AW: [Biturbos4] All causes to turbo failures known?

[Josef Hebenstreit]

Hi all

Just to give you a short reminder. 
My car: S4 2,7l 5V Biturbo, 265 hp, purchased 1999, chipped 1999 from
265 to 310 hp, only chip-tuning no modification on the motor and exhaust
system.
I had a dramatic turbo failure before eastern. Just in short terms:
accelerate -> big bang -> big cloud behind me -> my S4 lost all his oil
within seconds. This was very dramatic for me and the guy behind me!

Since then I am looking for the cause of the turbo failure. I am only
willing to get my car repaired, if I know the cause. 
The newest information from my garage I got is that the turbine wheel or
shaft has broken!

The argument from the garage is quite convincing:
S4 chipped (tuned) -> higher boost pressure -> higher turbo revolution
-> higher mechanical stress -> turbo charger breakdown

But then I am asking you: How can you drive 5 years and more then
160.000 km with the same turbo chargers? 
Is it normal wear or is it boost pressure overload which caused the
turbo failure? 
Does the boost pressure overload cause the seal to get leaky and give
the oil the free way down the exhaust system?

I have only a vage idee how boost control works:
When you accelerate you reach at about 2500 rpm the maximum boost
pressure which may be:
1) untuned: 0.8 bar relative to air pressure
2) tuned: 	1.2 bar relative to air pressure
without taking overboosting into account. May be overboosting adds
additional 0.5 bar.

To continue. Between 2500 rpm and 5000/6000 rpm the boost pressure stays
constant by regulating the wastegate (bypass valve) in the exhaust
system.
If you take the next gear the rotational speed falls down to let's say
4000 rpm and the bypass valve still keeps the boost pressure constant
without changing the turbo charger's turbine speed simply by getting
more exhaust gas to the turbine.

Is this a feasible explanation for the boost control? Can anybody verify
it?

Has anybody precise date for the boost control of the K03 turbo charger?

What is the maximum boost pressure both statically and dynamically I can
apply to the turbo charger without risking a turbo failure? Especially
the K03 turbo charger type.

>From the tuning point of view what is the recommended boost pressure?

If I have more precise data I can probably answer the question, whether
the boost pressure was to high all over the time.

Thanks,
Joe

___________


-----Ursprüngliche Nachricht-----
Von: Keman [mailto:keman at interwolf.net] 
Gesendet: Mittwoch, 5. Mai 2004 02:31
An: Josef Hebenstreit; biturbos4 at audifans.com
Betreff: Re: [Biturbos4] All causes to turbo failures known?

>> I have a S4 2,7 Biturbo/1999 and about 160.000 km. The chip-tuning
>> pushed the performance to 310hp instead of 265hp. Even though I put
>> my car to the service on a regular basis a turbo failure was
>> inevitable!

What kind of failure?

>> 1)    Wear

With infrequent oilchanges and poor quality oil, a turbos bearings can
certainly wear out. They don't have to, Gerret for example will tell you
that their turbos will outlast any engine you attach them to so long as
nothing breaks that controls the turbo.

>> 2)    Boost leak causes over-spinning of the turbo -> rpm to high ->
>> mechanical breakdown

IMO there's your #1 cause. I highly recommend a good boost gauge .. I've
had
one since 10k miles, even though my S4 still isn't chipped.

>> 3)    Defect or mal-functioning air-mass gauge causes ECU to read out
>> lower values for the air-mass flow -> ECU responses with lower fuel
>> injection -> lean fuel-air ratio -> high temperature on turbine of
>> the turbo -> thermal breakdown

The O2 sensors detect this though and that's how you know the MAF is
gone,
there are codes for reaching the adaptive limit rich set and a quick
glance
at the MAF under heavy throttle shows much less grams/second measured
than
what is really coming in.

>> 4)    Dump valve or bypass valve

When the bypass valves fail, they can cause an invisible boost leak
since it
can dump air back in front of the turbo, but still behind the MAF
sensor.
Aftermarket valves are a good idea.

The wastegate diaphram can rupture, overspinning the turbo. Rare, but
nothing is impossible. I've seen it once.

>> 5)    Insufficient oil supply for the turbo - oil lines clogged or
>> leaking

Crappy oil and infrequent changes, sure.

>> Normally the third case shouldn't be possible due to
>> exhaust-gas-temperature sensor sitting between turbo and lambda
>> sensor. ECU should detect high temperature (not only peak but also
>> average value) and protect the turbo using the waste gate and/or
>> switching over to limp program.
>>
>> What year of manufacture of the S4 has this exhaust-gas-temperature
>> sensor?

All biturbo S4's have two EGT sensors, one for each side just inches
after
the turbo outlet.

>> Is a K04-type turbo charger a solution to cope with the mechanical
>> and thermal breakdown, independent whether the car is tuned or not?

I honestly don't like the K04's. They're just a K03 housing with a
different
turbine wheel. I've compared them side by side now. There are some
ballbearing alternative turbos going around now that fit our cars which
are
a much more appropriate design and are more efficient. If you're going
through the hassle of replacing the turbos, you might as well go with
something else.

>> To you think the air-mass gauge is a problematic sensor? Do you have
>> a remedy?

It's problematic more often than not, but you'll get a check engine
light
before it becomes a serious issue.

- Keman

>> Thanks,
>> Joe