a custom insert thats been machined down to run on a stock intake pipe.

Rest of the setup is
2.5 forged
740cc injectors
HDI Fmic
Standard ported headers
Cobra sport 3" decat system
340lph pump
Forge actuator
newage coilpack conversion
Alcatek + toucan
Peiterberg 3 port boost solenoid (new)

The problem I'm having is holding boost. On the original FP actuator power will peak at 1.4 bar at just over 4000rpm but then instantly drop and hold 1 bar for the rest of the rev range. On dyno we held the wastegate shut and it instantly hit 1.9 bar and held. Therefore we diagnosed a weak actuator.

Fitted the forge with a 1.2 bar spring, expecting it to hold boost but it does the same, despite testing with a mityvac and the wastegate opening at 1.2 bar. Turbo spools very quickly around 3k and peaks at 1.4 bar around 4200 then instantly drops to 1 bar.

We decided to run some mechanical tests by bypassing the 3 port and running spring pressure from the housing to the actuator. In theory we were hoping this would therefore make 1.2 bar steady until forcing the spring but it only holds 0.7 bar.

I do have 1.5 and 1.65 bar springs that I can try but I cant get my head round why its not making a base 1.2 bar. Only thing thats been suggested is the fact its a small exhaust housing mated to a large inducer/cold side so its spooling and surging more than the spring can cope with. This is where i'm not sure because running via the 3 port should vent off the excess til it makes 1.5 bar as per the map.

The whole system has been pressure tested for boost leaks and is all good so I'm wondering if theres anything else that could cause the valley of death? I've read various threads/posts about this type of issue but most relate to restrictor pill being missing the wrong size etc.

Its got me :wonder:

Perhaps this is an elementary question but when you installed the 3 port, did you replace the tubing that contains the boost pill?

Sorry nothing to contribute but would like to know how this is fixed...! Having a similar issue on Evo!

3 ports dont require a pill mate BUT I am going to try one while doing some testing to see if it makes a difference