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post #1 of 18 (permalink) Old 03-23-2011, 05:13 AM Thread Starter
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Question about Compounds

When a compound turbo is setup you have a larger turbo (atmosphere) feed a smaller secondary turbo, my question is how does the larger turbo not choke the smaller one. Also, how do the exhaust feed both turbos, I would expect there to not be enough exhaust pressure to spool the first turbo, let alone both even if the first one lights.

Thanks in advance.

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post #2 of 18 (permalink) Old 03-23-2011, 05:31 AM
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Re: Question about Compounds

Thinking of twins on the ol ram

Lol, I believe that the small turbos exhaust side feeds the larger turbo. And Vise versa on the intake side...

I may be wrong, probably are but I tried

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post #3 of 18 (permalink) Old 03-23-2011, 05:33 AM Thread Starter
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Re: Question about Compounds

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Thinking of twins on the ol ram
No, just trying to learn...

Now what do you mean the small turbo is exhaust side, I didn't think it could suck air through the larger turbo's housing and make boost, wouldn't the larger turbo have to spool first? Then again wouldn't spitting compressed air into the smaller turbo without it spinning damage it?

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post #4 of 18 (permalink) Old 03-23-2011, 05:46 AM
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Re: Question about Compounds

The exhaust gases from the cylinders go into the small charger which feeds the larger charger, aiding in spool up... The large chargers compressor side feeds the small charger which feeds the intake manifolds... I believe the small charger is waste gated so you dont grenade it..

Im not sure what you are asking about the small charger sucking air through the large charger.. The exhaust side of the small turbo feeds the larger turbos exhaust side.. And the large turbos compressor or cold side feeds the small turbos cold side which supplies the motor with cold air through your intercooler.. Thats why the small charger has a wastegate. The way I see it the cold side of the small charger might get bypassed in a sense ??

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post #5 of 18 (permalink) Old 03-23-2011, 05:53 AM Thread Starter
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Re: Question about Compounds

That explains it a lot more for me, thank you, I did not realise the larger exhaust side turbo feeds the smaller one, I thought they were split like true twins (reason I thought it would difficult to spool) and the only difference was that the cold side were connected.

With that being said is it difficult for Compounds to spool up considering the larger turbo it connected to the exhaust side first, then the smaller one. You said the smaller turbo spools first, but wouldn't you experience a drop in pressure by the time exhaust gases went past the large turbos exhaust housing and finally hit the smaller one?

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post #6 of 18 (permalink) Old 03-23-2011, 05:56 AM
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Re: Question about Compounds

all setups are different...but i will use the stock 6.4 for an easy example....

The stock 6.4 uses a low pressure and high pressure turbo. the high pressure turbo is a VGT turbo, the low pressure turbo is a conventional turbo. This combination can be changed with wastegates to make what you need though.

The boost flows through the low pressure turbo...then through the high pressure turbo....to the intercooler and then the intake and cylenders. The low pressure turbo provides quicker spool up and it helps the high pressure turbo function since it would have an extreme amount of lag without the "helper" turbo.

The exhaust side of the turbos is the opposite. It flows through the high pressure turbo, then the low pressure turbo, then out through the exhaust. On the 6.4, the variable vanes on the high pressure turbo controll the overall back-pressure available to both turbos. Since the high pressure turbo (not including the variable vanes) is a smaller exhaust restriction than the low pressure turbo, it has to be first in line to get the exhaust or it would not get enough drive pressure to operate. The vanes allow the turbos to have a controllable amount of drive pressure (a wastegate can do a similar thing).

on a wastegated system, you would use boost pressure to counteract drive pressure to keep it at the proper ratio (approx 1.2-1 to 1.5-1 drive pressure to boost depending on who you talk to and what setup) the vanes do a similar thing in a VGT setup.

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post #7 of 18 (permalink) Old 03-23-2011, 06:03 AM Thread Starter
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Re: Question about Compounds

So the smaller turbo spools first, but wouldn't that mean the turbo has to suck air through the larger turbo's housing? Diesel said the larger turbo's cold side it connected directly to the smaller turbo's cold side, and then goes into the motor. Well if thet are in fact connected wouldn't it have to suck air before the larger one lights? Also if the larger turbo is to big, wouldn't the second one restrict it instead of helping?

>> = large turbo
> = small turbo
+ = motor

Keep in mind this is on the cold side

>>-->--+

With that being said, the smaller turbo sucks air though the largers housing before it lights.

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post #8 of 18 (permalink) Old 03-23-2011, 06:10 AM
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Re: Question about Compounds

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Originally Posted by KFA-W/-7.3's View Post
So the smaller turbo spools first, but wouldn't that mean the turbo has to suck air through the larger turbo's housing? Diesel said the larger turbo's cold side it connected directly to the smaller turbo's cold side, and then goes into the motor. Well if thet are in fact connected wouldn't it have to suck air before the larger one lights? Also if the larger turbo is to big, wouldn't the second one restrict it instead of helping?

>> = large turbo
> = small turbo
+ = motor

Keep in mind this is on the cold side

>>-->--+

With that being said, the smaller turbo sucks air though the largers housing before it lights.
the smaller turbo sucks air in through the air intake, pushes that air through the bigger turbo, and into the intake. when the larger turbo takes over, it just spins the smaller turbo faster than its capable of making pressure, thus making it not a restriction is the easy way of wording it.

on the exhaust side it similar but opposite. The larger turbo controlls the backpressure (through the VGT). but the exhaust flows easy through the large turbo so it doesnt spool or cause an exhaust restriction. the smaller turbo however is spooled by the drive pressure and provides boost. THen the smaller turbo eventually gets out of its MAP range and no longer can provide efficient boost. before this happens the larger turbo has already taken over. The combination of the incoming air and the drive pressure makes the low pressure turbo not a factor........and the high pressure turbo takes over providing higher pressure boost....

does that explanation make more sence??

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post #9 of 18 (permalink) Old 03-23-2011, 06:18 AM Thread Starter
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Re: Question about Compounds

Yes and no, because if you go to this link http://blogs.dieselpowermag.com/6750...64l/index.html it says the larger turbo sucks air though the intake first, where you said the smaller one does, so I must be missing something.

Scroll down (bottom of parts breakdown) you'll see where it says the larger turbo is the atmosphere one. This link started me wanting to know how compounds work, along with Bigrpwr telling me to start a thread asking about it.

Now if your correct, I think someone needs to tell Elite their setup is backwards.

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post #10 of 18 (permalink) Old 03-23-2011, 06:28 AM
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Re: Question about Compounds

that link doesnt take me to a particular article...just the front page of the website......

But your right and wrong in theory. It depends on your goal. you could have your high pressure turbo first on the intake side......but then it would not get drive pressure from the low pressure turbo to help it spool..... The high pressure turbo then would have to rely on the air drawn past it instead of the air pushed by it to spool. It would also have a restriction in the air intake (the low pressure turbo) so i honeslty dont know how it would react. I will leave this answer to the big boys running these turbos on thier race trucks......

the first senerio i gave you was the stock 6.4 setup. Its designed for purely street/towing use. Its much more efficient for that. If your racing, you may want to go with the setup you were describing .....im honestly not sure.....But im sure someone else will chime in here with more info.

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