Here's the situation: The most boost I'm able to get out of the 505 wagon is about 24psi. It goes very well at this level, but blows a fair bit of black smoke. From low revs and zero boost, when you put your foot down, it blows a little smoke which quickly disappears as boost builds, running completely clean (to the naked eye) from about 5psi up to 18psi. Beyond this however, it starts to blow smoke again, and by the 24psi limit it's belching.
So after some discussion with some turbo diesel experts, the conclusion that we've come to is that after 18psi, the wastegate spring is not strong enough to hold the wastegate shut, and the pressure in the manifold and turbine housing is escaping past the wastegate, so the amount of fuel required to increase the boost above 18psi increases exponentially, causing the smoke.
Now - I have a setup on the wagon whereby I have reduced the wastegate tension from the factory 7psi to about 4 psi, to minimise pumping losses when cruising on the highway and increase efficiency. I have a valve on the throttle so that when you get to about 50% WOT, the wastegate hose is 'disconnected' from the manifold boost and therefore goes into an 'overboost' condition - effectively unrestricted boost, limited only by the amount of fuel injected. And apparently now, by the strength of the spring in the wastegate.
So, I have possibly come up with a solution.
Can anyone think of any reason why I couldn't connect the vacuum source to the wastegate above the 50% WOT mark? The theory here is that applying a vacuum (in this car, approximately -13psi) to the wastegate, it will effectively add an extra 13psi 'spring strength' to the wastegate actuator.
The only thing I can think of is that since the wastegate is not designed to operate this way, I might damage a diaphragm or something. But I also think they should be fairly robust.
Has anyone heard of this being done before? Google has been of no help to me on this one!