S4 is centrifugal I believe, SC'd low down, and had the turbo take over @ 4000rpm...
later changed for two smaller turbochargers which provided 25psi of boost at higher engine speeds, designed to spool-up at 3500rpm just as the supercharger boost was being bled-off.
You might find something of interest on here too:
http://www.eng-tips.com/viewthread.cfm?qid=48738