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Overboost?- Long

Boost meisters,

Here are my thoughts regarding the Overboost discussion. Most of this
relates specifically to MAC11/12/14 ECU operation as used in the 10V Audi

I define "Overboost" as any boost level that is higher than what is
programmed inside the ECU boost maps, or any boost level that is higher
than what the Waste Gate spring pressure will provide on its own. Whether
it lasts for 1 second, 2 seconds, or 15 seconds is not important with "my"

For Example: 
If you program a MAC11/12/14 ECU to have a boost map with 1.8 bar boost
values, and you install a 1.8 bar Waste Gate spring, and you have a
functioning boost control system (i.e. waste gate solenoid, throttle switch
etc) under certain conditions, you will see an "Overboost" condition where
the boost level produced will overshoot the programmed 1.8 bar value. On my
1987 5000TQ and my 1989 200TQ this "Overboost will typically go up to ~2.05
bar briefly, and then it will come back down to ~1.8 bar.

The reason for this "Overboost" is the Feedback Control system which is
used to control the boost. This consists of the Waste Gate and Spring, the
waste gate solenoid and hoses, the throttle switch, the ECU boost control
system with its internal boost pressure sensor and the flow characteristics
of the engine/turbo/exhaust and the vehicle weight.

Electrical Engineering folks typically work with amplifiers with negative
Feedback that can be set up to have a specific output response assuming a
step voltage input. An over-damped response, a critically damped response,
and an under-damped response.  

The following boost profile graph at the top of page, was captured with a
pressure transducer and Oscilloscope. It shows both the under-damped and
over-damped response as exhibited by a turbo charger boost control system
on a 1989 200TQ with a modified 1.8 bar ECU and 1.8 bar WG spring.


If you disconnect the upper hose on the waste gate, you get a over-damped
response, the boost ramps up slower with a rounded profile on the leading

If you allow the ECU to "attempt" to control the boost, and you have a
specific waste gate spring tension, and other specific engine/turbo/exhaust
parameters, you can see the under damped response, (~0.6 damping factor)
where the boost actually overshoots the intended set value of 1.8 bar and
briefly rises above up to 2.0 bar, but then is damped out over time and
returns to the 1.8 bar set value. 

This "Overboost" response will typically happen on the Mac14 equipped cars
when you mash down the throttle when the engine RPM is 2500-3000, on the
earlier Mac11 cars this "usually" happens above 3000 RPM due to the
different boost output profile of the turbo. This is more noticeable in the
lower gears, 1st through 3rd, but can also happen in 4th/5th. The weight of
the car, the type of turbo, and exhaust used etc. can alter this
"Overboost" behavior.

If you have an analog boost gauge, this "Overboost" can easily been
observed as the boost needle will initially swing past 1.8 bar (~12psi) and
go up to 2.05 bar (15.2psi) and then swing back down to 1.8 bar

The Mac11/12/14 ECU code is actually setup to allow this brief "Overboost"
for a short length of time depending on the air temp before it shuts down
the fuel pump relay. If I remember correctly, this is only ~1.0 second.
(Orin can add more details)

Normally the fuel pump cutout will occur when the boost reaches ~1.95 bar
on these modified 1.8 bar ECU's but the "Overboost" time out, allows some

I have experimented a bit with increasing this allowed "Overboost" time
"slightly" during one test, where my car was hitting the overboost fuel
pump cutout, after installing a bypass valve, which allowed the boost to
shoot up quicker past the 1.8 bar set point during a 3rd/4th gear shift.
For safety purposes, you do not want to increase this time too much. A
better solution would be to use a 2.5 bar pressure sensor for an increased
control range.

A slight "Overboost" can be seen on these MAC11/14 vehicles running stock
boost levels, (1.4 bar) and will also show the same under damped response,
with the ECU controlled boost, and the over damped response if the upper WG
hose is disconnected. 

This "Overboost" is a nice feature as it gives the car a shot of additional
torque to help move it down the road.

The SAE Technical paper #860103 "The Third Generation Turbocharged Engine
for the Audi 5000CS and 5000CS Quattro" alludes to this "Overboost" mode
with the following passage.

"The dynamic pattern of boost pressure build-up can be influenced by the
boost pressure control system. For rapid acceleration, "excess boost" can
be obtained for a limited period."

This SAE paper also shows a boost profile graph with the two boost
responses, one response with only the mechanical Waste Gate, and the other
response with the ECU controlled boost system.

The "Overboost" as found and so named in the 1992> Audi S4/S6 allows ~15
seconds of boost up to 2.15 bar, but in this case the boost is not
exceeding the range of the internal ECU Pressure Sensor, as the S4/S6 ECU's
already have a 2.5 bar Pressure sensor installed, so the ECU is allowing
boost well within its control range, but only for a specific time to
increase engine/drive train longevity.

Some of my friends with 944 Turbo track cars run a particular tuner ECU mod
which includes installing a orifice (carb jet) in the boost line running to
the waste gate which delays the opening of the WG and causes an "Overboost"
condition where the boost shoots way up to 18psi briefly before coming down
to the 12psi set values. Assuming you don't have headgasket problems, this
brief "Overboost" gives an amazing amount of torque.

Scott Mockry