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Old 03-27-2020, 01:11 AM   #1
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CAT turbo design?

Hey y'all,


I have been reading threads like this one: https://www.skoolie.net/forums/f38/c...tml#post374017 where the OP is upgrading the turbo on his CAT engine.



What I know about a turbo's functioning can fit inside a thimble with LOTS of room to spare. So I started this thread to help me and others like me ferret out the details.


The function of a turbo is to pack the air into the intake manifold so that there is more air available for combustion. It does this my using the exhaust gases to spin the turbo's compression wheel. The compression of the intake air makes the air denser and hotter. The air out of the turbo usually goes through an intercooler to make the intake air temperature cooler, increasing the density of the air to be used in the cylinders for combustion.


The pressure of the intake air is (in the case of my 3126) about 21 PSI. This is the pressure after the turbo has done its thing on the air coming from the air filter. This is the MAX pressure when the turbo is in full compression mode. At low engine RPMs, the exhaust gases are not pushing out as hard and thus the turbo does not spin as fast nor produce as much compressed air. So the air pressure at the intake varies with the engine speed.


So what happens if the turbo speeds up faster than the original designers planned?
Does the engine produce more torque/power quicker? More sustained power? What?


How about the size of the turbo output? Does a wider turbo outlet allow for more air to be "stuffed" into the air intake?


How does one determine if/when a replacement turbo will do the engine good or help with that little bit more power needed to easily cruise at highway speed or tow that vehicle?


What turbos should one look at for what benefots to the engine?


You see, I know very little about a turbo in reality ... but I want to learn.
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Old 03-27-2020, 08:24 AM   #2
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Old 03-27-2020, 10:54 AM   #3
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I'll try my best to explain it. This will likely get long winded, there is a lot to say about it, so I apologize in advance.

For starters, lets identify the specific components of a turbocharger. The turbo has 2 housings, 2 wheels, and a common shaft. The housing and wheel on the exhaust side is called the turbine. The housing and wheel on the intake is called the compressor.

A turbocharger creates air flow by placing the turbine housing and wheel in the exhaust gasses. That placement creates restriction/pressure in the exhaust housing which causes the turbine wheel to spin. The turbine wheel is connected to a shaft and that shaft is also connected to a compressor wheel. So if the turbine wheel is spinning, the compressor wheel spins, creating air flow and pressure on the intake side of the engine.

Both turbine and compressor wheels/housings have a specific pressure/flow relationship. Turbo manufacturers have charts or "maps" that show this and you can see an example of those on the right side of the page here.Garrett Turbo GT3782

I'm going to refer to this turbo specifically throughout this post.

Here is it's compressor map


You can see on that compressor map the rpm and efficiency of the compressor at various air flows and pressure ratios.

On the bottom of the graph shows the air flow. Air flow is based off of engine displacement and rpm. Larger engines flow more air, and running the engine at a higher rpm flows more air.

The left side of the graph shows the pressure ratio. That is the pressure difference between the air going into and out of the turbo. So if you're at sea level where you have 14.7 psi of atmosphere pressure, and you have 14.7 lbs of boost, you have a 2.0 pressure ratio.

The different efficiencies are the ovals or "islands" on the graph, and the wheel rpm is labeled to the right of all the islands and shows wheel speed lines crossing the islands.

What you do is find the specific air flow and boost pressures and plot that point on the graph and you can see how efficient the compressor you're using is. For example, if your engine is ingesting air at 35 lbs/min air flow and has a 2.2 pressure ratio, the compressor wheel is spinning around 89,959 rpm and is on the 78% efficiency island.

That shows that for that specific point in time, you chose the correct wheel and housing for what you're currently doing.

Now, if you raise the rpm, you change the air flow, and put the point at a different location on the chart, which changes the compressor efficiency. Changing the boost pressure will also do the same thing.

So, OEM's will take all the different air flow and pressure points, put them on a chart, and then find a turbo whose compressor map lays over all the dots the best. They do this for each engine and power rating.

Every compressor wheel and housing out there will have a different graph showing it's air flow/pressure/efficiency/rpm relationship. So there are thousands of these maps out there to cover all the different engine options out there.

Now, lets move onto the exhaust side and look at the map for the turbine wheel/housing.

Like the compressor chart, it shows the relationship between pressure and air flow. So that when a specific amount of air is going through it, it creates a specific amount of back pressure. You need some pressure in the exhaust to get the wheel to spin, and without pressure it won't do that. You can see in the graph, it really won't flow more then 32 lbs/min. So trying to force more air through it will only create more exhaust back pressure, which isn't good. Too much exhaust back pressure can ruin an engine.

So, like the compressor, you plot the different air flow data points from the engine, and put them on the turbine housing chart. You then pick a turbine wheel and housing that best lines up with the data points you have. You have to run a housing that flows just enough to keep the back pressure down. Running too large of a turbine will create a slower turbine rpm, where too small of a turbine will create too much back pressure and too fast of a turbine rpm. Like compressors, turbines also have different graphs for each wheel and housing, so you have many different options to choose from.

Now, what you may realize, is that it's hard to pick a turbine housing that covers all your data points the engine runs at, and that's where waste gates and variable geometry turbo chargers come into play.

What those do, is modulate the exhaust housing pressure. So you can run a smaller turbine, which allows the wheel and shaft to get up to speed quickly at your lower engine revs, but then bypass the extra air flow you'd have at higher rpm, eliminating any excessive exhaust pressure. This allows a turbo to work over a very broad rpm range.

Now, here's some things to note.

With all the testing, computers, and technology available now a days, turbo manufacturers have created better turbine and compressor designs. In doing so, a design from 1990 might only have a peak efficiency of 75%, whereas a design from 2010 could achieve a peak efficiency higher then that, or have larger islands on the graph. This is also where your billet wheel options, like the "wicked wheel" for example, can come into play as they are an upgraded design installed on older turbos.

Another thing that some gloss over, and it's a pet peeve of mine, is that the turbine wheel and shaft spins 100k+ rpm. I think it's imperative to have the whole assembly balanced, and that installing a new wheel changes that balance and requires the whole thing to need re-balanced. I know some companies say it isn't necessary, but I disagree.

Another thing I see stated, and you didn't say it, but I'm going to address it because a lot of people do, is thinking that a turbo is all free power. It's not "free" power by any means, because it creates back pressure in the exhaust that the piston has to push against. But with that backpressure, it also creates boost, and boost is more air for the engine to ingest that it wouldn't normally have done so naturally aspirated.
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Old 03-27-2020, 11:12 AM   #4
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If, after reading all of that, you're not confused, some companies have software you can play with that will create your engine data points and allow you to overlay different turbo options and optimize the turbo that is best for you.

Borgwarner has the matchbot software which is about the best I've played with.

You can find it Here

They have youtube videos there you can watch to learn how to operate it.
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Old 03-27-2020, 11:51 AM   #5
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Quote:
Originally Posted by Native View Post
So what happens if the turbo speeds up faster than the original designers planned?
Does the engine produce more torque/power quicker? More sustained power? What?


How about the size of the turbo output? Does a wider turbo outlet allow for more air to be "stuffed" into the air intake?


How does one determine if/when a replacement turbo will do the engine good or help with that little bit more power needed to easily cruise at highway speed or tow that vehicle?


What turbos should one look at for what benefots to the engine?


You see, I know very little about a turbo in reality ... but I want to learn.
And, I realized I didn't answer your questions.

Anything to the right of the compressor island is under "choke" and the turbo is too small for the usage. The air leaving the tips of the blades is near supersonic, which is really hot and inefficient. Not something you want to do.

To the left of the compressor island is under "surge", which means the compressor is too large for the usage. Running it under surge can ruin wheels and break shafts.

Changing the turbo can allow the engine to be more efficient. Whether that results in more power or better economy depends on the difference. Power created is tied more to fuel quantity injected, and less about the turbo. You might gain a few hp by a better turbo, but nothing really monumental.

Turbo outlet size is all determined by the turbo manufacturer. Pick the right turbo according to the chart and let them determine the size. Putting a larger outlet won't increase air created.

You determine if you need a different turbo by plotting your data points on those charts, and seeing if there is anything better available then what you have. If your current marks are off the turbo graph, a new one might bring it back and make it good again.
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Old 03-28-2020, 12:34 AM   #6
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Thank you very much. You put alot into your replies, I am grateful. It would appear a lot more study is required to see if the old circa 2000 turbo can be improved upon with a more modern model.


Part of my reason for the thread is that I suspect my turbo is showing its age. I have a BlueFire adapter and I can monitor boost pressure. After reading your posts, I surmise that the boost pressure should respond quite predictably and consistantly. The boost pressure does not behave consistantly nor predictably at times. I will say that this could be sampling errors in the BlueFire adapter since at a steady highway speed, the boost pressure is fairly steady. I'll be monitoring the boost pressure indication more closely for a while.
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Old 03-28-2020, 09:36 AM   #7
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Quote:
Originally Posted by Native View Post
Thank you very much. You put alot into your replies, I am grateful. It would appear a lot more study is required to see if the old circa 2000 turbo can be improved upon with a more modern model.


Part of my reason for the thread is that I suspect my turbo is showing its age. I have a BlueFire adapter and I can monitor boost pressure. After reading your posts, I surmise that the boost pressure should respond quite predictably and consistantly. The boost pressure does not behave consistantly nor predictably at times. I will say that this could be sampling errors in the BlueFire adapter since at a steady highway speed, the boost pressure is fairly steady. I'll be monitoring the boost pressure indication more closely for a while.
I suspect... while nothing wrong with your learning more about turbo theory...
Instead of learning how to build your own wheel (reach of a pun there...)
Research 'build' threads of folks using 3126 or C7 engines and see what mods they did to improve their engines. The 3126 was used in a slew of on and off road machinery...

While a turbo swap for our mills is easy -- that doesn't mean our pistons/rods can take the heat... I've only read a little bit about our 3126 and it seems the higher HP versions have several different parts swapped for heavier duty versions than what ours might have come with...

Please share whatever knowledge your google-fu gleans you!
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Old 03-28-2020, 10:18 AM   #8
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All 3126's with over 210 hp have steel topped pistons, for one.
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Old 03-28-2020, 03:30 PM   #9
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I suspect... while nothing wrong with your learning more about turbo theory...
Instead of learning how to build your own wheel (reach of a pun there...)
Research 'build' threads of folks using 3126 or C7 engines and see what mods they did to improve their engines. The 3126 was used in a slew of on and off road machinery...

While a turbo swap for our mills is easy -- that doesn't mean our pistons/rods can take the heat... I've only read a little bit about our 3126 and it seems the higher HP versions have several different parts swapped for heavier duty versions than what ours might have come with...

Please share whatever knowledge your google-fu gleans you!
Interesting that you suggest reading threads/builds where folks have made mods to their 3126's for that is what prompted me to make this thread in the first place. In particuar, I was reading about the BorgWarner S200SX turbo that DtotheAmbrose was adding to his 3116 (due to swapping his AT545 transmission to an MT643/653, see the link in the first post) and a couple others many months ago when it struck me that I would really like to know how the turbo really works. After all, am an engineer, so KNOWING HOW it works is just as important to me as knowing that it works.


I'll be happy to share the knowledge.
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Old 03-28-2020, 03:52 PM   #10
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All 3126's with over 210 hp have steel topped pistons, for one.
In particular, mine is the 210HP version. I am looking at the as-buit for the engine onthe Cat internal site (you get to this through your DTNA login) and can see the part number (238-2729) and a diagram. However, there is no description as to the alloys used in the construction.


I performed a google search and found a few links to parts.cat.com which about all I can get from them is that they are 5.22 inches in height and 6.56 inches long ... whatever that means. I could understand if one of those dimensions were diameter.


The link to the part listing is: https://shop.cat.com/en/usa-parts/238-2729
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Old 03-28-2020, 05:04 PM   #11
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Interesting that you suggest reading threads/builds where folks have made mods to their 3126's for that is what prompted me to make this thread in the first place. In particuar, I was reading about the BorgWarner S200SX turbo that DtotheAmbrose was adding to his 3116 (due to swapping his AT545 transmission to an MT643/653, see the link in the first post) and a couple others many months ago when it struck me that I would really like to know how the turbo really works. After all, am an engineer, so KNOWING HOW it works is just as important to me as knowing that it works.


I'll be happy to share the knowledge.
I was meaning outside of Skoolie land...
Tractor pulls
logging, skidding,
Pirate 4x4 -- somebody's probably put one of these engine's in a p'up...

I've seen a chart -- a CAT chart showing all the incarnations of the 3126 and what changes as the HP is turned up -- I'll find it again...

As for your turbo -- if you index all the parts before disassembly you can rebuild it yourself. It's just bearings and seals...
If you change the alignment of pieces or replace a turbine you will need to have it balanced as 50,000 rpm is quite conceivable...

A little painful to watch but some good info on the different 3126's...
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Old 03-28-2020, 06:17 PM   #12
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Here's the other vid I was thinking of --

Same guy as before (Deboss Garage) but in collaboration with Adept Ape...

This vid mentions the re-rate chart I was talking about before...

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Old 03-28-2020, 07:08 PM   #13
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I'd be inclined to do ALL of his suggested mods before chasing more HP with one of these engines. Longevity first then more power. Note the oil cooler. Its newer and that's because getting a TRULY flat, square oil cooler mounting is a big deal on these.
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Old 03-28-2020, 07:21 PM   #14
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I'd be inclined to do ALL of his suggested mods before chasing more HP with one of these engines. Longevity first then more power. Note the oil cooler. Its newer and that's because getting a TRULY flat, square oil cooler mounting is a big deal on these.
I agree -- you gotta start with a healthy PM'ed engine before you turn up the heat...
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Old 03-28-2020, 11:37 PM   #15
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We are fortunate that our CAT came with the "round" HEUI pump canister and the steel oil feed line.
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Old Yesterday, 06:20 AM   #16
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We are fortunate that our CAT came with the "round" HEUI pump canister and the steel oil feed line.
What about the blueprinted oil cooler plate and added heui prefilter?
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Old Yesterday, 05:50 PM   #17
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What about the blueprinted oil cooler plate and added heui prefilter?

I do have access to the CAT as-built for the engine, so I suppose I can find out if it shipped with them. Thank you for the reminder.
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Old Yesterday, 06:02 PM   #18
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I'd bet money on it being bone stock. No way they did aftermarket mods or bought multiple oil cooler plates.
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Old Yesterday, 07:37 PM   #19
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I'd bet money on it being bone stock. No way they did aftermarket mods or bought multiple oil cooler plates.
For some reason unknown to me, I can not access the CAT site right now!
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