This one is interesting. It has only been a few years that these
papers have been declassified and placed on the internet.

420.Turner, L Richard Desmon, Leland G Performance of blowdown turbine driven by
 exhaust gas of nine-cylinder radial engine naca-report-786  1944 

Engine power at sea level was only decreased by 1% while the power out
of the turbine was an additional 9% of engine power.

The altitude performance is stunning! At an ambient  pressure of 
10" Mg the turbine is generating about 115 free HP with no increase
in fuel burn. The basic engine has lost no HP. This is a 22% gain
in HP with no increase in fuel burn. That means the BSFC of a 
current Mazda rotary engine would go down to about .37 from 
about .47. Even better on the RX8 engine.

The total HP of a current Mazda 250HP RX8 rotary with this feature 
would be about 305 HP with an unprecedented low BSFC. 

As a light plane engine this combination would have no equal
in the world today.  Range would increase by by about 22% for 
the same fuel load. Best of all, without the worst failure mode of
all the early turbo compound engines. Namely exhaust valve failure.

This with a turbine RPM of 75% of its redline or 56% of max blade stress.
Obviously not yet optimized at this very early stage of development. 

Paul Lamar
 
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