Selecting Turbos for a Twin Setup

Looking for some thoughts/advice. I want to start with: Yes, I know an off the shelf bolt on single turbo will be easier, that's not what I am into.

Through a series of events, I have a B8 A4 with an engine built to withstand 700 crank hp, and a transmission that should be able to withstand 600-700 crank ft-lb of torque. It is ready for an upgraded turbo setup so I can enjoy the capabilities of it. It is a near daily driver, so I don't want it to be crazy powerful, instead I want something that is very responsive with minimal lag on the boost. Being the enthusiast that I am, I don't want what is easy and readily available, I want something different.

So in order to fulfill these requirements I am looking at going twin turbo. Specifically, I am looking at the Garrett GBC22-350 or the GBC20-300. Boost should theoretically come online pretty quick and I should still be making good boost at 6500-7000rpm. This should give me a relatively predictable power curve and should be capable of 500 wheel hp (after drivetrain loss). I would prefer a turbo with water cooling instead of just oil cooling, but the turbos I could find with water cooling all required too much displacement.

I wasn't able to find a ton of info on how selecting a turbo for a twin setup differs from a single setup. I just cut the engine displacement and hp goal in half and looked for turbos that met that.

Planning to utilize my knowledge from the "3d Modeling & CAD for Motorsport" and what I've been able to learn from those on YouTube (check out "Couch Built" on YouTube who is turbo rotary swapping a BMW i8) to get a pair of metal 3d printed exhaust manifolds, and plan to merge the compressed air just before the intercooler so I can use off the shelf parts for the rest of the system.

I think that is all that I should include to get useful advice from y'all, so: What do you think? Thoughts?

I guess there is a smidge more background information that I could include:

B8 A4 comes with a 4cyl engine, mine has been bored out so total engine displacement is 2008cc.

The car is equipped with Quattro, so I have zero concerns with 500whp in a daily.

Looks like twin gbc would get you there but you are in peril of over overspeed on the 20 with any significant intake loss or altitude and the 22 compressor map isn't fabulous. You would also have to replace the factory wastegate actuator with dual port or weld the factory gates and run external ones to get the boost required. There are proportionally higher parasitic losses and efficiency compromises on really small turbos which effect boost threshold, response and overall power capacity for flow/available air & fuel.

4 bar absolute MAP on a daily driver is still nothing to sneeze at.

Personally I'd go compound if I wanted 2 turbos on something that size, sticking with Garrett products G25-550 with 0.72AR Vband turbine housing primary and a G30-770 or G35-900 with probably second from largest turbine housing if you want something bigger for looks. If looking at GTX line up that might become GTX28 and GTX35. That should provide a lower boost threshold, better response, less total heat generation and EMP allowing a wider tuning margin and more power from the same fuel due to lower pumping losses than small parallel twins.

If you are comfortable with more complex boost control strategies to optimise pressure ratio splitting go nuts, but as a fall back position you could run a ~1bar or touch higher gate on the larger and 1+bar on the second, the trick being choice of reference points and switching references allowing a safe fall back lower boost setting of around 1+bar. When in high power mode you reference 1st stage compressor or intercooler outlet for the gate on the large turbo, top port to atmosphere. 3 port solenoid on the reference, in low power mode reference the secondary stage compressor outlet or post intercooler pressure, I'd personally make that on the normal open for power fail safe (can be energised by healthy output of WM system or eth content etc). The second gate, venting from exhaust manifold across the primary turbine, reference top port via Mac valve to atmosphere on NO and primary compressor output on active (AI safe/active, eth % adequate etc). Use conventional boost control to trim the final target to ~4 bar absolute with an additional solenoid on the high pressure turbo, this should see the most efficient pressure ratio splitting without going nuts on multi-stage control strategies.

Alternatively if you wanted only full pneumatic control step each up in spring combination gradually till you are hitting your target. That would see you using ~1.5bar spring combination in both to still allow a pump fuel "low boost" setting. You could add BOV reference switching to drop even further if you preferred/allow spring combinations for best ratio splitting on the gates.

Thank you for such a detailed response, you have given me lots to consider.

If I were to stick with a twin turbo setup, is there a non-Garrett turbo you would recommend, or are you firm on the recommendation to go compound turbo? I am really attracted to a twin setup due to its overall simplicity compared to other dual turbo setups.

I haven't really looked into utilizing a compound setup before, I will need to do some extensive research into packaging and doing the proper plumbing on a compound setup. I have never had a compound setup and neither has anyone in my circle. You have given me a lot of useful feedback, thanks.

Parallel twins will do it, but the power delivery will be more narrow. Doesn't matter as much if it's 6+ gears

My original post disappeared, I realised my conclusion about ballpark required boost doesn't have any context. I have a spreadsheet which I use with a few assumptions on required airflows and efficiencies, to make ~500whp on an Australian Mainline or Dyno dynamics chassis dyno on a 2L motor with peak power rpm around ~6200rpm (this was assumed from the "performance" variants of factory Audi engines, they may be able to peak significantly higher with better flowing turbines) it looks like around 4 bar boost. Fuel /AI choice and if the cams/heads/manifolds allow peaks at higher rpm will obviously lower required boost. Ethanol tends to increase charge density better up to ~10% than pump with majority water AI for example.

The Garretts may prove the best option still, it's probably worth going over the catalogues of a few manufacturers and playing with BW matchbot and any calculation scheme Garrett recommends to get a ballpark idea.

The car has a zf8 trans in it.

I don't really want to rev the engine out very far, I may rev it out a bit on a drag strip, but probably a 6600rpm limit 99% of the time.

I'll be running 94AKI pump (the pump nearest my house consistently measures 8-9% ethanol content) and I am still debating how i want to implement it, but I will be running either a water/meth or pure meth injection on the engine over a certain rpm or boost level.

The ballpark number for boost I was aiming for was 3.3bar absolute, so your 4 bar guesstimate sounds close.