# gasoline engine compound turbo discussion



## elRey (May 15, 2001)

Hello all,

I wanted to start an open and on-going discussion about compound turbocharging gasoline engines. I'd like to keep it limited to gas engine tho one member on here has a great tdi setup. And I would still hope he would contribute to this thread.

The reason I want to exclude diesel is even though they have more experience and more builds, their method of boost control is for the most part not applicable. Also their target pressures are well beyond what we can achieve.


I'll reserve the first few reply spots for any conclusions we come to and links to existing information / build threads. 

*naming conventions:*
large turbo / low pressure (LP) turbo / primary turbo
small turbo / high pressure (HP) turbo / secondary turbo

*flow paths:*
intake -> large (compressor) turbo -> small (compressor) turbo -> intercooler/engine
engine/mani -> small (turbine) turbo -> large (turbine) turbo -> exhaust

_restated (to illustrator the counter flow direction):_

intake -> large turbo -> small turbo -> intercooler/engine
exhaust <- large turbo <- small turbo <- engine/mani




My first question (and reason for start this thread) is: 

I know the direction of the intake is optimized with intake air first feeding large turbo and then small turbo.
However is the exhaust direction crucial? Other than minimizing internal volume in front of the small turbine to promote faster spool, is there any other reason to put the small turbine first? edit: I just realized this may heavily depend on goals: street vs track) 

Thanks,
Rey


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## elRey (May 15, 2001)

reserved


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## elRey (May 15, 2001)

also reserved




*Links:*

http://www.dsmtuners.com/threads/my-compound-turbo-set-up.336541/
www.youtube.com/watch?v=y-Me6yWApzY
http://www.solsticeforum.com/forum/f63/lipsticks-800hp-first-lnf-compound-turbo-engine-build-64983/
http://forums.vwvortex.com/showthread.php?5714295-Audi-TT225-Compound-Turbo-Build
http://www.evolutionm.net/forums/pr...nd-turbo-set-up-gt2871r-gt4094r-turbos-3.html
http://www.dsmtuners.com/threads/compound-charged-talon-wins-dse.444669/
http://www.dsmtuners.com/threads/compound-turbo-setup-holset-style.426339/
http://www.dsmtuners.com/threads/compound-turbo-exhaust-routing.424333/
http://www.dsmtuners.com/threads/compound-turbo-thread.450569/page-5#post-153337093
http://www.yellowbullet.com/forum/showthread.php?t=216811
http://forums.vwvortex.com/showthread.php?5524872-1000-awhp-or-bust


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## G60 Carat (May 14, 2004)

There is some interesting exhaust routing in these pics.

http://www.yellowbullet.com/forum/showthread.php?t=1093130


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## elRey (May 15, 2001)

G60 Carat said:


> There is some interesting exhaust routing in these pics.
> 
> http://www.yellowbullet.com/forum/showthread.php?t=1093130


:thumbup: more than your typical compound turbo, and a recent/current build no less


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## Pat @ Pitt Soundworks (Nov 14, 2008)

That layout is very typical of a high performance diesel compound turbo. When you need to move large volumes of airflow, some times you have to run two of your hp turbos as lps....



Regarding turbine orientation: 
By placing the small turbine after the large turbine, you are using most of the exhaust energy to turn the large turbine wheel. Assuming we're talking about a street-driven compound set up (where the turbine wheel is not so large that it cannot extract energy from the exhaust at mid-low rpms - think 85mm turbine on a 2l), that energy is "wasted" on trying to turn the lp turbo while it could be used more efficiently by spinning up the hp turbo. 

There are only two reasons to compound a gasoline street car:
1) to spool a turbo that would otherwise be lazy on the street
2) to run high boost pressures on a motor that would otherwise have to spin ridiculously high rpms to make power

An example of point 1 would be any <2.4l 4-cylinder motor running a hx35 (in a non-mitsubishi turbine housing) or gt35r. Normally you will spool up around 5krpm. By compounding the big turbo with a small turbo, such as a 28rs that will make positive pressure by 2500rpm, you can move enough exhaust energy to spool the hx by 3500rpm. If you run a 2:1 pressure ratio on the hp turbo on a 2l, the engine will move a volume of air roughly equivalent to a 3.8l. 

These maps show the response of a 2l motor vs a 4l motor using a gt35r









The maps also can function as an example for point 2, as well. You can see that on the 4l motor, in order to make 700whp, you will have to spin the 2l motor to 9k+, where as in the 4l motor, you only need to spin the motor to 7k. 

I personally don't see compounding useful on a track car - any almost any track car. On a drag vehicle, nitrous easily achieves the same results. On a time attack vehicle, gearing can make up for the decreased displacement. On autox, you wouldn't be running a turbo that big anyway - even with the faster spool, it's still going to be too slow. I could see a use in hillclimb events, like pikes peak, however.

FWIW, I have a customer who I'll be building a 28rs/hx40 compound set-up for in roughly two weeks.


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## elRey (May 15, 2001)

Thanks for the great info.



Pat @ Pitt Soundworks said:


> Regarding turbine orientation:
> By placing the small turbine after the large turbine, you are using most of the exhaust energy to turn the large turbine wheel. Assuming we're talking about a street-driven compound set up (where the turbine wheel is not so large that it cannot extract energy from the exhaust at mid-low rpms - think 85mm turbine on a 2l), that energy is "wasted" on trying to turn the lp turbo while it could be used more efficiently by spinning up the hp turbo.


OK. This is where, admittedly I lack experience and knowledge. I can't imagine the large turbine zapping all the energy from the exhaust to prevent a small turbine from spooling. I guess I have a specific setup in mind. Now I can see that as you increase the size of the small turbo, this would start to happen.

I also wouldn't consider it a "waste" to run exhaust into the large turbine at any point. I WAS thinking any exhaust going into the large turbine would do some work to spool it, no matter how little. But then as I think about my current BT setup... I realize at lower RPM, no matter how long I hold it there (i.e. break boosting), it never raises boost beyond what that RPM worth of exhaust can produce with my turbo. 

But really? How much less energy will the exhaust have after flowing thru the large turbine? I guess the only real way is to try it? I'm thinking gt28 or smaller and t3/t4


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## Pat @ Pitt Soundworks (Nov 14, 2008)

It's not wasted-wasted. But more like improperly allocated. I don't know enough of the math to calculate loss, but I can give an anecdotal example. The force that actually spins the wheel is pressure difference between the turbine inlet and outlet. Wheel size, turbine size, exhaust size, engine displacement, and fuel type all play a role in how much pressure is before the turbine, but the simple statement is more pressure before the wheel, less pressure after the wheel. 



















These maps are examples of exhaust restriction, but they also show is where the turbine wheel is able to extract energy from the exhaust gas. More restriction on the low end means more energy converted from the exhaust. The 28rs can extract more energy down low, 5lb/min at 1.125:1 turbine pressure ratio where the 35r requires more pressure to extract work. Now the maps are not complete because they cut off at the lowest flow points. Even if the compressor is not making pressure, the turbine is still extracting work from the exhaust gas because the shaft is still spinning. And removing work from the exhaust gas means a reduced pressure on the turbine outlet when compared to the inlet. 

So if the large turbo is first, you're removing exhaust energy from the stream and simultaneously lowering drive pressure the small turbo could be using to spool up. It may be .5psi. It may be 2psi. Without trying it and throwing a drive pressure gauge on it, it's hard to say. But the large turbo will spool with increased exhaust gas. The easiest way to get more exhaust gas is getting the small turbo spooling sooner. Increasing small turbo spool up 200rpm moves way more exhaust gas in to the big turbo sooner than pulling 75% of the energy out of 5lb/min exhaust gas when the small turbo is out of boost.

How much difference would it actually make on small turbo spool up? Dunno. I'm sure with a large enough turbine it wouldn't matter. The motor couldn't flow enough energy for the large turbo to extract any of the exhaust energy. I would also think it would require more wastegating to bypass the small turbine at high flow levels since the small turbo will have a smaller inlet and have a much higher restriction than the big turbo. 


I've always though of compounding as "putting a turbo kit on your turbo kit". The small hp turbo adds "synthetic displacement". 2l engine at 2atm moves similar air to a 4l engine at 1atm. So now you can spool a turbo that would normally go on a 4l engine. If you look at motors like Kevin's DSM, he was spooling a 72mm compressor with an 85mm turbine in a t6 housing on a 2l. He's admitted to making it past the 1/8th mile mark without seeing positive pressure without nitrous or compounding. Mike is running a bw472 (I think) on his drag gti. But he's not making boost until 6krpm.


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## elRey (May 15, 2001)

Hmmm. So, it's the pressure delta across the turbines that I need to keep in mind.

My rationale for the question was to simplify the system by relocating the biggest restriction to the tail end of the system where it can be more easily by-passed while also relieving excessive back pressure (seems contradictory to goal above) and lowering EGTs.


Seems now it would just be the same old trade-off, low-end spool for more free flowing top-end


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## Pat @ Pitt Soundworks (Nov 14, 2008)

Unfortunatley, yes.

With a large enough bypass on the HP turbo and a manifold that gives the wastegate flow priority over the HP turbo, the increased restriction is not enough to worry about. You are actually going to have a much larger drop in back pressure by switching to the largest LP turbine housing and wheel that you can fit. You kind of have to size the turbo backwards. A 3071 is a 500hp compressor, but as a LP turbo, the exhausts wheel is going to be the restriction. To hit 500hp with a compound, you need to size a 700hp turbine and whatever compressor is attached is well work.

With enough bypass gating, the big LP turbo well be the restriction. That may mean a 44mm gate on a k04 sized turbo, but it depends entirely on manifold design. The compound TT I an designing well be a log manifold with the HP turbo of off of runner 4, but with 1-3 all sweeping to a wastegate "collector".


You can also increase spool by using the largest exhaust pipe as possible after the HP turbo. 3"-4", the LP won't care.


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