More Boost info

[scott.john.mockry@bangate1.tek.com]

Some additional considerations when you divide down the boost signal on the 86-88 1/2
5000 CS T's and Q's. I used an op amp and  a divider network to reduce down the boost
 sensor voltage on my previous 86 5000CS Turbo (FWD). I found a spring for my WQ 
at a local hardware store that allowed higher boost levels. 

The dynamic boost control (via the wastegate solenoid controlled by varying the dutycycle)
may not function as originally intended depending on how well you scale down the boost signal
with the zener or resistors etc. This may not be all that bad, but .....if the stronger wastegate 
spring allows a slightly  higher boost  level than what is expected by the ECU boost map at 
that particular RPM/TEMP etc the wastegate solenoid won't be controlling the boost. 

The boost will be controlled only by the wastegate spring tension. Normally a weaker spring 
is used and this is combined with the extra pressure applied on top of the wastegate 
diaphram to dial in the total boost at a particular RPM/TEMP etc. The boost map graph I 
have for the 86-88 1/2 5000CS TQ shows the two different boost output pressures.
One uses just the mechanical spring and the other using the combination of the spring and 
the additional  pressure applied to the top of the wastegate diaphram. This combo boost level
only comes into play when the throttle is opened past the 2/3 mark and the "full throttle"
switch is closed.

I noticed this behavior on my moded car when I looked at the wastegate control signal
(duty cycle) on an oscilloscope while under acceleration. The boost was not being controlled
by the wastegate solenoid at all. I never took the time to get the divider network setup 
correctly. The car ran great but the boost came on like a Saturn V rocket!

I used the op amp to isolate the signal going to the stock gauge so it would read correctly
 (although with slow response and questionable accuracy). An analog gauge is more fun to 
watch as that boost increases FAST! I also used a comparator circuit to keep the overboost 
shutoff system intact that allowed the boost to rise up to around 12psi with a 14psi 
shutoff value. When it sensed 14psi it would feed the higher boost sensor voltage 
(not divided down) to the ECU input and then shut down the fuel pump. I did not spend 
a lot of time putting this circuit together and likely could have gotten it dialed in 
better to allow the factory boost map control  to take place but with higher boost levels.

The SAE article # 860103 entitled "The Third Generation Turbocharged Engine for the 
Audi 5000CS and 5000CS Quattro" written by D. Stock at Audi AG, Neckarsulm, Germany
has some nice descriptions on how the whole system works. It  shows the difference
in temperature at the turbo bearings with and without water cooling. It has a graph 
showing temperature of the boost air versus vehicle speed (40-240 km/h). There is a
graph showing the Boost air cooling effect and IC pressure loss versus RPM. It mentions that
the turbocharger used is a K 26-2664-GA 6.95 unit from KKK. It also talks about 
the two timing maps are stored in memories with 16 X 16 memory addresses and that they
are complemented by specific ignition timing characteristics or values for cold starting,
warm up, idling and overrun (deceleration). (Note: The factory service literature
"1986 New Model introduction" also describes the boost control system)

With help from RDH the search for more boost continues on my 89 200TQ........... 

Scott M.