Has anyone tried compound turbos? This setup is common for diesel pickups over here to increase the pressure ratio above 3bar.

I've got a GT2256V mounted to my engine right now and a stock KKK K26 leftover from the conversion. I'm not looking for more than 30psi of boost, but the GT22 won't make much more than 22psi on my OM617 without choking.

Compounding would allow me to get to 25psi quickly while keeping the GT22 in it's efficient range and getting the K26 off my shelf. 12psi fed to the GT22's inlet plus 14psi from it could net me 26psi without extreme exhaust pressure and intake heat.

I know I could do this with a GT2359V or GT2559V but they are still rare.

Two-stage turbocharging is fascinating. In a case where your primary stage works at 12PSI (gauge pressure), and second stage works roughly at 2.0 PR ratio (i.e. doubling the ABSOLUTE air pressure it receives) will eventually work out to 40PSI inlet manifold charge pressure.

If instead you're after all looking to net a mere 26PSI (gauge pressure), you only gotta have your primary work at whopping 4PSI, and run the secondary hard at mind boggling 9PSI.
Remember, compressors are absolute pressure multipliers.

You're gonna need something a hair larger than a K26 to keep your secondary stage air demand happy. 3-liter engine doing 5000rpm @ 2bar/30PSI boost will consume air approx. 50lbs/min. That'll call for approx. 55mm compressor inducer diameter.

EDIT - miscalculation on airflow values. Corrected to more realistic values.

Thanks!