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Just beginning to put together parts necessary for a compound system, should i start with one or two wastegates, one on each turbo?

TIA
s.r.
 

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Just beginning to put together parts necessary for a compound system, should i start with one or two wastegates, one on each turbo?
There's no easy answer to that question. What works and doesn't work depends a lot on how you route and position the gates. I've outline a couple of arrangements that have worked for me, and at least one that has not. I'm sure others can share what has worked for them as well. For generic guidelines though it's usually safe to assume that you need a LOT of wastegate bypass on the small turbine if you route it to the big turbine inlet, as you typically would. Two 44s or one 60mm would be typical here, with pros and cons to each option there. The big turbine is much easier to bypass, just treat it the same way you would as a single (large gate to run less of its capacity, small gate if you intend to use most or all of its capacity).
 

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My goal is get more boost at significantly less rpm, now I have to run 10k plus to get sufficient boost to get decent short times, makes around 18lbs on the two step. When I do anti-lag at lower rpm the banging in the exhaust is hard on the valve train, blowing exhaust valves open, knocking valve shims out, etc... So I'm hoping this will be a better option.
 

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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.

Hopefully something in there helps.
Tnx for taking the time for such a good answer.

I understand that you have to bypass a lot / most past high pressure turbine.
You refer to the importants of spool on SI engine's.
Is that couse the torque cuve is fixed a AFR ?.
So no lbs / spool no torgue ?

And if HPT choke's, does it allow more shaft RPM still ?
Or try to ask it other whise.
If HP turbine choke's and you have great bypass capabilitys.
Are you still able to flow more LPT vollume on the intake site still ??

Just trying to wrap my head around what happens if this occurs.

Gr maurice
 

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I have just started to tune my compound setup and noticed that the high pressure turbo manifold is glowing red after just a couple of wot runs. Can someone recomend a safe egt.
 

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And if HPT choke's, does it allow more shaft RPM still ?
Or try to ask it other whise.
If HP turbine choke's and you have great bypass capabilitys.
Are you still able to flow more LPT vollume on the intake site still ??
Again, I regret that my turbine knowledge is limited, but as I understand things, when the turbine/housing chokes, that doesn't mean it won't admit more shaft RPM. It will raise back pressure, which will then force more mass flow through the same restriction/choke point, which will raise shaft speed. A turbine side that provides more choke flow will operate with a lower back pressure (at the expense of spool, usually). The wastegate can really only bypass what the turbine does not require to generate shaft power to drive the compressor to what you're asking of it. That's a pretty loaded statement there, but it's key to understanding what the wastegate is really doing. If you understand that you can't simply open the WG more to reduce back pressure without boost falling, you probably get it. You can look at the WG position or WG bypass as the result of the turbine wheel, turbine housing, and work you're asking of the compressor, rather than the cause of it.
 

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Quote:

when the turbine/housing chokes, that doesn't mean it won't admit more shaft RPM. It will raise back pressure, which will then force more mass flow through the same restriction/choke point, which will raise shaft speed.

Now i understand it bether, tnx

Quote:

but it's key to understanding what the wastegate is really doing. If you understand that you can't simply open the WG more to reduce back pressure without boost falling, you probably get it.

Yes i do, tnx for your patience
 

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Interesting.

Engineering theory suggests there is a lot more to come from the compound set up in any class which allows it, not just 4-pot.

Mind you, engineering theory also suggests bees can’t fly ? ?
 

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So besides compound turbos, I've noticed people have tried compound prochargers as well as turbo + procharger. However, the example of the turbo + s/c I saw in this thread had the turbo blowing into the s/c. This seems backwards from my understanding since a centrifugal s/c is unable to be fully spooled in the lower rpm range and the larger turbo would take a while to spool.

Instead, I'm planning a build consisting of a Rotrex C38R-112 blowing into an EFR 8374 or 8474. The EFR would act as the "small" turbo, while the Rotrex is the "big" one that provides the flow since its spool is tied to rpm. This setup would simply plumbing vs a compound turbo.

I'm looking for some feedback in regards to my turbo sizing (is it too close in size to the C38R?). Which turbo would be a better fit?: EFR 8374 or 8474 or other? This is going onto a 20B (3 rotor) btw. Thx!

*I am prolly going to stick w/ the Rotrex C38R-112 since it's the largest, most efficient centrifugal s/c I can find.

Tldr...
Primary (low pressure): C38R-112
Secondary (high pressure): EFR 8374/8474
EFR 8374.png EFR 8474.png Rotrex C38R.png
 

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Those probably are too close in size. If you can spool the 85 lb turbo it will work but it doesn't seem worth dragging the SC along for the right for another 200 hp. I would consider a small turbo or quicker spoo if that SC is enough power, or a larger SC if that 85 lb turbo is easy to spool and you need more power. Always a judgement call with this stuff based on what you really need it to do.
 

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Those probably are too close in size. If you can spool the 85 lb turbo it will work but it doesn't seem worth dragging the SC along for the right for another 200 hp. I would consider a small turbo or quicker spoo if that SC is enough power, or a larger SC if that 85 lb turbo is easy to spool and you need more power. Always a judgement call with this stuff based on what you really need it to do.
Yea I was thinking it would be too big / close in size to the s/c, but I have dynos showing the 8374 making 28psi by 3200 rpm on a TWO rotor (13b). So my reasoning was that the same turbo would spool 50% quicker on a 3 rotor (20b). I guess the next size down would be the EFR 7670, but I'm a bit worried it would be too small turbine-wise. Ur right tho, a larger s/c would be sweet, unfortunately the the C38R is the biggest compressor Rotrex offers.

So I guess my choices are...
8474 - 95 lbs/min
8374 - 79 lbs/min
7670 - 64 lbs/min
...while the Rotrex C38R flows 112 lbs/min.
EFR 7670.png
 

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Regarding order in TC-SC ordering I have some experience here:

When you order the chargers with the turbo blowing in to the supercharger, you can use either a positive displacement OR centrifugal charger. Positive displacement really works better here. If you're using TC->SC ordering you're mainly looking to run more turbo than the motor could reasonably spool. You want the positive displacement here because it's simply a cfm pump. It does not care at all what the inlet pressure is; as long as the SC can supply more air volume than the engine can consume it will continue to function.

I built a setup on a 1.8L VW motor that used an eaton M62 supercharger being fed by a holset hx35 turbo. The 1.8 with the hx35 began to spool in the mid 4000rpms and would see full boost around 5000rpm. With the supercharger on there I was able to fully spool the same turbo by 2500rpm and have positive pressure between the two chargers by 1800rpm.

The same result can be achieved with a centrifugal charger, however the benefits are eliminated because, like you said, the centri charger doesn't make boost at low rpms. They also don't typically need help in generating higher pressure levels since they use more efficient compressors.

IF you choose the other route, with the turbo being fed BY the supercharger then all you need is the blower to move at least as much volume as the turbo compressor. Going larger will increase parasitic loss on the blower exponentially, but you will get some of that pressure compounding effect. This set up reduces spool time of the turbo significantly, however typically won't make more power than the turbo can support by itself. If you look at the boost curve of any centri supercharger, even at low rpms, you're still in positive pressure, which is what helps you spool faster. The really neat part about this set-up is as the pressure ratio across the blower drops parasitic loss drops as well. The supercharger compressor is essentially freewheeling when the turbo comes online. The motor sort of acts as if it has variable displacement in relationship to the turbocharger.

I would never run blower over turbo, but I can see it's merits.
 

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@para pay, isn't the 1st scenario u described basically called a "twincharged" setup? I think VW even has some mass produced examples of this (if we're talking positive displacement). Any specific reason u wouldn't run a s/c into a turbo? I'd agree if its a positive displacement s/c, but as far as centrifugal s/c goes, I was thinking of it as basically a compound turbo setup except the large turbo is belt driven instead.

Also the reason I went w/ a Rotrex for my s/c is cuz they're typically the most efficient with the least parasitic drag penalty vs any other centrifugal s/c out there. I went w/ a turbo for my smaller secondary compressor cuz I wanted a buttload of torque at the low-mid rpm range. I'm shooting for a 9000rpm red line as well, which I hope isn't too unrealistic if it's a billet rotary.

Lastly, how come the s/c would be freewheeling once the turbo spools up? Is it cuz turbos typically spin faster or u think it'll outflow the s/c? Is this true even if the centrifugal s/c is noticeably larger (and spins nearly as fast) than the turbo? Thank you, any and all feedback is invaluable and much appreciated. ^.^
 

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The first scenario is definitely "twin charged". The VW OEM set up was a sequential setup where the supercharger was on a clutch that would deactivate when the turbo came online.

I would never run a SC over TC because it does everything worse. The only benefit it has over TC over SC, or compound turbos is that it might package better. But it will be outperformed by both compound turbo and TC over SC. to get the most out of the centrifugal charger, you need to already be at a high rpm, which your turbo should already be online at.

The centrifugal charger only has noticeable parasitic loss when it's actually compressing air. Parasitic loss scales exponentially with pressure ratio. If the turbo is drawing the same amount of air that the supercharger is supplying, the SC is not actually doing any work. if the SC is creating boost between the SC and TC, then you'll have parasitic loss.

If you want torque, TC over PD SC is the way to go.
 

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Hmm, I never thought that the centrifugal wouldn't outflow or create boost to the turbo. If the centrifugal isn't doing work, doesnt that basically mean its too small or the turbo is too close in size to it? Also how did u deal w/ the inefficiencies of the PD s/c in ur twin charged setup? The <60% adiabatic efficiency has always put me off from using one, especially since there's typically no intercooling after the PD s/c.
 

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Well like I said, every time I've seen SC over TC, the chargers have been closely matched. I've never seen a situation where SC over TC was with a tiny turbo trying to use the turbo like a PD SC.

PD inefficiency is easy: use e85 where the intake temperatures don't matter, and meth injection if absolutely necessary.
 
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