Compound turbos

Sky Render · Oct 21, 2017

OP- what setup are you designing?

1. Twin turbo (2 turbos same size, one from each half of the engine)
2. Sequential Turbo (one small, one large turbo, small one is used at lower rpms then is blocked off when exhaust flow is great enough to spool up second turbo; think FD RX-7)
3. Compound Turbo (one turbo's output feeds into the input of the second turbo)

tjm73 · Oct 21, 2017

I like the idea of experimenting. The simplest path the end result is the way to go.

However two turbos are overly complex IMO. Does the big turbo have a divided inlet on the exhaust side?

Investigate quick spool configurations. The idea is basically all the exhaust is funneled to one half of the exhaust inlet flange until engine speed is high enough to open a valve to allow the engine to drive both sides of the exhaust inlet. Will it work? I don't know, but it's damn interesting idea to me.

Seems to be two schools of thought.

A.) some Toyota Supra guys run something like this from what I can tell.

B.) This looks interesting...

Heaten m90 · Oct 21, 2017

This is what i was thinking....

Sky Render · Oct 21, 2017

Shouldn't the small turbo feed into the big turbo (inlet air, not exhaust)?

702GT · Oct 21, 2017

Heaten m90 said:
This is what i was thinking....

Little backwards. The smaller turbo (LP-Turbo) drives the bigger turbo (HP Turbo) through the inlet of the bigger turbo. This feeds air directly into the inlet side of the impeller and into the motor, while at the same time spooling up the bigger turbo. Once the pre-determined boost has been achieved, the LP-Wastegate opens up, bypassing the LP-turbo and feeding the hot side of the HP-Turbo, which is already pre-spooled by the LP-Turbo. The HP-WG will control final boost.

This is why impeller design and turbo sizing is critical. But the big turbo in no way feeds anything to the smaller turbo. The big turbo is the star of the show, the smaller turbo just makes the introduction.

Heaten m90 · Oct 21, 2017

My thougt was, in this configuration the small turbo will likely be supporting the load untill say 5500RPM then when the big turbo comes online it will help exaust gas velocity by force feeding the boost thorough the small turbo. Im hoping to spin this thing to the moon. This is just an idea.

Heaten m90 · Oct 21, 2017

Maybe im confused, as why the small turbo should be the high pressure turbo

Because the output from the LP side occupies a smaller volume than the input, therefore the HP turbocharger is fed with a lesser volume (but higher pressure) of input gas and can be smaller.

For the same reason, on steam locomotives the high pressure cylinder is smaller that the intermediate cylinder which may also be smaller than the LP cylinder.

A multi ring (or stage) compressor on a gas turbine is typically the same, although it's more difficult to visualise.

I feel dumb

Heaten m90 · Oct 21, 2017

Within a compound turbo system, you have a low-pressure turbocharger (the larger one) and a high-pressure turbocharger (the smaller one). Air from the atmosphere flows through the low-pressure turbo, from there into the high-pressure turbo, and from there into your intake manifold or intercooler. This "compounds" the boost effect..... if someone could expanded that would be great. This is just what ive been reading

tjm73 · Oct 21, 2017

Looks to me like the little turbo gets exhaust first to spin it up ASAP. It pulls what it needs through the large turbo. The small turbo exhaust then goes to the large turbo exhaust and passes through until enough flow spins it up. At that point the big turbo pushes it's pressure through the small turbo. I could be wrong.

Heaten m90 · Oct 21, 2017

this is how i understand it as well.

I've been trying to find a forum i read awhile back, a dude running this setup on his 240sx with an SR20 DET. He said it felt like a PD blower and top ended like his big turbo setup. He also ran 2 IC's before and after the high pressure turbo.

Sky Render · Oct 21, 2017

The sheer amount of piping you'd need to run would probably negate any responsiveness gains you'd get.

Usually compound turbos are only found on tractor pulling rigs, where you need massive power over response.

tjm73 · Oct 21, 2017

I re-read your first post.

Do you know if.....
A.) the engine will retain(contain) 30-40psi?
B.) you will be able to provide enough fuel?
C.) you will be able to cool the 30-40psi heated air charge?

Let's assume you are successful at producing 30psi. That is roughly a 3 fold increase in power from N/A output. So a stock spec 4.6 3V would put out about 900 hp. (at 40psi it would bump to about 1100hp)

So you have to plumb this system. You will have to cool the air charge sufficiently. Then you have to be able to provide enough fuel to the boost level you run. Then you have to ignite the mixture. Then the engine has to contain the pressure of the combustion. The pistons have to be up to the task of containing this pressure. The rods have to be able to not buckle under this pressure. The crank bed has to be able to stay in the block at these pressures.

All of which is uncharted territory in your "project" scheme.

I think this is all a big pipe dream.

702GT · Oct 22, 2017

tjm73 said:
Looks to me like the little turbo gets exhaust first to spin it up ASAP. It pulls what it needs through the large turbo. The small turbo exhaust then goes to the large turbo exhaust and passes through until enough flow spins it up. At that point the big turbo pushes it's pressure through the small turbo. I could be wrong.

I stand corrected, this diagram is accurate with some provisions of hot side routing. The LP turbo *is* the larger turbo and the HP turbo is the smaller turbo. The smaller turbo spools fast and draws air through the LP turbo and boosts it directly into the engine, doing what turbos do best and recycling that quick spooled boosted exhaust through itself as well as feeding the hot side of the LP turbo. If sized correctly, the combination of drawing air through the LP turbo impeller as well as being fed the downpipe of the HP turbo will spin up the larger LP turbo much faster than it would otherwise on its own.

As for pipe routing, honestly other than looking like a maze of pipe, there really isn't all that much extra length in this type of setup. It just looks over-engineered. Turbos really aren't my thing, I usually differ to Jeremy's words of wisdom in the matter, but one principal has always held true over all FI so far. The more CFM a mechanical device can push, the less pressure is needed to push it. Boost is a measure of resistance the air encounters along its path. A tiny turbo on a WRX can make 20-30 psi and push nowhere near the CFM a larger turbo at half that pressure will. That's why sizing is so critical. In this instance, the HP turbo will need an inlet sized to flow what the LP turbo is going to throw at it in CFM rating, as well as being able to spin like a sonovabitch, cuz that turbo is going to be going like a bat out of hell when the LP turbo starts chugging.

Cooling the air charge is another matter, but this isn't exactly unexplored territory, I'm sure any reputable turbo outfit can point the OP in the direction of the IC setup he would need. Beyond that, there's always Meth and E85. For how much boost the engine itself will be able to withstand, I've ridden in a drag cobra pushing 42psi through 4v "C" heads, so physically the engine can hold it. I'm just not sure if the 3v can flow it.

I get where all the realists are coming from with the traditional suggestions of single and twin turbo setups, but as I'm reading from the OP, he really doesn't want to follow a traditional setup and obviously accepts the consequences regarding budget and the struggle of the build. Personally, I'm fascinated to see him build it and if it holds up to theory, and how far he can really push it before failure or his butthole puckers and decides where good enough really is.

Heaten m90 · Oct 22, 2017

Well, one of main benefits of this configuration is how efficiently the air is compressed. less heat generation is a characteristic of this setup. I'll also be running X85( racing ethanol ) or straight methanol. It'll work in a 2 stage pump configuration, one 465LPH(with a BAP) then an 800LPH will will kick on under load. All feed through 5/8th hose. Ill be running higher compression as well 10:1. I think the head sealing will be a non issue. But yes, this configuration will be complicated and im unsure if it would be possible to dial in. But im willing to put the thought and effort into trying.

stv_huff · Oct 22, 2017

I just don't see the point of this apart from being a thought experiment.

1 Large turbo > basically anything else. Learn everything you need to know from the Supra world. And nitrous is the answer to lag as far as Im concerned.

tjm73 · Oct 22, 2017

What is the largest turbo in your picture? What is it from?

Heaten m90 · Oct 22, 2017

The MTU is a Garrett GTX5533R 98mm
The cummins is a HX82 117MM

Yes the piping will be difficult but the responsiveness might make worth it. The small turbo will hit hard and the big turbo will top end like a motherfucker.

Heaten m90 · Oct 22, 2017

He is the link for the exact turbo .81 AR, rated at 1800HP. Dual ball bearing.

https://www.amsperformance.com/cart/gtx5533r-turbocharger.html/

weather man · Oct 22, 2017

There are no off the shelf engine parts that would support the power this would output. Billet crank, filled block, custom rods and pistons and a shop that has built engines for this level. $50 to $70 grand if not more.

JeremyH · Oct 22, 2017

tjm73 said:
I like the idea of experimenting. The simplest path the end result is the way to go.

However two turbos are overly complex IMO. Does the big turbo have a divided inlet on the exhaust side?

Investigate quick spool configurations. The idea is basically all the exhaust is funneled to one half of the exhaust inlet flange until engine speed is high enough to open a valve to allow the engine to drive both sides of the exhaust inlet. Will it work? I don't know, but it's damn interesting idea to me.

Seems to be two schools of thought.

A.) some Toyota Supra guys run something like this from what I can tell.

B.) This looks interesting...

(A) will be going on my car over the winter finally, have been meaning to test it for awhile and we can compare to my conventional single setups. I have going from a open 70mm to twin scroll 72mm with a 1.15ar so about .58ar per scroll. Going to feed one side till about 10-12psi. Then feed the whole thing. From previous testing on my motor I know a 65mm turbine with .68ar will max out around 14psi and power starts to drop. While I don't expect to see any real radical difference in spool I do expect to pick up around 100rwtq at 3500rpm and get a little more seat of the pants feel on down shifts.

tjm73 said:
Looks to me like the little turbo gets exhaust first to spin it up ASAP. It pulls what it needs through the large turbo. The small turbo exhaust then goes to the large turbo exhaust and passes through until enough flow spins it up. At that point the big turbo pushes it's pressure through the small turbo. I could be wrong.

That is indeed how compound is done. And to be effective everything needs to mounted extremely close so basicly the turbo's will be side by side.

Heaten m90 said:
this is how i understand it as well.

I've been trying to find a forum i read awhile back, a dude running this setup on his 240sx with an SR20 DET. He said it felt like a PD blower and top ended like his big turbo setup. He also ran 2 IC's before and after the high pressure turbo.

This isn't a single bank motor with half the displacement and cylinders, you would be misguided to try and compare any of these platforms to what you have.

The small turbo hitting like a pd blower is about the dumbest thing I ever heard. That's just not how it works on a load based wheel compressor no matter how undersized it is for the engine and no matter what you think it feels like. When driving your still transitioning from no load where your at 500-1000 shaft rpm to around 100k shaft rpm when at peak boost and then the turbine is waste gated. A belt driven screw blower, be it roots or pd is always moving or compressing air while driving and is transitioning from 10-15k rpms at cruise rpm while being bypassed to 50-60k rpm tops while at wot and peak rpm. The physics involved from inertia of the compressor and rotor acceleration time between these setups in their respective operating ranges is just no comparison. You can def get an increase in low end torque doing stuff like this but its not ever going to come in instantly at most you may shift it 200-300 hundred rpm to the left on a v8 that operate from 3k to 7krpms.

702GT said:
I stand corrected, this diagram is accurate with some provisions of hot side routing. The LP turbo *is* the larger turbo and the HP turbo is the smaller turbo. The smaller turbo spools fast and draws air through the LP turbo and boosts it directly into the engine, doing what turbos do best and recycling that quick spooled boosted exhaust through itself as well as feeding the hot side of the LP turbo. If sized correctly, the combination of drawing air through the LP turbo impeller as well as being fed the downpipe of the HP turbo will spin up the larger LP turbo much faster than it would otherwise on its own.

As for pipe routing, honestly other than looking like a maze of pipe, there really isn't all that much extra length in this type of setup. It just looks over-engineered. Turbos really aren't my thing, I usually differ to Jeremy's words of wisdom in the matter, but one principal has always held true over all FI so far. The more CFM a mechanical device can push, the less pressure is needed to push it. Boost is a measure of resistance the air encounters along its path. A tiny turbo on a WRX can make 20-30 psi and push nowhere near the CFM a larger turbo at half that pressure will. That's why sizing is so critical. In this instance, the HP turbo will need an inlet sized to flow what the LP turbo is going to throw at it in CFM rating, as well as being able to spin like a sonovabitch, cuz that turbo is going to be going like a bat out of hell when the LP turbo starts chugging.

Cooling the air charge is another matter, but this isn't exactly unexplored territory, I'm sure any reputable turbo outfit can point the OP in the direction of the IC setup he would need. Beyond that, there's always Meth and E85. For how much boost the engine itself will be able to withstand, I've ridden in a drag cobra pushing 42psi through 4v "C" heads, so physically the engine can hold it. I'm just not sure if the 3v can flow it.

I get where all the realists are coming from with the traditional suggestions of single and twin turbo setups, but as I'm reading from the OP, he really doesn't want to follow a traditional setup and obviously accepts the consequences regarding budget and the struggle of the build. Personally, I'm fascinated to see him build it and if it holds up to theory, and how far he can really push it before failure or his butthole puckers and decides where good enough really is.

Well said.

Compound turbos

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