Volvo D5 twin-turbo diesel revealed for S80

This is a Bi-turbo or Two-stage not Twin if i'm not mistaken the twin terminology is used when both units are the same size.

wow, i have never seen that setup, the same air gets charged twice, large turbo feeds to small. that much air compression must really get hot.

Looking at the diagram it seems like a very efficient set up, it not only uses exhaust to spool up the bigger turbo but also the smaller turbo uses it as an intake which results in it spooling faster. There is ONE problem i can see with this set up though, in the pipe to the intercooler from the bigger turbo there is a piston like pressure valve that once the larger turbo has spooled up it opens up and based on the volume of the piping between the two units only enough air to keep the smaller turbo boosted goes to it, now lets say that valve breaks, you now have a much higher boost hitting the small turbo and you can say goodbye to your compressor wheel. I think on the pipe linking the intake side between the two units they should have put a blow off valve just in case, if i got this engine that would be the first thing i would do, find out what the upper turbo is rated for and get a piston type BOV with the proper spring for the pressure. Just my two cents.