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How the turbo works
Here's an ASCII-fied drawing of how the turbo control works on my 86 5000CS TQ.
I believe this is a representation of how most computer controlled Audi's
operate. Some of this is in the Bently book, but a lot was discovered by
tracing out the vacuum lines and pulling hair. I needed this information in
order to "intelligently" modify the boost control. *Beware* There's lots going
on here, so don't change anything unless you understand what you're doing. You
can obtain uncontrolled full boost and blow your engine by removing just one
wrong vacuum hose.
Theory
------
The wastegate contains a diaphram and a spring to allow control of boost. The
spring is a static/failsafe boost control mechanism, while the upper portion of
the diaphram allows "tweaks" by the computer. The wastegate spring "balances"
against pressurized air exiting from the turbo. This is the only boost control
non-computer controlled cars have. The computer monitors boost on the
downstream side of throttle. If the computer thinks the boost is too low, it
switches a valve (frequency valve in Audi lingo) to allow boost pressure to be
applied on top of the wastegate diaphram. This has the effect of making the
wastegate spring appear stronger. If the boost gets too high, the computer
switches the frequency valve to vacuum and lowers the pressure on top of the
wastegate diaphram. This has the effect of making the wastegate spring appear
weaker. Where do you get vacuum when a turbo engine is running at full boost?
Only one place; the air intake to the turbo (just like a vacuum cleaner).
It's important to note that there is pre-throttle boost, and post throttle
boost. Application of the correct "boost" are the proper place is essential to
safe operation. Have fun with this...
Wastegate spring
/ computer controlled valve
WASTEGATE / +-----+/ C = A or C = B
+-------====---/----+ | +------------------------+
| ---- / | <-> | A +----------------------+^|boost at
|push| ---- / +-------+ | <--- |^| open
|down| ---- +-------+C +----+ |^|throttle
| \ / ---- | | B +--+v| +------------.-+ +------
| v ---- | +-----+ | | | > > > > / to intake>
|_-----------------_| |v| | ^ +------.------ -----
/| || | | | | ^ | | |
diaphram/ | || +---------------------+ ^ | throttle +------>
| || < < < < < < < < < < < ^ | to sensor in
| | || ^ +---------------------+ ^ | computer
| | || | | | | | ^ |
+----- v || | up --+ constant |v| | ^ | Throttle valve
down || less boost vacuum | | | ^ |
more boost|| \ |v| | ^ |
|| || | | | ^ |Boosted air outlet
/|| +----------||--------------+ ^ |
/ || | > > > > ||> > > > > > > > ^ |
boost regulator / || | ^ +------||------------------+
+--------+ ^ + || | |
| | intercooler |v|
| TURBO | | |
\ | "AIR | |v|
-------------+ PUMP" | | |
AIR INTAKE> > > > > | constant |v|
---------+ +--------------+ vacuum | |
/ |^| \ |v|
|^+--------------------------------+ |
+------------------------------------+
...bill
DEC
Maynard, MA