by dr20t » Tue Apr 19, 2016 12:12 pm
Viv - i hope you dont mind but I'll respond here instead of pm as a future guide to anyone else
The 3 port solenoid will assist in holding / regulating boost, so a worthwhile investment.
I've never been a fan of these hybrid vf34/38 turbos. The main reason being the vf38 rear twin scroll housing is quite small and therefore restrictive. This is fine for the early spooling vf38 with the titanium wheel, but as any stock preface GT owner will tell you, they run out of efficiency very early. This is primarily due to the turbine housing size causing back pressure.
In saying that, I'm prepared to suggest its one or more of the following issues:
1. Pre turbo backpressure due to the small twin scroll housing on the vf38. This could be choking the vf34 wheel, creating more backpressure that limits ignition and valve timing advance.
2. Other tune parameters not being optimized (turbo dynamics, wg duty cycle etc). Whether in isolation or as a result of the above two issues (limitation of 2 port and turbo back pressure), it will limit your ability to spool and rev.
It is very unlikely that your mid pipe is the issue, as your back pressure limitation will be in the turbine housing first, before the mid pipe. If you were talking about a larger turbo / turbine housing with lower backpressure, then the mid pipe may become a restriction at that point. It's the same as having a 2 inch hose on the end of a straw and saying if you went to a 3 inch pipe instead of 2 inch, that it would help your straw breathe better. The straw is still the bottleneck, and the vf38 rear housing is analogous to the straw in this example.
The fact that its alot laggier without the top end that you should have with the vf34 tells me its restricted.
Take some logs and post them up here for further discussion. Happy to help out if need be.