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I've been thinking a lot about this backpressure issue. If you look at Kjewer1 setup, he is running a 96mm turbine wheel to drive his 75mm compressor. On the BMW setup you are running an engine 1.5X the displacement with a 88mm compressor being driven by a 92mm turbine wheel. It would be my best assumption you need a larger. Turbine wheel on the large turbo to get the backpressure numbers in check.
 

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What a/r are you running? I know the 2jz appications with the 4788 are generally out of turbine wheel before they maxout the compressor. You are obviously going to have to make some a/r adjustments as you are going to have pressure drop across the wastegate and small turbine. You have to add up these losses before you count in the drive pressure of the large turbo.
 

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I'm suprised by that since Kjewer1 is running such a small T3 60 trim in compound with his S475. Something seems a miss in your setup. Thanks for trying it out atleast! That 8285R is a killer turbo!
 

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Discussion Starter #188
The small turbo will see whatever back pressure it normall would at the PR and corrected airflow (actual airflow divided by big turbo PR) you run it at, as long as there is enough WG capacity. But, the large turbo backpressure will be multiplied by the small turbo's turbine PR. The compounding works in both directions. IMO, the hotside on the small turbo is of little consequence (as long as it "makes sense" and there is enough WG), it's the large turbo backpressure that hurts more since it gets multiplied. They both batter of course since the small turbo is the multiplier, but either way, the small turbo doesn't give you much choice, you need whatever turbine setup it needs to spool. The big turbo gives you many more options, and in my opinion, you really can't go too big here. To use an extreme, impossible example to illustrate the point, imaging a big turbo with a turbine side so big that it gave no back pressure. You would effectively double your flow for the same BP you'd get from the small turbo on it's own.

In my case the T3 .82 housing on the stage 3 turbine T04E 50 trim is a reasonable turbo for a 2 liter. Back pressure would certainly be acceptable on its own. The trick is the big turbo though, I only see 9-10 psi backpressure from it. This is what keeps the total back pressure from getting over 1:1. The same big turbo with nitrous to spool would be far less than 1:1 of course, but that defeats the purpose. :)

In short, if you are able to run the large turbo on the motor alone, without compounding it, it's probably way too small on the turbine side for compounding.
 

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Discussion Starter #190
That's good to hear! I know it can be a pain in the ass doing testing on a race car when you need to have it up and running, preferably when the season starts. I'm considering moving some projects to my street truck for that same reason.

I would still size the big compressor for a little more than the total flow you need, and just go huge on the turbine side. There may be cases where you have to go up in compressor to get to the big turbines, but that's the basic idea.

Another way to look at turbine sizing to help choose is to use the small turbo PR to correct for displacement and decide if that would spool the large turbo. For example, my 2 liter engine, with the small turbo at 2:1, moves as much are as a 6 liter. Consider the engine and the small turbo the engine package. Would that 6 liter package spool my S475 with the big 1.32 T6 housing and 96mm wheel? You would expect it to. Especially considering the RPM range I cover, where the big turbo needs to be "on" from 6500 to 9500 rpm. Use your particular displacement and small turbo PR as a gross reality check on any large turbo you decide to try to be sure it will still spool. Bigger is better, as long as you can use it. :)
 

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Hey Kev can you see any way of using c02 to control wastegates with a compound setup?
I am all set up for c02 but with the need to add the boost pressure from the big turbo to the top of the small turbo gate I just cant see how it could be accurately done with c02. I really like the thought of being able to ramp in the boost with an ams but I dont want a boost controller nightmare which it is starting to look like being.
 

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Discussion Starter #192 (Edited)
You know, I went through the same thought process, wanting to use the AMS1000 to control both turbos. It can be made to work. Putting the big turbo boost on the small turbos WG is just one way of bringing up both turbos together. There's no reason you can't program a boost controller to do the same thing, it's just more manual. Ultimately I think I had decided to leave the small turbo on automatic control (hard lines to the WG, no co2) and use the boost controller to control the big turbo, which ultimately controls total boost anyway. I never got around to trying it. If you need to reduce launch boost though you'll probably end up wanting control of both turbos, since minimum boost will be roughly small turbo WG spring plus large turbo back pressure. If that ends up being less boost than you intend to launch with, controlling the big turbo only will work. It all comes down to your setup and your car's needs, and your preference for more automatic or more manual control. If you decide to set up a 2 channel boost controller to run both turbos separately, you'll want to consider the boost curve of each turbo in terms of what you acutally want them to do vs what they actually will do. Trying to coordinate two ramp rates where the second depends mostly on the first might take some work but it shouldn't be too bad. For the sake of discussion a set of curves I might consider for my setup, knowing already what the turbos are capable of, might look something like this. I'd set the small turbo to launch at 35 psi, and the large turbo at 10 psi (25 for the small ultimately, but small turbo outlet is manifold pressure, need to keep that in mind with this strategy). Then I'd ramp them both to their max in about 1 second, with 45 psi from the small turbo and about 18 from the large turbo. If the small turbo wasn't making enough steam to spool the larger turbo yet, you'd have to set the small turbo ramp rate ahead of the large turbo's ramp rate. For a scenario where you'd be launching on the small turbo only, it works the same, but you'll probably want to let the small turbo ramp before the large turbo. Or just set the ramps you want and let the turbos do what they can to keep up. There are a million ways to skin this cat. The nice thing about it is that you have full control over the PR of each turbo, no need to change springs or settle for a compromise, which is the main reason I wanted to go this route. If you try it, be sure to post your experiences. :)
 

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Because my car is completely new I think I probably should just try it without c02 initially for ease of setting it up and see how it goes. If it destroys the tyres then I will have to look at leaving on the small turbo only which will be around 700hp on its own and then bring in the large turbo at around 60 foot. I could do this by using c02 only on the large turbo and opening the large turbo wastegate until I want it to start boosting. This would still allow me to run the large turbo boost pressure to the top of the small turbo wastegate so that will make it easier for me to set up initially aswell. I will definately try and do it with dual c02 when the AMS2000 comes out as I have spoken to seb and he says it will be possible with the second channel to independently control both wastegates, I think that is the ultimate setup. I also have a dual water meth injection setup which will be interesting to play with and log the results. No doubt I will be picking your brain in the near future when I get this thing going ;)
 

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Discussion Starter #194
Feel free, I'll help in any way I can. :) That sounds like a good plan to start out with. I ran a small spring in the big turbo WG in the beginning so I could turn it down if I wanted to until I got a feel for how it wanted to launch. Of course in my case it just boost creeped to 40+ psi anyway and ran 9.2 on the first pass, lol. But it was a good plan. :D If you have better control of big turbo boost you can run a 3 psi spring and essentially leave on the small turbo and run it that way for the whole pass.
 

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My setup will have excellent boost control of both turbos, I made sure of that ;)The small turbo is a split pulse t4 manifold with twin 44mm gates that each have 50% priority of the exhaust flow, its a true Y intersection leading to them so they should be able to keep up with the gross flow no problem. The outlets of both then dump into the main dump pipe where there is a 60mm wastegate downstream to control the big turbo boost. The turbo system alone is very complicated then you add the dual water meth + dual c02 boost control and its getting out of hand fast LOL.
 

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Another way to look at turbine sizing to help choose is to use the small turbo PR to correct for displacement and decide if that would spool the large turbo. For example, my 2 liter engine, with the small turbo at 2:1, moves as much are as a 6 liter. Consider the engine and the small turbo the engine package.
I see your reasoning here, but wouldn't a 2L motor with 2:1 on the small turbo be moving as much air as a 4L motor, not a 6L? You'd need a 3:1 PR on the small turbo to make the exhaust of a 6L assuming a single-turbo setup, which is the way I read your post.
 

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I see your reasoning here, but wouldn't a 2L motor with 2:1 on the small turbo be moving as much air as a 4L motor, not a 6L? You'd need a 3:1 PR on the small turbo to make the exhaust of a 6L assuming a single-turbo setup, which is the way I read your post.
You are correct assuming the same rpm and intake temps.
 

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Discussion Starter #199
I see your reasoning here, but wouldn't a 2L motor with 2:1 on the small turbo be moving as much air as a 4L motor, not a 6L? You'd need a 3:1 PR on the small turbo to make the exhaust of a 6L assuming a single-turbo setup, which is the way I read your post.
You guys are right, that was a mistake. Should be 3:1, which is what I run on my small turbo (28 psi). It does become 2:1 at the same pressure after the big turbo comes up and puts another 15-20 psi on the small turbo's inlet, which is what I was apparently thinking about at the time. Things gets tricky when the PR is a moving target on the same WG spring. :) In my case the small turbo starts at ~3:1 and drops to ~2:1 as the big turbo comes up to ~2:1 for a total of ~4:1 (45 psi). Sorry for the confusion, hopefully that clears it back up.
 
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