Doug,
Had a long talk with my friend at Garret now Honeywell and 
he gave me a few tips. Make sure you use a ceramic ball bearing turbo charger
and don't put any side load on the shaft itself. The biggest problem
they have is the whirl vibration mode of the shaft. The ceramic ball bearing 
shaft systems are much stiffer and that problem goes away more or less. 
He also confirmed the TO4 or the even larger T45 would be the right 
starting point.

He also gave me some contacts to get some turbine maps of flow verses efficiency 
and pressure ratio for existing products. The A/R to use for a given
radius is not necessarily the highest. Turbine blade angle is
varied to optimize it for flow and pressure ratio. One turbine design
is used with a wide range or A/R housings to adjust the turbo charger for
a given engine configuration. Therefore a change in the A/R makes small single 
digit changes in the efficiency.

Paul Lamar
 
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