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The point of a compound turbo system like that is that it provides very -very- high boost levels. Let's take an example:
The larger turbo runs at say a 1.5:1 pressure ratio, consuming something on the order of 1200 CFM of air, compressing it down to something on order of two thirds that (somewhat more than that, given heat pollution, but this is example math, so don't crucify me.). So we have 800 CFM at the outlet at 1.5 Bar pressure.
This feeds into the inlet of the smaller charger, which runs at say, 2.5:1 pressure ratio. The turbo doesn't care what pressure the inlet is at, it just compresses it further. So our charge gets compressed to ~320CFM at 3.75 bar, or around 55PSI absolute; think about 40 psi of boost.
It's not really a device to increase response like a twin setup, it's a device to get around the boost/airflow limits of turbochargers. The larger turbo flows enough air for the entire system at a reasonably low boost level, while the smaller turbo, instead of having to worry about massive airflow numbers (The numbers I used are probably par for 1/3 to 1/2 throttle in a big diesel application at that boost level) can compress a smaller charge further.
Heat buildup, however, is pretty intense, since unless you absolutely match the turbos with regard to efficiency ranges, they'll both be running outside of their efficiency ranges most of the time. Even if they are well matched, you're getting heat from two turbochargers in all of the intake air. Hence why they generally run massive intercoolers in diesels.
In short, if your goal is to run > 30psi or so of boost, this might be something to look into, and if you want to run more than about 45, it's almost a necessity, but otherwise, it doesn't do what you're looking for.
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