Compounds - Explained - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 13 (permalink) Old 02-11-2010, 11:55 AM Thread Starter
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Compounds - Explained

I just need some clarification here because I'm not quite grasping the concept.

Compounds are a combination of a large turbo and a small turbo. The larger of the 2 is feeding the smaller. There is less smoke and lag with this combination.

If we're feeding a smaller turbo with a larger turbo aren't we exceeding the maximum boost for the smaller turbo once the larger one is spooled?

Example - GT42 feeding a stock H2E. Hypothetically speaking, the H2E is about to grenade @ 60psi, but the GT42 is certainly capable of pushing that sort of pressure through the H2E. Manifold pressure is above what you would normally see with just an H2E, so how is it surviving?

Are the drive pressures multipled or diminished when you run compounds?
What is the difference between a compound setup and sequential?

Thanks!

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post #2 of 13 (permalink) Old 02-11-2010, 12:13 PM
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Re: Compounds - Explained

If you think about it they are feeding each other.

The exhaust side of the small turbo is feeding the big turbo and the compressor side of the big turbo is feeding the smaller turbo.

But I am no expert and would love to hear someone that really knows you brought up some good questions.

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post #3 of 13 (permalink) Old 02-11-2010, 12:15 PM
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Re: Compounds - Explained

I know nothing about compounds, but from what I read...

Drive pressures are kept in check by external wastegates and can be adjusted from inside the cab. A GT42 is too small for an atmospheric turbo. Need to bump up to a GT47 or something similar/larger depending on the size of your smaller turbo.

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post #4 of 13 (permalink) Old 02-11-2010, 01:04 PM
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Re: Compounds - Explained

In a nut shell and I am not an expert but;

They compound each other. For example lets say both turbo's are the most efficient at increasing their incoming air 30 psi. Once both turbos are spooled the atmospheric turbo builds to 30 psi and delivers to the smaller secondary turbo, which then builds the air another 30 psi, delivering 60 psi to the engine. Both turbo's are still operating efficiently by building 30 psi each, but the engine gets 60 psi. Same priciple as a two stage air compressor, the atmospheric cylinder is bigger than the secondary cylinder.

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post #5 of 13 (permalink) Old 02-11-2010, 02:25 PM
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Re: Compounds - Explained

Quote:
Originally Posted by superih View Post
In a nut shell and I am not an expert but;

They compound each other. For example lets say both turbo's are the most efficient at increasing their incoming air 30 psi. Once both turbos are spooled the atmospheric turbo builds to 30 psi and delivers to the smaller secondary turbo, which then builds the air another 30 psi, delivering 60 psi to the engine. Both turbo's are still operating efficiently by building 30 psi each, but the engine gets 60 psi. Same priciple as a two stage air compressor, the atmospheric cylinder is bigger than the secondary cylinder.


You have the correct concept, however the two turbo's do add up directly. Someone that knows more will have to explain it but instead of adding 30 + 30 and getting 60 you much more than 60. One thing you are doing with compounds if keeping the pressures down that each charger is building, and keeping them with in the map of the compressor..

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post #6 of 13 (permalink) Old 02-11-2010, 02:40 PM
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Re: Compounds - Explained

Someone put this in smack so Charles (aka chillin)can chime in......

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post #7 of 13 (permalink) Old 02-11-2010, 02:42 PM
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Re: Compounds - Explained

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Originally Posted by farm boy View Post
Someone put this in smack so Charles (aka chillin)can chime in......

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post #8 of 13 (permalink) Old 02-11-2010, 02:43 PM Thread Starter
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Re: Compounds - Explained

Due to popular request, I'm going to make the same thread in smack talk. If mods feel necessary, please let me know where it should go.



As far as these comments, thanks for the input. The concept of taking a base psi and adding X psi to it, makes perfect sense. I guess if a turbo is capable of taking atmosphere and multiplying it by x, then the higher the atmosphere is within the turbo doesn't matter really, it is going to be multiplied either way.

What about drive pressures?

Why can't it (or doesn't it) go the other way? I could put another turbo after my stock one way cheaper and easier than putting one before it.

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post #9 of 13 (permalink) Old 02-11-2010, 03:13 PM
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Re: Compounds - Explained

Quote:
Originally Posted by Jack_Toepfer View Post
I just need some clarification here because I'm not quite grasping the concept.

Compounds are a combination of a large turbo and a small turbo. The larger of the 2 is feeding the smaller. There is less smoke and lag with this combination.
Larger feeds the smaller on the intake side. On the exhaust side, the smaller is first.

Quote:
Originally Posted by Jack_Toepfer View Post
If we're feeding a smaller turbo with a larger turbo aren't we exceeding the maximum boost for the smaller turbo once the larger one is spooled?
Yes, and no.
Turbos aren't rated in 'boost'. They're rated in pressure ratio. Pressure ratio = outlet pressure/inlet pressure.
A single (small) charger making 30psi boost at sea level: PR=14.7+30/14.7 = 3.04
A single (large) charger making 60psi boost at sea level: PR=14.7+60/14.7 = 5.34


Quote:
Originally Posted by Jack_Toepfer View Post
Example - GT42 feeding a stock H2E. Hypothetically speaking, the H2E is about to grenade @ 60psi, but the GT42 is certainly capable of pushing that sort of pressure through the H2E. Manifold pressure is above what you would normally see with just an H2E, so how is it surviving?
Let's take the two chargers in the examples above, but work it out differently.
We want 60psi 'boost' at sea level going into the engine, which is 74.1psi absolute. In general, most good compound systems will roughly evenly divide the pressure ratio between the two turbos.
So, Total PR = 14.7+60/14.7 = 5.34, now divide that between both chargers and you have a PR of 2.67 for each.
The first (large) charger will be: 2.67*14.7-14.7 = 24.5psi outlet pressure.
The second (small) charger will have the same pressure ratio. But, it will have a different inlet pressure. So: 2.67*(14.7+24.5)-(14.7+24.5) = 65.464psi boost pressure. The extra 5 psi came from rounding during the calculations.


Quote:
Originally Posted by Jack_Toepfer View Post
Are the drive pressures multipled or diminished when you run compounds?
Either, but overall they should be diminished for the same boost pressure since both turbos should be running closer to 100% efficiency than they would individually.

Quote:
Originally Posted by Jack_Toepfer View Post
What is the difference between a compound setup and sequential?
Thanks!
Literally, compound means
Quote:
composed of two or more parts, elements, or ingredients: Soap is a compound substance.
OR
Quote:
Machinery. noting an engine or turbine expanding the same steam or the like in two successive chambers to do work at two ranges of pressure.
The sequential is a more specific term.
Quote:
1. the following of one thing after another; succession.
2. order of succession: a list of books in alphabetical sequence.
-In other words, the definition of a 'compound' can be intrepreted as having two seperate turbos of the same size, one on each bank (in the case of a V motor), and having the compressor outlets tied together to the inlet of the engine.
-The sequential is very specific that there is no branching, and that the supply is from the same source. This is probably the most correct term.

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post #10 of 13 (permalink) Old 02-11-2010, 05:19 PM
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Re: Compounds - Explained

need to track down charles he can explain them pretty well

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