[BiturboS4] RE: boost & engine load

Edwards, Dave Dave_Edwards at iOra.com
Fri Nov 15 08:56:56 EST 2002

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I would guess that the reason is simply to avoid unnecessary stress on the
engine: If there is no load then there is no need to provide boost. I
suppose if the boost were maintained then there would be a risk of engine
surges during gearchange before the throttle actually closes.

However you are correct - the ECU controls the wastegates and the DVs. This
is different from some other cars where the DVs are simply controlled by the
pressure in the inlet manifold.

Have you had your "new" turbos fitted now?


 -----Original Message-----
From: Michael Kuriger [mailto:michael at isuzu-mods.com]
that exhaust you can feel with your hand at the tail pipe is not nearly
enough to spin up a turbo and create boost.  if you were to remove the turbo
from your exhaust manifold, and hold your hand here while under full
throttle and undel a load you would lose your hand.

the diverter valves open when there is a vacuum (which only happens at
cruise or decel) if a diverter valve fails to open it will not have much if
any affect on boost under load.  It's purpose is to release the pressure
inside the intake when you let off the gas.  otherwise this pressure has
nowhere to go except back through the turbo backwards.


David wrote:

I still don't see any mecanical reason for the boost to stop when in
neutral, or when there is no load. Try to put your hand at the output of the
exhaust when you are accelerating in neutral (I know, you should have a
really long arm, or a friend to help you). You will feel a big increase of
the gas flow. This flow is supposed to generate, through the turbo, the
boost. I guess the reason is electronic : both wastegate valves and diverter
valves are controled by the Motronic system.
Quoting the "internal-use-only 2.7t self-study program", :
"The charge pressure of both exhaust gas turbochargers is controlled via the
common charge pressure control valve N75 (...).
The Motronic ME 7.1 activates the solenoid valve for charge pressure control
N75 and regulates the charge pressure via the wastegate".
Actually I do remember that when I had boost problems a few months ago, my
ABT tuner in Paris told me to have the N75 checked (and many other things,
of course).
And about the diverter valves, the manual says :
"Divert air control in overrun :
The Motronic ME 7.1 activates the electric divert air valve for the
turbocharger and open the pneumatic divert air valves using this vacuum".
Doesn't it sound more realistic ?
1999 S4 Avant ABT
Barcelona - Spain.

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