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Discussion Starter #1
Hello all
What are the advantages of compound turbos over twins? I found a lot of info on the compounds turbos for diesel but not much on twins. With smaller exhaust housings, wouldn't you be able to achieve the same results? I know the split exhaust manifold would be an issue so lets just it would be fab'ed up.

Thanks.
 

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compounds make a lot of boost and are lot more involved with fab wise. they are a type of twin style set-up (others being parallel and sequential).
 

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Almost all diesels with two turbos are compound setup.

The only setup that would make sense for two individual turbos would be a V8 type arrangement with one turbo mounted on each bank.
 

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Compound turbocharging (multistage) is THE way to go on a B series engine.
I have been doing some reading on this subject and also believe that this is the case. As you have far more experience in this area than I do, I would like your opinion on this. If I were to perform this modification to my truck and did not want to use a wastegate and only wanted 40-45 psi max boost, what size turbo would you recommend? I would like to keep the Holset that is on there now as the large turbo, and add a smaller one for quicker spool and more power, along with the added benefit of better economy. If it would not be feasible to reuse the stock turbo, what would you recommend?
 

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Discussion Starter #6 (Edited)
Charles you're the man!!
Thanks.... I'm still trying to get a handle on how compounding actually works. All the info i found on twins dealt with V type engines.
So the turbo thats on the manifold (second stage) is feed by the first stage (big turbo)? What are the capabilities of each stage; stage 1 gets you to X psi and stage 2 get you XX psi boost?
The exhaust side is what has me a little confused. I the second stage provides quick spool up (on the manifold) isn't there a big potential for over speeding it?
Most of the books dealing with turbos are gas specific, there's a book coming out
http://search.barnesandnoble.com/bo...rmance+diesel+builders+guide&z=y&cds2Pid=9481
that seems to be pretty informative (diesel specific); It's on order. I need all the help i can get.:D
 

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I have been doing some reading on this subject and also believe that this is the case. As you have far more experience in this area than I do, I would like your opinion on this. If I were to perform this modification to my truck and did not want to use a wastegate and only wanted 40-45 psi max boost, what size turbo would you recommend? I would like to keep the Holset that is on there now as the large turbo, and add a smaller one for quicker spool and more power, along with the added benefit of better economy. If it would not be feasible to reuse the stock turbo, what would you recommend?
Why do you not want to wastegate?
IMO it's a necessity.

Look at it this way. With a compound setup and the small turbo wastegate open, you're spreading the exhaust flow between the two turbines. It's almost like running the turbines in parrallel and is the best arrangement for minimising backpressure.

Without the wastegate you're probably going to have the small turbo choking the flow, greatly increasing backpressure which reduces performance and economy.

The single turbo you've got now is best suited to being the small turbo. The size of your large turbo depends on the density ratio you're after (combination of boost pressure and boost temp out of the large turbo). I've educated Charles on this before so it's good to see him passing it on.:grinpimp:
 

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You have to look at boost as a function of atmospheres (bar) to make it easy to understand. If the first turbo is working under atmospheric pressure at a pressure rate of 2:1, then it is pushing 2 atmospheres or 29.4psi. Now where it gets interesting is that the second compressor(first turbo from the engine) is running at 2bar or 29.4psi from the first compressor(second from the engine) at a pressure ratio of 2:1, so you would multiply 2 x 29.4 for 58.8psi.

Alot of compressers are comfortable and fairly efficient up to 3 bar so you would get 14.7 x 3 = 44.1psi x 3 = 132 psi to the manifold.

The other thing to keep in mind that I have not seen mentioned is that the compressor efficiency doubles in the opposite direction. So, if you are 65% effcient on the first and you add X amount of heat, then you mulitply times X for the second compressor which gets you up pretty good in the heat department. All of this is of couse dependent on ambient.

I would not run a compound setup without water injection. You will need a high pressure pump or a good way to pressure reference the tank to make it work at 120+ psi.

Charles you're the man!!
Thanks.... I'm still trying to get a handle on how compounding actually works. All the info i found on twins dealt with V type engines.
So the turbo thats on the manifold (second stage) is feed by the first stage (big turbo)? What are the capabilities of each stage; stage 1 gets you to X psi and stage 2 get you XX psi boost?
The exhaust side is what has me a little confused. I the second stage provides quick spool up (on the manifold) isn't there a big potential for over speeding it?
Most of the books dealing with turbos are gas specific, there's a book coming out
http://search.barnesandnoble.com/bo...rmance+diesel+builders+guide&z=y&cds2Pid=9481
that seems to be pretty informative (diesel specific); It's on order. I need all the help i can get.:D
 

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You have to look at boost as a function of atmospheres (bar) to make it easy to understand. If the first turbo is working under atmospheric pressure at a pressure rate of 2:1, then it is pushing 2 atmospheres or 29.4psi. Now where it gets interesting is that the second compressor(first turbo from the engine) is running at 2bar or 29.4psi from the first compressor(second from the engine) at a pressure ratio of 2:1, so you would multiply 2 x 29.4 for 58.8psi.
Not quite.
If both turbos are running at a pressure ratio of 2, then the output of your first turbo is 1 bar gauge pressure, the output of your second turbo is 3 bar gauge pressure.
3 bar is approx 45psi.
The 58.8psi you mentioned above is absolute pressure, you forgot to take away atmospheric from the result.

To calculate air densities and charge temperatures is quite involved, there is a thread in here about 6 months back where I detailed how to do it step by step.

IMO a good air/air or air/water intercooler is a far better solution for most people than water injection. Pumping water into your intake is not for the faint hearted. I believe it's a solution more suited to sled pulling and 1/4 mile racing than for daily driving and towing.
 

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Yes, I understand you always take away one atmosphere for what you are reading psig, but psia is, IMHO, a little easier to understand....spent too much time in controls engineering...

Depending on the engine use, compounds are not always worth the effort at 45psi(gauge) since you can do the same thing with a single compressor with a small sacrifice in efficiency. Most of which can be improved(system efficiency) with a charge cooler and water. I am too lazy to go through the math on this, but it would be interesting to see what compound/single system efficiencies would be in that pressure range. Depending on needs, there are compressors out that are happy to run in the 4:1 range (~44psig):
http://www.rbracing-rsr.com/turbo/TurboMaps/GT42compress.jpg

I guess if you are trying to build the ultimate tow rig, compounds at that level make sense, but most guys just want the peak HP numbers. Once you get it rolling the single will not be much different than compound. Of course it will be a little lazy off idle.

Another issue with compounds is pumping losses in the exhaust side. With a larger single you get alot more available turbine power on the first(and only)stage so you also have a much lower deltaP on the turbine. Not a huge deal, but has some merit.

Water can actually be done nicely so that you are not flooding the engine. On my turbo gas puller I use a series of solenoids and pressure switches to stage up water as boost pressure increases. You can buy nozzle assortments to tune water volume. Basically, if it bogs when the water comes on you need less water or higher pressure setting.

Another interesting debate point is whether or not water can burn.....we all know the thermodynamic benefits, but it also involves itself with combustion. Not sure how the chemstry would be with diesel, but I suspect it is similiar....http://www.not2fast.com/thermo/water_injection/water_chemistry.txt
 

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Yes, I understand you always take away one atmosphere for what you are reading psig, but psia is, IMHO, a little easier to understand....spent too much time in controls engineering...

Depending on the engine use, compounds are not always worth the effort at 45psi(gauge) since you can do the same thing with a single compressor with a small sacrifice in efficiency. Most of which can be improved(system efficiency) with a charge cooler and water. I am too lazy to go through the math on this, but it would be interesting to see what compound/single system efficiencies would be in that pressure range. Depending on needs, there are compressors out that are happy to run in the 4:1 range (~44psig):
http://www.rbracing-rsr.com/turbo/TurboMaps/GT42compress.jpg

I guess if you are trying to build the ultimate tow rig, compounds at that level make sense, but most guys just want the peak HP numbers. Once you get it rolling the single will not be much different than compound. Of course it will be a little lazy off idle.

Another issue with compounds is pumping losses in the exhaust side. With a larger single you get alot more available turbine power on the first(and only)stage so you also have a much lower deltaP on the turbine. Not a huge deal, but has some merit.
Yes I can think in absolute pressure too. But it sure as hell confuses some people. Tell them there NA engine already has 14.7psi boost.:dustin:

It's true that you can get high boost from a single turbo, but the drivability is the major issue. When you size a turbo for big boost, you've given away half your rev range. A compromise I'm not willing to live with.

Regarding pumping losses, if you wastegate the small turbo the situation becomes close to having two turbines in parrallel driving two compressors in series. Quite an advantage when comparing turbo drive pressure.
But if you don't wastegate the little turbo, you've got a long and tortuous path for the exhaust to escape.

I've measured backpressure with a three turbos on my engine. All settled into a steady 2:1 backpressure/boost ratio. The turbo that was too small spiked to 4:1 under full load and full revs, the turbo that was biggest had a small range of 1.5:1 backpressure/boost at partial boost levels.
The turbo that gave the best economy was not the one with the least backpressure. Interesting huh?
 

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It seems like compounds are more suited to over 45psi applications? Where the something like the ATS MST come into play. Its a multi stage turbo, basically a VGT type turbo.

http://www.atsdiesel.com/ATSWebsite/ATSUDodge/TurboSystems.asp

Keep it coming. The info here is nothing short of phenomenal.:beer:
I feel single stage fixed geometry turbos lose their appeal past about 20psi boost. VGT you can get 30 odd psi but they still use a single stage compressor which has issues providing large boost at low flow rates.

I found this gem on another website today.
 

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Discussion Starter #16
I feel single stage fixed geometry turbos lose their appeal past about 20psi boost. VGT you can get 30 odd psi but they still use a single stage compressor which has issues providing large boost at low flow rates.

I found this gem on another website today.
So the its essentially in turbo with two compressors:eek: ...Do you have a link to that site? thanks
 

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So the its essentially in turbo with two compressors:eek: ...Do you have a link to that site? thanks
Yep, twin stage compression. Very nice idea.

I found it on the forums at Schumann Automotive, only a little discussion about it. I think you need to be a member there to view photos and attachments.
No idea of the original source.
 

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Discussion Starter #18
Vgt

Hey Dougal,
With the VGT ( at least the ATS one) would the variance in the exhaust eliminate the issue low flow/low boost? If the compressor side was big enough, say capable of 55-60 psi, why wouldn't the exhaust side be able keep up speed?
Is the variance generally not that great? Thanks.

Clarance
 

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Hehe...yes, all engines run at least 14.7 psi boost....hehe

It is very interesting that you got better economy with higher exhaust/intake pressure ratio. The 4:1 does not surprise me, but was that 4(small turbo turbine):1(largest compressor) or was that on the same turbo. On the same turbo, that is an insane difference, but on a compound does not surprise me too much. To get deltap/t across all three turbines it takes alot of heat and pressure.

I guess wastegating into the larger turbine from the smaller would make a big difference. I have never built or tinkered with a compound setup, but it would be alot of fun.

Yes I can think in absolute pressure too. But it sure as hell confuses some people. Tell them there NA engine already has 14.7psi boost.:dustin:

It's true that you can get high boost from a single turbo, but the drivability is the major issue. When you size a turbo for big boost, you've given away half your rev range. A compromise I'm not willing to live with.

Regarding pumping losses, if you wastegate the small turbo the situation becomes close to having two turbines in parrallel driving two compressors in series. Quite an advantage when comparing turbo drive pressure.
But if you don't wastegate the little turbo, you've got a long and tortuous path for the exhaust to escape.

I've measured backpressure with a three turbos on my engine. All settled into a steady 2:1 backpressure/boost ratio. The turbo that was too small spiked to 4:1 under full load and full revs, the turbo that was biggest had a small range of 1.5:1 backpressure/boost at partial boost levels.
The turbo that gave the best economy was not the one with the least backpressure. Interesting huh?
 

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The main problem is that, if they will get there, compressors are horribly inefficient in that pressure range. There are just not compressors(single stage) that will run at 5 bar efficiently. You will be adding so much heat you will not gain much if any power. Discharge temp will be so high that density will drop dramatically. You will not be getting any more air than at lower boost levels and the engine will not respond well to the hot "empty" air.

These guys on that link only show their ignorance with there comment about surge being a turbine problem due to lack of drive power or something....that is crap. Surge is a mechanical limit of the compressor where the air basically stalls and the turbine freewheels and produces a dangerous pulsing state in the air. It is due to mismatching of compressors and can lunch a turbo quickly. Google gave this as a general definition for compressor surge:

A disturbance occuring in gas compressors, especially centrifugal compressors, due to excessive pressure across the compressor in which the compressor blades stall and air flow suddenly drops. The flow can fluctuate and the compressor be damaged. It is the point of instability at which a compressor surges across, trying to reestablish its point of stability.

Hey Dougal,
With the VGT ( at least the ATS one) would the variance in the exhaust eliminate the issue low flow/low boost? If the compressor side was big enough, say capable of 55-60 psi, why wouldn't the exhaust side be able keep up speed?
Is the variance generally not that great? Thanks.

Clarance
 
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