Oookaaaay (E.T. says very hesitantly) after trolling thru heaps of sites trying to sort shyte from clay I believe I have the adjustments sorted in my head. My understandings below were gleaned mainly from the Factory Manual, Garret and other VNT manuf's, several forums and Chaz's website
Boost Pressure
Easy bit first. Ex factory the ZD30 Boost pressure is controlled solely by the ECU electrically sensing the boost pressure at the intercooler outlet plenum and modulating a vacuum signal via the boost control solenoid to the turbo actuator which then mechanically increases/reduces the angle of the vanes directing gases into the turbine.
This engine control process is what the Dawes Valve partially replaces by limiting max boost pressure and, hopefully for those who fit them, eliminating overboost. Note that as Chaz says in his site, the Dawes alone will not fully control proportional boost settings just maximum.
Turbo response (often also referred to in forums as 'spool up'. I'm not going to argue other than say the manuf's of VNT's seem to distinctly differentiate the two terms)
Adjusting the actuator rod length (if your game). Adjusting the rod length doesn't change the max boost pressure and it also doesn't change the max spool up rate as such. What it does do is make the actuator more sensitive to the modulated vacuum signal from the boost control valve.
Shorter rod length means the same actuator movement for the same ECU commands will move the vanes over a larger angle earlier up until you hit the grub screw stop.
Now the important bit... it appears that this ONLY affects the turbo when the ECU is modulating below max boost. IE if you 'metal the sucker' everything goes max and the vane angle is once again limited by the stop screw. Intermediate settings with an incorrect rod length however and the turbo could possibly hunt like a bitch
It is the turbo response from the ECU via the bost control solenoid that the Needle Valve will affect and here is the interesting thing.
The needle valve adjustment can only make the turbo less responsive not more. Again, using Chaz's illustrations the Boost at a given RPM is often significantly lower with a needle than when controlled from the ECU.
If you look at typical VNT curves they show much higher boost available at lower engine RPM which is exactly why VNT turbo's are used in almost every modern small diesel. Also I discovered that the fluctuations observed in VNT curves are usually attributed to EGR control modulations not the VNT turbo operation itself.
Adjusting the grub screw. This adjustment adjusts the maximum vane angle available to the turbo nozzle. This affects the maximum spool up rate as it allows more exhaust gas to be directed away from the bypass plenum and into the turbine by the variable vanes as they can open further. Larger vane angle, more gas, faster spool.
Now the important bit again... neither the Dawes nor Needle valves can affect this setting as it is 100% mechanical.
Bottom line... no wonder Dawes and Needle mods are so popular rather than dicking with the actuator on the Di's.
CRD's however with their higher factory boost pressures (arguably up to 22 PSI manifold), different EGR plumbing and different ECU maps maybe not needed so much.
In no way am I saying "Do" or "Don't Do" any mods etc. I am simply saying I like to understand what is happening and previous posters caused me to rethink a few things and very rightly so on their part.
Also like any subject I could still be rrr... rrrrrrr..... rrroooonnn... incorrect
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