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Nice to have some sharp people on this forum.
With a turbo alone, exhaust pressure between the exhaust valve and the turbine (p2) is always equal to, or higher than inlet pressure (P1). With a belt driven supercharger P1 is always the higher of the two (provided it's even remotely well designed). That's the reason the supercharger provides a scavenging advantage. When you combine the two you still retain the supercharged P1 and P2 differential. For example with 5psi from a supercharger, and a great exhaust system at full boost manifold pressure will be 19.7psi (with standard atmospheric pressure). The pressure in the exhaust port will be 14.7 for a 5psi differential. Add 10 psi of turbo boost and the inlet pressure will be 29.7psi but P2 will rise to 24.7psi. We still have our differential. Remove the supercharger but keep an inlet pressure of 29.7psi by spinning the turbo faster and exhaust port pressure will rise to 29.7psi at least. It will probably rise more depending on exhaust manifold design and other factors.
This is not just my opinion, it's right from G.E. They built literally tens of thousands of these systems. They say the reason they used a compound system rather than a turbo alone was for this scavenging advantage, I am inclined to believe them. They certainly could have used a big enough turbo to get the boost levels they wanted without a supercharger. In fact quite a few aircraft did this, and quite a few did this with a supercharger alone, but that's another story....
If anyone has any evidence that compound setups were used on WW2 or later aircraft because turbos couldn't deliver enough air on their own, I would love to see it.
Why don't other manufacturers do it this way. THEY DO! You mentioned Lancia, well the Delta S4 does it exactly this way. I fact I think the Delta S4 setup should be the basic guide of how to do it on an Alfa because the goals and designs of each are similar.
The VW system is totally different and is not comparable to the Delta S4's system. The Delta S4 was designed for power and winning races. The VW twincharger is designed for mass production and fuel economy. The VW's system has the supercharger BEFORE the turbocharger. This is the opposite of what G.E. and Lancia did, and the opposite of what you want for a performance application. I think it's opposite of they way you plan to set it up. Explaining the reason VW did this would take me pages and pages of writing. It was not for horsepower.
I have not seen Mazda or Audi's compound systems so I can't comment on those.
Greg
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