I think that we need to define the difference between compounding and staging. I consider staging as multiple blowers in series, the diesel tractor pullers do this to get their 125 psi + manifold pressure. I consider compounding to taking power from the exhaust gases and connecting it to the engine output shaft. Just like the PW R 3350 engine. You can actually do a compound motor by staging a big turbo into a roots blower that is directly driven by the engine. If the turbo is big enough the inlet pressure to the roots blower will be higher than the outlet pressure and therefore it has turned onto a motor which is driving the engine directly. This is why the GMC two stroke diesel engines work so well.
Some day when I have an "pile" of money I am going to put one of the large centrifugal blowers on an engine and also the run the engine exhaust into a large turbo hot section, then connect them with a drive shaft. Instant compound motor with available parts. Once the turbine makes enough power to turn the blower every thing above that is extra horse power that would go right back into the engine crank by the drive belt. Remember at 60000 rpm you only need to transmit 100 inch lbs of torque to make 100 horse power so we are not talking about big stuff here.
As I have always said: " Only money and time prevent me from completion"
Rex
Mike,
Looks like you must have worked at the "Bomb Factory" with Van Dyne, did you know Alan Brickey? I worked with him at Interscope. Drake Engineering did a small 60 inch 3 cylinder engine for one of the drones and and it had either a double or triple staged turbo for operation at 70,000 feet and I remember that the engine weighted around 100-125 lbs and the blowers weighted about 150! Might be a little off in the weights but the blowers definitely out weighted the engine.