Choosing correct turbo size in a sequential diesel setup

Hi, I am looking to upgrade the LP turbo on my BMW E60 535d.

This car stock, has a Borg Warner R2S setup, with a KP39 HP turbo and K26 LP turbo. HP turbo works from idle to 1500rpm. From that to 3250 the LP is working in tandem, but most gases are routed to the HP. From 3250 to redline most of the gases are routed to the LP. The common upgrade on these cars are custom hybrids of the LP turbo, but I dont want to do that because they doesnt deliver enough air for the powerlevel I am chasing and they dont do anything regarding the oem wastegate, which at high boostlevels create excessive amounts of EMP. The size of the compressor/turbine on most hybrids is:

Comprssor: 56/78mm, Turbine: 65/55mm

What I was thinking was to remove the K26 and use an Borg Warner EFR instead as they look to be suited for the application (internal wastegate, new technology and high efficiency at high boostlevels).

Now, I want to have somewhat similar powerband as oem, but I assume I have to sacrifice some spool when going to a bigger turbo. The choice is between an EFR7670 or an EFR8374, but I was hoping someone has some input here, and also about choose the proper turbine housing (a/r).

Kind regards,

Peter

Can't offer direct help, but it would help if you gave the expected power level you're looking for?

Most sequential designs are either plumbed exclusively in series, or in parallel with a change-over system in place, but from your description it seems a bit of both?

Hi, sorry. I was thinking of having atleast enough air for around 600-650hp and 11-1200nm of torque.

Most hybrids for sale are rated at approx. 450hp, but I think if the emps were more manageble, one could exceed that a little bit.

It is kind of both as you say, a little bit complicated system, but works really well. 540nm of torque at 2000rpm is lovely in a big sedan.