DaveLeonard wrote:

Paul, I am very inclined to agree that there are probably some other
factors
involved.  But I think you used the wrong RPM in the above
calculation.  All
the turbo maps I could find for a similar sized turbo in similar
conditions
seemed to indicated that the turbine would be turning closer to
70,000 RPM
under those conditions.

Dave Leonard

Well perhaps but that is a small turbine housing.
If it is blade bending resonance that is not going to change the
situation.

All that will do is reduce further the already small amount of stress
due to
centrifugal force alone. That in turn also increases the likelihood
of a
blade bending resonant condition as the cause of failure. The main
forcing
function for that is still the exhaust pulses at 12,000 per minute or
200 Hertz.

If UCLA has a shake table we could put the turbine on it and see
what the blade resonant frequency is. Heck you could tap it with a
small
hammer and use a micro phone hooked to a storage oscilloscope and take
a picture.

Paul Lamar

     Just checked my Turbonetics catalog and a T3 "60" trim which is a
similar size
     to the stock 2nd gen turbo goes to 154,700 max RPM.

     Did you have any small leaks in the turbo intake manifold that
you know
     about? Unlikely but that would increase the turbine RPM. Also if
the compressor
     is undersize for the amount of engine combustion air required
that too will
     drastically increase the turbine RPM.

     As I said however I don't believe this is the root cause of
failure.

     Paul Lamar

Tracy Crook wrote:
     Not a significant factor in this discussion but for the
discussion of any centrifugal pump (air, water, whatever) it should be
remembered that the load on the pump increases with mass
     flow.  In other words, the turbo would slow down if there were a
leak in the intake manifold down stream of the turbo.  Load on a
centrifugal pump is at minimum when the backpressure is
     highest.  Sounds counter-intuitive, but true.

     Tracy Crook

I do agree that a high back pressure on the compressor such as a closed
throttle would cause the
the turbo to speed up if the HP going into the turbine was constant.

BUT why would the back pressure be higher with a leak. I think it would
be lower.
The turbo might slow down because the turbine is getting less HP
because the boost is down.
We are working with a complex closed loop system here. Less intake
pressure less HP. Less HP lower
engine RPM. Less engine RPM less requirement for combustion air.

Paul Lamar

The simple demonstration of Tracy's point is your vacuum [cleaner].  Start it up,
note the tone of the motor.  Plug the downstream leak (if you have a
vacuum that has an output like mine) and note the increase in rpm.  The
motor is doing less work so it speeds up.  Once you unplug the hose, the
air is now free to move again and the motor slows down as it begins to
work.

My question is this.  Is it possible to overspeed a turbo where the
compressor side is just pumping out into the atmosphere, in other words,
a high volume, low pressure situation?

Chris Cosman

The vacuum cleaner example does not apply because there is no positive 
feed back to the electric motor power input from the air output.

In other words the electric motor HP could be the same. Torque is lower but
RPM is higher so HP could be the same.

It would if the turbine was small enough to recover enough HP from
the exhaust of a normally aspirated engine.

Paul Lamar
 
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