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+--- Thread: Curiosity question for those familiar with VGT's (/showthread.php?tid=961)
Curiosity question for those familiar with VGT's - E300TSC - 12-18-2009
Guys, I was wondering if any of you have observed boost at idle or at low RPM's if you force the vanes to the full closed position.
I know on a 6.0 PSD, the vanes can be forced closed and it will spool up and generate boost at idle. Just wondering if it's possible here too.
RE: Curiosity question for those familiar with VGT's - MB TECH - 12-18-2009
I did with mine, but I have a mechanical control on the vanes, I only did it once and it made some odd noises and then blew the boost hose off the intake. But it did make boost at idle in park. Its probably not good for it though.
RE: Curiosity question for those familiar with VGT's - ForcedInduction - 12-18-2009
Mine will make about 1/4psi at idle, just enough to keep the needle off the peg.
Boost at idle is nice, but the performance is worse since the vanes restrict the exhaust so much. You have to tune and find the happy balance between spoolup, off the line torque and compressor surge.
RE: Curiosity question for those familiar with VGT's - E300TSC - 12-18-2009
So the vane control can't adjust fast enough to keep a balance between boost and back pressure? Or, does the back pressure spike before the control can make an adjustment?
RE: Curiosity question for those familiar with VGT's - tomnik - 12-19-2009
First there is something with an adjusting screw limiting the vane lever.
On servo controlled VTG there is no outer adjustment screw all is done by the electronic. On vac actuator control there is this screw. The VW guys report that this is turbo factory adjustment and will cause over peaks when not correct. I believe that I there is a garage method to roughly adjust this screw. Therefore a different in boost of some (100) mbar must be there at a fixed rpm and the vanes manually operated (open - closed).
On my VW TDI the vanes wait in (nearly) closed position at idle. In this case there is very low back pressure. Then according to pedal position different set boost values are given and the vane control reacts. The more you press the pedal the higher the set boost.
Lifting the foot after hard acceleration (now we have back pressure) the vanes open and slowly close as the car slows down.
It seems that the vanes try to adjust a certain boost value that might be approx. 0.05 or 0.1 bar. This might also be the value (set boost) when the car idles.
The observation in my TDI (boost and control vac pressure monitored) correspond with a VW technical information about the VTG (in German and paper only). They write also about the active back pressure reduction for better economy as a further improvement of the VTG compared to wastegates.
Tom
RE: Curiosity question for those familiar with VGT's - ForcedInduction - 12-19-2009
(12-18-2009, 10:27 PM)E300TSC Or, does the back pressure spike before the control can make an adjustment?
Backpressure raises until the desired boost is reached. The vanes are controlling the velocity at which the gas hits the turbine, backpressure is a natural result of that restriction. Its the same idea as putting your thumb over the end of a garden hose, the pressure inside the hose goes up the faster you make the water shoot.
The vanes can be controlled with exhaust pressure as a control input variable, but no OEM does this that I am aware of (They set the boost maps based on data collected during R&D and the computer sticks to it). I personally don't worry about it much, compressor surge during acceleration is much more of an issue to me.
(12-19-2009, 12:26 AM)tomnik First there is something with an adjusting screw limiting the vane lever.Right. It limits the minimum angle the vanes can close.
Quote:The VW guys report that this is turbo factory adjustment and will cause over peaks when not correct.Thats due to the computer's reactive control system. Installing a boost valve (what my system is entirely based on) eliminates those spikes since it reacts to limit boost much faster than the computer and solenoid valve (N75).
RE: Curiosity question for those familiar with VGT's - E300TSC - 12-19-2009
Thanks for the info guys, that helps a lot!