5KT waste gate question...

[Nate Stuart]

The boost is controlled both by the ECU, and by simple physics.

The wastegate has two chambers with a diaphram seperating the two. The
diaphram moves up and down in accordance with the pressure differences
on its two faces. This diaphram is connected to a valve, that controlls
the amount of exhaust gases to vent around the turbo. The further open
this valve is the more gasses escape, and less boost is produced.

So the determining factor in controlling the boost is controlling the
opposing pressures on that diaphram. The most basic meathod of doing
this is with the wastegate spring. This is placed in the 'low' pressure
side of the wastegate, and is pressing down on the wastegate diaphram.
This causes the wastegate to remain shut until the boost pressure on the
other side of the diaphram overcomes the spring load, then the diaphram
will move upward and open the wastegate, lowering boost. The next
meathod Audi uses is to assist the spring load with additional air
pressure. It does this via the wastegate frequency valve. What this
valve does is allow boost pressure into the top of the wastegate when
the ECU signals it to. This raises the pressure in the upper chamber,
and reduces the pressure difference across the diaphram, keeping the
wastegate closed. When the ECU says that the engine has reached it's max
boost levels it dumps the pressurized air that it was feeding the
wastegate back into the intake stream, and the wastegate valve opens.

Hope some of this helped.
-Nate
'89 90tq
www.newtsplace.com/90tq

Bill Phelps wrote:

>Can anyone give me a quick and dirty explination of how boost is controlled
>in the 5KT? Is it tied into knock or simply boost pressure or a combination
>of both. The Bentley is a bit vauge on how exactly it works. Any idea of
>what the base boost is in absence of the control of the frequency valve (ie.
>just the spring)?
>
>Thanks.
>
>
>