RE: K24 and boost

[<isham-research.freeserve.co.uk@pop.pol.net.uk>]

> WG springs on 10vt.  M. Peterson hasn't presented this concept quite right.
> The WG frequency valve either does nothing, or adds boost pressure above
> spring pressure to the top of the WG.

Even that isn't quite right.  The WGFV switches between what passes for
vacuum on the car (from the turbo inlet) and 'boost' - which may come
from the unthrottled boost chamber cast in the inlet manifold or from
the inlet manifold itself (throttled) depending on the exact level of
the engine.  Later engines use throttled boost to the WGFV - I haven't
yet settled on an explanation for Audi's change.

Anyway, when it's applying 'vacuum' - the force tends to lift the
wastegate.  On the MAC12D and MAC14, the WGFV operates whenever the
WoT switch is closed, although the duty cycle varies considerably with
the actual boost level.  A diode light (I use a design that Orin came
up with) is a 100% method of testing WoT function.

Of equal importance - does the ECU actually _abandon_ the WGFV (leave it
switched to vacuum) at any point?  This phenomenon is common close to
redline on shimmed wastegates - it's a good indicator of too much shim.

What is surprising, the first time you see it, is the rate at which the
valve runs - several Hz.  With a very lightweight mechanical pressure
gauge in the line from the WGFV to the top of the wastegate, the
indicator needle just becomes a blur.  I had expected a quite crude
on/off switch.

--
 Phil Payne
 UK Audi quattro Owners Club
 Phone +44 7785 302803  Fax +44 870 0883933