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Anyone ever tried an intake bypass but without dropping boost on the HP turbo completely??

Say for example HP gate at 15psi, once LP reaches 17psi the intake bypass opens but the HP still contributes "15psi" instead of dropping completely.
 

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Discussion Starter #1,470
If I understand you correctly, you could achieve that by simply putting a 15 psi spring in the HP gate and let it always add 15 psi to primary boost. That would give a very low PR at higher boost, but that's what bypassing the HP compressor generally does.
 

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These small engines must be stout as can be to make big power starting at low rpm. The torque figures for that low rpm, and on only 4 slugs to boot, must be goofy high at 3500 rpm like one of them stated. Heck, using n2o to get things moving on a heavily loaded engine at low rpm makes me cringe too.
Really cool to see people trying different stuff. Pretty innovative.
 

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If it weren't for my torque converter, I'd be scared of what my turbo setup could do. We know we can make 30-40 psi down around 3500 rpm. But fortunately, the converter doesn't really allow that to happen under real load. On an engine dyno for example, things could get ugly.
 

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Discussion Starter #1,473
If it weren't for my torque converter, I'd be scared of what my turbo setup could do. We know we can make 30-40 psi down around 3500 rpm. But fortunately, the converter doesn't really allow that to happen under real load. On an engine dyno for example, things could get ugly.
 

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Silly question, is manifold pressure a direct reading of both turbos combined, or is it a reading of the high pressure turbo only since low pressure one supplying x amount pressure ratio?
 

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Not sure the question

But the it is the sum of the individual PRs from each turbo

Say 60psi boost single.... PR of 5

Now a twin set with the same 60psi in the intake overall and 30 psi showing between compressor stages

That is a PR of 3 on the low pressure (big) turbo and a PR of 1.6 on the high pressure (small) turbo

That PR on the high pressure turbo is what hardly anyone seems to understand
 

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Not sure the question

But the it is the sum of the individual PRs from each turbo

Say 60psi boost single.... PR of 5

Now a twin set with the same 60psi in the intake overall and 30 psi showing between compressor stages

That is a PR of 3 on the low pressure (big) turbo and a PR of 1.6 on the high pressure (small) turbo

That PR on the high pressure turbo is what hardly anyone seems to understand
I just want to know if manifold gauge reading is the of total boost being backed up in the plenum.
 

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Total boost in a compound setup is big turbo boost + small turbo boost.

Example:
Small turbo reaches 15psi(2:1pr) first, then the large turbo reaches 15psi(2:1). Now since the large turbo has come on and is affecting the inlet pressure of the small turbo its PR drops to 1.5:1 but its boost output stays at 15psi. Total boost is 30psi or 3:1PR.
 

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I'm currently building a compound turbo setup for my little 1.8 vw golf like to see 60psi of boost at the manifold. I'm using 3x 44mm wastegates, 2 for the HP bypass and 1 for the LP.

Love it, i'm a VW enthusiast.

@ kevin i read this topic 3 times but i've came across a quote on a other forum which raised questions to me.
On your site i've read :

Q. It'll never work!

A. OK, that's a statement and not a question, but I've heard it often enouegh to give it the number one spot here. The answer of course is that it does work. But not always. I've seen more setups that did not work well, than setups that did work well.

Could it be that the following quote is one of the main reasons that makes matching difficult ?
I can not renember al lot of discussion about this subject.

Quote :
The biggest overseen factor in compound setups is the turbine choke flow maps, they are really the driver for turbo selection, then choose the appropriate compressor that will do the job most efficiently.

would like your opinion, mosty if you explain the subject it's easier to understand
 

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Discussion Starter #1,480
Boost pressure in the manifold is the same as small turbo outlet pressure, yes. There is no difference there (aside from any IC pressure drop of course). To try to touch on what I think is confusing to most people here, using the small turbo's Pressure Ratio you can see what the small turbo "feels" like it is producing. To use my car as an example, at 60 psig my small compressor is running at a PR of about 1.5, or the same that would be required to make about 7 psi boost as a single. In other words, it's out for a Sunday drive. Temperatures involved make the work a little higher, but it's a good way to visualize what is going on. Looking at individual boost pressure, those add together due to the nature of how wastegate/boost control systems work. From that angle, the small turbo is adding 25 psig to the atmo turbo's 35 psig. But because of the way Pressure Ratios multiply the small compressor adds that 25 psig while doing about the same work it would as a single making 7 psi. You can shift the PR burden quite a bit between compressors using boost control of course. This is just where I've run my PRs most recently, this isn't an ideal split or anything like that.

Same deal on the exhaust side. Exhaust manifold pressure is the number most people are quoting, and that is of course the total from both turbines, since it's measured in the exhaust manifold. The two turbine's Expansion Ratios are multiplied in the way that compressor Pressure Ratios are multiplied. The large turbine's ER is multiplied by the small turbine's ER, and the small turbine's ER is the multiplier for the larger turbine's ER, so having a low ER on both turbines is necessary to have a good total back pressure in the exhaust manifold. This is why I always recommend going with the biggest hotside you can get for your chosen big turbo.

Regarding the question about turbine choke flow, I'll make the disclaimer that I'm not an expert on the turbine side at all, it's the hardest part of all of this for the layman to get any good info on. I tried for years to predict exhaust back pressure in my fancy spreadsheet and have mostly failed. I can predict it once I get data from a given setup, and can look at other scenarios with that particular setup, but that's it. That said, turbine choke flow is clearly important for making this work. The problem is that we can rarely choose a turbo based on that. The small turbo has to be chosen for the required spool, and for SI engines in particular, and that always means a turbine that is going to choke and require a lot of wastegate bypass. Diesels have more leeway here. The large turbo we have more freedom with to oversize, but there is a limit there too (my 50 tim and S475 combo on a 2 liter was borderline as discussed in the past). That largely comes down to the spread between the small compressor and the large turbine though, meaning that a larger small compressor will help spool a larger big turbo/turbine, giving some room to play there. Really on both turbos turbine selection comes down to running the biggest turbine you can live with. As long as the compressors are efficient enough to not cause excessive back pressure on their own, the hotsides are the main factor. This is why my compressor side rule of thumb of the amto compressor being about 2:1 with the small compressor's flow is so basic, it's just not that critical on the compressor side in my experience.

I also feel that people place too much emphasis on the back pressure. It's important, but think about why you're considering running compound turbos in the place (earlier spool, most likely, if you're in this thread). I'm happy to run at 1:1 to 1.5:1 for a setup that spools like yesterday without nitrous and generally works very well. Any single turbo that I could spool without nitrous would do far worse and come nowhere near the power requirement. Using nitrous I can easily get below 1:1, but that alone does not make it worth it for me. In other words, have reasonable expectations for back pressure and do your best to hit them, but another 10 psi of back pressure may not be the end of your world. 1:1 is pretty doable and a great place to be on a setup that spools very well. On some of the much higher power setups using compounds to more easily get boost over 100 psi on SI engines, as much as or more than they are being used for easy spool, below 1:1 is likely possible just due to the much larger size of both turbos being used in those cases.

Hopefully something in there helps.
 
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