You aren't going to get anywhere with a .81ar on a turbo that big on these motors. The turbine housing itself regardless of wheel size wont support the flow needed. You would max that turbo around 800-900hp tops.
I'm bored and its the mid watch so here goes my complete thoughts on the matter. lol
Still haven't seen you post the motor combo or specs. While this is all fun to think about, that is what you will need to build the setup to fit. The capability of the motor and ve will not support the pressure ratio of 30-40psi regardless of fuel. This isn't a 4-6 cylinder that revs to 8,10 or 12k rpms. Or a diesel motor that revs to 3-4k rpms.
Flat out I would not try to compound a gasoline mod motor v8 you won’t get this setup into a high enough pressure ratio with load to work anywhere decent and be tune-able or drive-able or get the amazing results you want.
To throw some data at you these kind of setups (spool valves, vgt, compoung, sequential etc) typical will see around 25% faster acceleration time when done perfect (ie the single turbo or multiple turbos are sized perfectly) for the motor ve and achievable pressure ratio).
It takes about 2-3 seconds to spin a turbine and compressor shaft from 1000rpm to 100k rpm on a v8 to around 10psi. Lets say the turbo or first turbo in question is well matched so 2 seconds, now lets implement a way to increase acceleration time by 25% be it which ever method we are discussing. So 1.5 seconds to 10psi vice 2 seconds. Lets punch the throttle around 2500 rpms where there's lots of initial load and boom we hit 10psi at 3500rpm. So in 2 seconds and 1000 rpm we "spooled" and hit are target 10psi. That's a 100rpm every .2 seconds. Now lets say you made the turbo smaller to increase acceleration time and it now only takes 1.5 seconds. A .5 difference in time would only amount to 200-250rpm difference in "spool" time to target boost level.
Now that we did all this work for that huge gain. Let's hope the larger turbine wheel and ar size is good enough to take over where the little one leaves off and it doesn't start to stall due to being at too low of an rpm to grab air, while simultaneous creating reversion and sending air back to the smaller turbo and putting it into surge and making it a nightmare to drive and tune. Was it worth it? That's for you to deicide. But don't think its going to be some huge difference on our specific application.
On our motors that operate up to the 6-7rpm range at most if you somehow sized the setup perfect and it works right you might get into boost 200? 300? maybe 500rpm quicker? You will see down low torque gains and feel it but its not going to be some magic physics defying thing that somehow you were successful in achieving.
A plain parallel would be ideal for this high power level or a bi turbo sequential setup on each bank to try and create this concept of a smaller turbo on one bank and a larger turbo on the other bank could be done and would be a lot easier to do and get right and be tunable.
You can’t just slap on a giant turbo for the larger one it still needs to be properly sized for the max pressure ratio goal for what the motor can handle ve and flow wise. Or its going to do all kinds of weird shit when you switch over. There is such a thing as too large if you run a 2000hp capable turbo at 1200hp is going to be out of its compressor map efficiency range and it can stall/surge and create more heat and generate less power then when it’s at is most efficient island on the map for the pressure ratio. Pressure ratio is used to plot on a compressor map by way, it takes into account engine size, ve, rpm, flow and boost level essentially. You can measure this as you watch power output as you change boost level. A 1000hp 30psi capbale turbo on YOUR MOTOR may only see 15-20hp per psi in the 6-10psi and 25-30psi ranges. But may make 35-40hp per psi in the 15-20psi range for example. There is a sweet spot when its at its most efficient spot on the compressor map where your getting optimal flow at the shaft rpm and pressure and its creating very little heat. This is your goal when sizing the turbo. A power rating is extremely subjective as the turbine and compressor operate in tandem with the motor. A turbo that can make 1800hp at 40psi on a 2.5liter 4 cylinder. May only be capable of 1000hp and 30psi on a 5 liter v8. Like mentioned you have to take into account flow and the engine not just pressure. So again your motor setup needs to be known first.
For a real world example a billet 6765 is rated to around 35psi at 1000hp but what setup is that on? I will tell you it can only do about 700-750hp and 18-20psi on a 4.6 v8 with a 3v head that revs to 7k rpms. Stop looking at supra and 240's and diesels or anything. Look at what v8 mod motors have done specifically look at setups that have similar engine builds as you and then you can make some actual progress towards something with your setup.
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