Is that picture showing the hot and cold sides of the big turbo being closed off until 4000rpm? Thats a horrible idea. Is the exhaust manifold some common plenum, or is it just half the cylinders to each?

 

I see your reasoning with wanting to introduce some new complicated system simply to just keep your y pipe intact in case you want to return it to stock... But the reasoning doesn't justify it, especially considering how easy it is to change exhaust and do it the right way. Just glancing at the diagrams you posted, the small turbo will not be happy for very long...

Like I said initially, compound is useless unless you have an engine that wants over 30-40psi.. Your engine will die long before then. BUT, if you just have to do compounds...remove your y pipe and keep it in your garage somewhere. Then set up compounds in series, like they're meant to be.

 

even if you can keep the compressors out of surge do you have the ability to tune each bank of cylinders separately for the brutal difference in back pressures?