[BiturboS4] RE: boost & engine load

David goodson at menta.net
Wed Nov 13 20:43:58 EST 2002

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.

-----Mensaje original-----
De: biturbos4-admin at audifans.com [mailto:biturbos4-admin at audifans.com]En
nombre de Edwards, Dave
Enviado el: Wednesday, November 13, 2002 10:44 AM
Para: 'biturbos4 at audifans.com'
Asunto: [BiturboS4] RE: boost & engine load

Further to what Phil has written, if you rev the engine whilst in neutral
there is no boost.

Also, I have the Defi HUD gauge which as well as a digital readout has an
analogue display which is effectively a set of lights in an oval line around
the outside of the gauge. This is quite effective in showing the magnitude
of boost change.

When I am accelerating hard, getting over 1bar boost, this line is near its
end. Then when I change up a gear and shove the gas pedal again, I see the
line rapidly disappear to vacuum and then grow all the way back to above

I admit that this surprised me the first time I saw it. I had thought that
when the throttle is closed when changing gear, all that happened was that
the DVs opened. This can't be the case. If it were then as soon as I opened
the throttle again, the pressure would build up much more quickly than it

I assume that when the engine is not loaded, the wastegates are opened?

Anyway its changed my driving style when I want to go fast: Firstly I try to
stay in a lower gear (I have a bad habit of lazily driving around in 5th and
6th), and secondly I try to avoid changing gear.



Message: 11
Date: Tue, 12 Nov 2002 08:16:25 -0800 (PST)
From: Philip Pace <pjpace at yahoo.com>
Subject: RE: [BiturboS4] no siren
To: David <goodson at menta.net>,
	Michael Kuriger <michael at isuzu-mods.com>,
	Bill Neilsen <wneils at hotmail.com>
Cc: biturbos4 at audifans.com

I'll confirm it. I have a digital boost gauge and when you hit the throttle
out of gear, it will go all the way to 0bar (no boost, no vacuum). I've done
this a lot, as it's fun to see the car get perfectly to 0 and then drop back
down (ok
- I sit in traffic a lot).
P.S. Were you going to do some S4 website in Spain/Spanish? If so, how's
that plan going? If not, you know who that was?

--- David <goodson at menta.net> wrote:
> Really ? I don't understand that. Could someone with a boost gauge
> confirm that "there is no boost without load" ?. I thought there was,
> because in the workshop when they check boost pressure, they don't use
> any rolling-road. David. 1999 S4 Avant ABT - Barcelona, Spain.
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