Folks,
Few bits on turbos:

- All turbine wheels made in at least the last 30 years will be cast
Inconel unless otherwise stated, regardless of manufacturer. No
worries on melting stuff!

- Most automotive turbine housings are cast iron (obviously), though
some are made of higher temp stuff than others. The good higher temp
ones are high sil. moly or D5 Ni-resist, assume low grade D3 for
off-the-shelf after market. For some Garrett turbos after market
automotive companies have made stainless turbine housings of various
A/Rs, this is the way to go for a high duty cycle rotary IMO. Don't use
V-bands if you can help it, they are difficult to seal and are
expensive. Do use high temp bolts (B16 is terrific stuff!)

- First thing to worry about when matching is the compressor, it is
the most performance sensitive (turbines are like big paddle wheels
compared to compressors). Special consideration will have to be taken
for the high Pressure Ratios (PR) experienced at altitude however, so
you're going to have to choose where you want efficiency (take-off or
cruise)

- Rotaries are good at spooling turbos, but don't assume they are TOO
good when choosing a turbine A/R (you will end up with a supercharger
like boost curve. The Hitachi turbos used on s5 RX7s do use the
waste gate like any proper match, just after torque peak. All matching
work is validated on engine, so expect to try a few A/Rs before getting
it dialed in where you want (I agree with wanting a large AR for
pressure equalizing, you will have plenty of turbine power with the
expansion ratios you can get up there! Especially if you are not
concerned with take off power)

- After market waste-gates are not terribly reliable, using an
adjustable built-in is the way to go for A/C, but a standard one may
be difficult to keep closed at altitude unless you install a higher
rate spring (you can build an adjustable pressure actuator with a
turnbuckle and some clamp plates, I'll try to find a link to an
example)

- TEST YOUR LUBE SYSTEM!!! Make 100% sure it flows oil quickly and at
the rate specified by the manufacturer, poor oil supply (& dirty oil)
is a major killer of turbos. Return lines should be above the sump!

- Water cooling is mostly chosen in non-A/C applications to prevent
oil coking during a hot-shutdown, but I agree that it should be
considered mandatory for a high duty cycle application (with tight
packaging). Return lines should be as high as possible in the cooling
system to aide thermal siphoning

- Typically ball bearings are only chosen when transient performance
is crucial, as journal bearings will get you 80% of the performance
with better steady state efficiency, but ball bearings are better with
low oil conditions and can handle higher loads. IMO unless you have
the $$ burning a hole in your pocket standard bearings will do just
fine, and they have MANY more hours of design history behind them.
(eg. To my knowledge no production engine uses a ball bearing turbo
except the Garrett on the new Ford Scorpion engine.)

I saw someone ask about cooling system additives. Redline Water Wetter
is well thought of.
FWIW unused oil has a thermal conductivity of around 0.1 W/(m-K),
water is around 0.6, ethylene glycol around 0.2

Cheers,
Zach K
Guy that works in the industry

Thanks Zach. Some of the best info we have received so far.

Paul Lamar