Twincharging an engine that features a screw-type supercharger (ie it already has internal compression) in my mind has the following advantages and disadvantages:

Disadvantages:
It would still feature parasitic loss when the supercharger is "bypassed" and the turbo is powering away at higher rpm

Advantage:
The efficiency being better than a roots blower, would mean lower temps and more power

Can one of the gurus post their thoughts? PatPrimmer and Warpspeed had a lot to say on previous TwinCharge topics.

In short: what is the suitability of screw-type SC for twincharging project?

You've probably got it ... the screw type would give (theoretically) an efficiency loss during cruise, but a minor power improvement during the short period of acceleration until the turbo takes over.

The twin-charging application that I'm aware of (VW/ball valve) only uses the supercharging for the first moment of acceleration in which the turbo is running "off design". As soon as the engine builds enough revs for the turbo to take over, it does. This leads me to suspect that there is little purpose in using a higher efficiency supercharger, because it is really only used for maybe a second at a time. Also, those engines are intercooled, so whatever minor extra heat comes from using a supercharger that is less efficient than it could be, gets taken away by the intercooler anyway.

At least in that application, the need to minimize parasitic losses during cruise and acceleration when the turbo is active takes priority.

'course, not every application is the same, I'm speaking only of that one, where VW's intention was to make a downsized and more-efficient gasoline engine; it's best not to throw away a few points of efficiency driving an inactive supercharger.

Have you considered electric clutching the supercharger?  Mercedes, on the original SLK230 had an electric clutch on their 2.3l four cylinder, Eaton M45 supercharged, which in my mind could potentially open many options for a twin charged application.  Mazda, with the twin turbo RX7 of 1993 through 1995, had a very complex system of making the turbos sequential.  I'm not saying it was especially good, but when in a good state of tune, that system gave a nice flat torque curve, coming on low, which the little rotary really needed.  If designed properly, twin charging should be able to eliminate the parasitic loss of the supercharger, when no longer needed, or overtaken by the turbocharger.

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