I've noticed that a few of the boost curves that have been posted show boost overshoots and unstable boost plateaus with tunes that use the factory N75 as the boost control. I was also reading on Plautos's build thread (page 1 I think) how he used a series flow control with his N75 to take out boost irregularities, and after reading some of Julian Edgars stuff have a few questions.

AutoSpeed - Shooting the Overboost
AutoSpeed - The Audi's DIY Boost Control - Part 1
AutoSpeed - The Audi's DIY Boost Control - Part 2

Does the factory system run closed loop N75/boost control? ie if you make manual changes to how the N75 works eg with a pressure regulator or flow controller, does the MAP sensor feed the resultant boost changes back to the ECU which will then try to operate the N75 accordingly to pull boost back to where it expects to see it?

OR

is it open loop, and the MAP will see these boost changes and input back to the ECU which might adjust ignition or fuelling within certain parameters ,but not N75 function?

If I do go down the path of further breathing mods, then I expect that I may run into boost spikes, overshoots etc and have always been interested in using pneumatics with boost control. I just first up need to know how the ecu runs the N75 because I wouldn't want a situation where the ECU is just fighting any mechanical changes that have been made to how boost is controlled - which would be interesting!!

thanks

sam