> 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
 
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