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So many valves...

GregC Oct 6, 2004

  1. GregC

    GregC Member

    Can someone explain which valves are which and where I can find them on my 1.8TQ A4 B5?

    The BOV I have found stuck under the engine and I believe recirculates un-needed boost back through the system to keep the turbo moving in when I change gear or throttle back.

    The wastegate - allows the turbo to vent excess pressure outside the system?

    An N75? Every other thread seems to suggest changing an N75 - what is it and where is it?

    If there are other boost/turbo related valves anyone could tell me about it would be appreciated.

    Thanks. /ubbthreads/images/graemlins/smile.gif
  2. dunk

    dunk Member

    the wastegate physically controls the boost pressure by allowing excess exhaust gas to bypass the turbo resulting in less boost pressure, and it in turn is controlled by the n75 valve - this is in effect an electronic solenoid, which in turn controls the behaviour of the wastegate, and gives the ecu the electronic control of the wastegete.

    the wastegate has a weak spring and without the n75 valve a low turbo boost of usually 3 or 4 psi will be mechanically maintained by the wastegate leaking excess exhaust gases past the turbo.

    the n75 valve fools the wastegate into thinking the pressure is lower than it really is, by diverting some of the existing boost pressure away from the wastegate and back into the inlet system - the wastegate therefore sees a lower boost pressure than really exists and stays shut, thus allowing a much higher boost pressure to be generated before the wastegate maintains boost pressure.

    different types of n75 valve have different ranges of action, and so for any given range of electronic signal from the ecu result in more or less boost, or more or less agressive rise in boost, all depending on the behaviour of that particular n75 valve

    changing n75 valve type will change either the ultimate boost, the way that the boost rises, the overboost response to full throttle or any combination of these things.
  3. GregC

    GregC Member

    Cheers Dunk - great explination.

    When I first got my big turbo (well bigger turbo....) I was getting a wavering power delivery when the turbo started - suppose this could be the N75 not working properly (kind of stole this assumption from some other forums).

    Any idea where I can find out what the options are for N75 valves and where I can get upgraded ones. I imagine I have to be careful about blowing something by the wrong valve in.


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