Yes, I meant to quote you. I think all my of us are aware of the larger turbine on the 75 which may help lower EBP. My question was is that the biggest factor in determining EBP? I know that changing to just a 71 raises EBP compared to stock. When you consider the 75 will flow a good bit more air than a 71, the larger turbine of the 75 would have to cancel out the VGT restrictions which would be even larger with the 75....which is why I would have predicted the 75 would raise EBP compared to stock. I am not sure how much if any the exh gas velocity would influence EBP when comparing the 2.
Tuning and exhaust back pressure
- Thread starter HD-tech-NH
- Start date
Yes, I meant to quote you. I think all my of us are aware of the larger turbine on the 75 which may help lower EBP. My question was is that the biggest factor in determining EBP? I know that changing to just a 71 raises EBP compared to stock. When you consider the 75 will flow a good bit more air than a 71, the larger turbine of the 75 would have to cancel out the VGT restrictions which would be even larger with the 75....which is why I would have predicted the 75 would raise EBP compared to stock. I am not sure how much if any the exh gas velocity would influence EBP when comparing the 2.
Thanks for the posts.... I Was responding while you were so I didn't get to read those.
Morgan@Midwest Diesel & Auto
New member
Adding a 75 with the same vgt will lower ebp, period. It does not add ebp.
The only reason you have a higher ebp with a 71 is because you have the same size turbine and making more power. More power with the same size turbine means more ebp due to more flow. And lower shaft speed caused by the bigger compressor causes the lower turbine speed which causes another restriction to flow. Couple the slower shaft speed and the higher volume at the higher hp level = higher BP with 71-72-73-76 turbos from various companies.
I know we are just talking theories... so please don't take offense to any of my thoughts... or disagreements :grouphug:
I disagree to some extent...
Something to remember is that turbos, turbines, and compressors do not care so much about "pressure"... they care more about "pressure ratios". That is something important to understand.
Think of it from a compressor map perspective... under most circumstances both turbos would be operating in a much more efficient area of the compressor map. Depending on how you size the turbos they will be basically doing half the work as before to create the same amount of boost.
As long as the turbos are "sized" correctly. The HP compressor is not really a restriction. It is simply compressing already compressed air. Running a LP or Atmo turbo in a compound setup is kind of like changing the atmospheric pressure at which the HP operates at... kinda like the difference between running a turbo at 9000' (baro of 10.5psi) and sea level (baro of 15psi). The 10.5 and 15psi is literally the density of the atmospheric pressure pushing on the compressor wheel. The reason turbos run better at sea level is because there is more pressure and it changes the pressure ratio at which the hp turbo operates at. That is why it is commonly refereed to as the "atmospheric turbo". So adding an atmo/lp turbo will now increase the density of the air before it enters the hp turbo... so instead of seeing 15psi at sea level (which we all understand as being a good thing) now the turbo is operating at an atmospheric pressure of around 15-30psi which helps lower the pressure ratio required from the HP turbo which helps keep the turbo in a more efficient area of the compressor map... thus helping the system to run more "efficiently"
Also having a second turbine causing a parasitic loss seems incorrect to me. Yes you have to spin yet a second turbine/compressor... but you are simply catching exhaust that would otherwise be wasted out the tail pipe and creating more energy from it. That is kind of like saying that a single turbo is parasitic... yes to a certain extent, but the benefits outweigh the small losses.
From a turbine perspective... Remembering that this is all about "pressure ratio" and "corrected flow"... If the lp turbine side is too restrictive the the pressure "after" the hp turbo in the intermediate pipe gets too high then the pressure ratio on the HP turbine will go down moving it backwards along the turbine map and lowering the corrective flow capabilities... thus creating huge amounts of back pressure. That point is usually around a 1.25-1.5 pressure ratio where you move backwards/left far enough across the turbine map and the corrective flow starts to drop off... but above a 2 pressure ratio you start to flow a lot more air.
SO... for example.
FIRST ORANGE DOT
psi before hp = 70psi
psi after hp = 45psi
Pressure ratio = 1.55 and the flow is going to rock out and the system will do well
corrected flow = 27lb/min
SECOND ORANGE DOT
psi before hp = 70psi
psi after hp = 39psi
Pressure ratio = 1.79, you move right across the turbine map and the flow is going to go up and the whole system will breath better and the flow goes up.
corrected flow = 29lb/min
THIRD ORANGE DOT
psi before hp = 70psi
psi after hp = 35 psi
pressur ratio = 2.00, you move even more across the turbine map and the flow goes up even more... the system is even happier and flow continues to go up
corrected flow = 31lb/min
Now if the LP becomes a restriction (or you have too big of a HP)
ONLY RED DOT
psi before hp = 70psi
psi after hp = 55psi
Pressure ratio =1.27, you move way left across the turbine map and the flow starts to drop way down which will then raise back pressure even more and then drop the pressure ratio even more... this starts a chain reaction the the EBP/Drive pressure before the HP skyrockets... That is when people start to gate the truck. Gating to the intermediate makes the problem even worse... gating to the atmosphere will alleviate the issue.
corrected flow drops down below: 21lb/min and your hp turbo has now become a restriction and is probably starting to overspeed.
So you can see that by increasing the turbine size of the LP turbo you will lower the pressure in the intermediate pipe and help the the HP turbine pressure ratio go up and will increase the flow across the whole system. This in theory can cause your ebp to go down... or at least be able to flow more air at a lower ebp as compared to before.
Hope that helps :thumbsup:
I disagree to some extent...
Something to remember is that turbos, turbines, and compressors do not care so much about "pressure"... they care more about "pressure ratios". That is something important to understand.
Think of it from a compressor map perspective... under most circumstances both turbos would be operating in a much more efficient area of the compressor map. Depending on how you size the turbos they will be basically doing half the work as before to create the same amount of boost.
As long as the turbos are "sized" correctly. The HP compressor is not really a restriction. It is simply compressing already compressed air. Running a LP or Atmo turbo in a compound setup is kind of like changing the atmospheric pressure at which the HP operates at... kinda like the difference between running a turbo at 9000' (baro of 10.5psi) and sea level (baro of 15psi). The 10.5 and 15psi is literally the density of the atmospheric pressure pushing on the compressor wheel. The reason turbos run better at sea level is because there is more pressure and it changes the pressure ratio at which the hp turbo operates at. That is why it is commonly refereed to as the "atmospheric turbo". So adding an atmo/lp turbo will now increase the density of the air before it enters the hp turbo... so instead of seeing 15psi at sea level (which we all understand as being a good thing) now the turbo is operating at an atmospheric pressure of around 15-30psi which helps lower the pressure ratio required from the HP turbo which helps keep the turbo in a more efficient area of the compressor map... thus helping the system to run more "efficiently"
Also having a second turbine causing a parasitic loss seems incorrect to me. Yes you have to spin yet a second turbine/compressor... but you are simply catching exhaust that would otherwise be wasted out the tail pipe and creating more energy from it. That is kind of like saying that a single turbo is parasitic... yes to a certain extent, but the benefits outweigh the small losses.
From a turbine perspective... Remembering that this is all about "pressure ratio" and "corrected flow"... If the lp turbine side is too restrictive the the pressure "after" the hp turbo in the intermediate pipe gets too high then the pressure ratio on the HP turbine will go down moving it backwards along the turbine map and lowering the corrective flow capabilities... thus creating huge amounts of back pressure. That point is usually around a 1.25-1.5 pressure ratio where you move backwards/left far enough across the turbine map and the corrective flow starts to drop off... but above a 2 pressure ratio you start to flow a lot more air.
SO... for example.
FIRST ORANGE DOT
psi before hp = 70psi
psi after hp = 45psi
Pressure ratio = 1.55 and the flow is going to rock out and the system will do well
corrected flow = 27lb/min
SECOND ORANGE DOT
psi before hp = 70psi
psi after hp = 39psi
Pressure ratio = 1.79, you move right across the turbine map and the flow is going to go up and the whole system will breath better and the flow goes up.
corrected flow = 29lb/min
THIRD ORANGE DOT
psi before hp = 70psi
psi after hp = 35 psi
pressur ratio = 2.00, you move even more across the turbine map and the flow goes up even more... the system is even happier and flow continues to go up
corrected flow = 31lb/min
Now if the LP becomes a restriction (or you have too big of a HP)
ONLY RED DOT
psi before hp = 70psi
psi after hp = 55psi
Pressure ratio =1.27, you move way left across the turbine map and the flow starts to drop way down which will then raise back pressure even more and then drop the pressure ratio even more... this starts a chain reaction the the EBP/Drive pressure before the HP skyrockets... That is when people start to gate the truck. Gating to the intermediate makes the problem even worse... gating to the atmosphere will alleviate the issue.
corrected flow drops down below: 21lb/min and your hp turbo has now become a restriction and is probably starting to overspeed.
So you can see that by increasing the turbine size of the LP turbo you will lower the pressure in the intermediate pipe and help the the HP turbine pressure ratio go up and will increase the flow across the whole system. This in theory can cause your ebp to go down... or at least be able to flow more air at a lower ebp as compared to before.
Hope that helps :thumbsup:
Last edited:
sootie
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oh geez. you. it was all over as soon as i got to the word "theories"...
Morgan@Midwest Diesel & Auto
New member
I know we are just talking theories... so please don't take offense to any of my thoughts... or disagreements :grouphug:
I disagree to some extent...
Something to remember is that turbos, turbines, and compressors do not care so much about "pressure"... they care more about "pressure ratios". That is something important to understand.
Think of it from a compressor map perspective... under most circumstances both turbos would be operating in a much more efficient area of the compressor map. Depending on how you size the turbos they will be basically doing half the work as before to create the same amount of boost.
As long as the turbos are "sized" correctly. The HP compressor is not really a restriction. It is simply compressing already compressed air. Running a LP or Atmo turbo in a compound setup is kind of like changing the atmospheric pressure at which the HP operates at... kinda like the difference between running a turbo at 9000' (baro of 10.5psi) and sea level (baro of 15psi). The 10.5 and 15psi is literally the density of the atmospheric pressure pushing on the compressor wheel. The reason turbos run better at sea level is because there is more pressure and it changes the pressure ratio at which the hp turbo operates at. That is why it is commonly refereed to as the "atmospheric turbo". So adding an atmo/lp turbo will now increase the density of the air before it enters the hp turbo... so instead of seeing 15psi at sea level (which we all understand as being a good thing) now the turbo is operating at an atmospheric pressure of around 15-30psi which helps lower the pressure ratio required from the HP turbo which helps keep the turbo in a more efficient area of the compressor map... thus helping the system to run more "efficiently"
Also having a second turbine causing a parasitic loss seems incorrect to me. Yes you have to spin yet a second turbine/compressor... but you are simply catching exhaust that would otherwise be wasted out the tail pipe and creating more energy from it. That is kind of like saying that a single turbo is parasitic... yes to a certain extent, but the benefits outweigh the small losses.
From a turbine perspective... Remembering that this is all about "pressure ratio" and "corrected flow"... If the lp turbine side is too restrictive the the pressure "after" the hp turbo in the intermediate pipe gets too high then the pressure ratio on the HP turbine will go down moving it backwards along the turbine map and lowering the corrective flow capabilities... thus creating huge amounts of back pressure. That point is usually around a 1.25-1.5 pressure ratio where you move backwards/left far enough across the turbine map and the corrective flow starts to drop off... but above a 2 pressure ratio you start to flow a lot more air.
SO... for example.
FIRST ORANGE DOT
psi before hp = 70psi
psi after hp = 45psi
Pressure ratio = 1.55 and the flow is going to rock out and the system will do well
corrected flow = 27lb/min
SECOND ORANGE DOT
psi before hp = 70psi
psi after hp = 39psi
Pressure ratio = 1.79, you move right across the turbine map and the flow is going to go up and the whole system will breath better and the flow goes up.
corrected flow = 29lb/min
THIRD ORANGE DOT
psi before hp = 70psi
psi after hp = 35 psi
pressur ratio = 2.00, you move even more across the turbine map and the flow goes up even more... the system is even happier and flow continues to go up
corrected flow = 31lb/min
Now if the LP becomes a restriction (or you have too big of a HP)
ONLY RED DOT
psi before hp = 70psi
psi after hp = 55psi
Pressure ratio =1.27, you move way left across the turbine map and the flow starts to drop way down which will then raise back pressure even more and then drop the pressure ratio even more... this starts a chain reaction the the EBP/Drive pressure before the HP skyrockets... That is when people start to gate the truck. Gating to the intermediate makes the problem even worse... gating to the atmosphere will alleviate the issue.
corrected flow drops down below: 21lb/min and your hp turbo has now become a restriction and is probably starting to overspeed.
So you can see that by increasing the turbine size of the LP turbo you will lower the pressure in the intermediate pipe and help the the HP turbine pressure ratio go up and will increase the flow across the whole system. This in theory can cause your ebp to go down... or at least be able to flow more air at a lower ebp as compared to before.
Hope that helps :thumbsup:
I understand pressure ratios. I'm trying to be basic with why back pressure in a series is highly affected by the last point of restriction. And how compound stages of boost are highly manipulated by the second stages ability to process the flow from the Atmo through the hp.
In regarding compound efficiency, I understand your theory. But dyno proof that taking the exact same s475 used as a single and with a s363 (If I remember the sizes correctly) in compounds was nearly 50hp more peak as a single (I'd have to dig for the sheets as this was years ago). But, it sucked for everything else besides peak power and lower backpressure. You have to understand the velocity and heat loss that the first turbine takes from the exhaust before it gets to the second. And furthermore yes the compounds work in a better total efficiency. But, your losing the true potential of the atmosphere. There is no way around it. Although I realize the reason for compounds is the ability to run a large turbo with the spool of a smaller one. The trade-off is a little top end power loss. The rest of the gains make up for it IMO.
Like I've said, running a large lp through a small hp (such as an rcd 75 and stock cast 52mm vgt) is losing power on the compressor side. We proven it on the dyno. Going to a 59 billet vgt allows for more power even though the turbine side is identical.
You saying the same thing as me. Like I said, and proven by your map, using a lp with less turbine restriction will drop overall ebp. I've said this about five times now. I'm not arguing at all. To recap, lower ebp in the rcd 75 turbine makes lower ebp overall.
I think we are along the same lines for most of what you said.
HD-tech-NH
Member
How can I use this formula to calculate the theoretical drop in back pressure from the RCD 75 's larger turbine?
Cool. Sounds like we are basically saying the same things in different ways.
I would love to see the dyno graphs from the tests you did... or maybe thread link. Most experiments I have heard ended just like yours, that the large single makes more top end power. I just like reading and studying about it. Were you gating to the atmosphere or the intermediate?
Earlier I meant that they run more efficiently through most of the rpm range... except for at peak power like you mentioned.
Morgan@Midwest Diesel & Auto
New member
I'll see what I can dig up. It was on a cummins. I believe it used an internally wastegated s300?. This was not on my personal truck and we didn't setup the turbos either. Some pre-built kit.
But yes I believe we are on the same page. Really the only page to be on!!
08SUPERKING
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Their a lot of info on this thread...
Question: elite 59 with the bigger turbin and rcd 75 working as a team would
work better flow wise but will it hurt performance wise...
Thanks....
Question: elite 59 with the bigger turbin and rcd 75 working as a team would
work better flow wise but will it hurt performance wise...
Thanks....
Morgan@Midwest Diesel & Auto
New member
No, it will help. But like I said before. It's not the standard 59. Must be able to get the large turbine 59. But honestly anyone running a 75 and fuel should have a gate. Having the large turbine atmo helps efficiency and lowers ebp. But you still need to control overall BP with a gate. It doesn't matter even if you had the large turbine vgt. Still not large enough to remove the need for a gate. Maybe on stock fuel, but that's never been documented.
08SUPERKING
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Thanks
sideactiondiesel
Member
:thumbsup:
Last edited:
The quick answer is to tell you its not possible... You would need to get a hold of the stock vgt turbine map (which is not available), the RCD 75 turbine map (which is not available), and the stock LP turbine map (which should be close to a standard S300 turbine). Even if you had all of that info you would have to get some sort of baseline and start calculating backwards.
The simple answer is that the larger turbine on the LP turbo will allow you to flow more air through the whole system... but that does not necessarily guarantee lower EBP. The reason being is that you are also up-sizing the compressor on the LP turbo, thus increasing your flow/boost capabilities. So you might end up with more flow, more power, more boost, and more EBP... but you should see better/lower ebp numbers at a similar boost as before.
For example if your max boost before was 45 psi and 70ebp... then you might see 45 psi of boost and 60psi ebp with the RCD 75 upgrade.
On the other hand after the RCD 75 upgrade you might be capable of hitting 55psi of boost and 75ebp... so more boost, more ebp, and more flow.
Basically you more than likely will not see an "increase" in power and "decrease" in ebp. But you might be able to pull back a little on the tuning/fuel and maintain the same power with a lower ebp... But if you are fueling really hard then you are just going to push more air and your ebp might just go up with the power.
HD-tech-NH
Member
Thank you!
Morgan@Midwest Diesel & Auto
New member
Given the same boost you WILL see a drop in ebp at the same boost as before (ratio goes down). I say this from experience.
Can we quit using theories? Speaking from experience. Anyone who has not ACTUALLY ran these turbos personally or tuned engine and wastegate with one is purely guessing.
I am telling everyone the exact truth from experience. There is zero reason to argue whether the RCD 75 lowers ebp. IT DOES period, end of story. Done. No argument. So can we move on from people making a guess as to what happens?
Not trying to be an a**hole. But this is not a topic that is unknown. It's a proven fact.
Morgan@Midwest Diesel & Auto
New member
Charlie, that was not directed towards you.
I am just trying to make sure someone doesn't take your statement the wrong way and say that the 75 increases back pressure in relation to boost.
I am just trying to make sure someone doesn't take your statement the wrong way and say that the 75 increases back pressure in relation to boost.
Last edited:
I understand what you are saying... sorry for getting too much into theories. I get a little carried away sometimes. I love talking turbos, compounds, turbo theory, triples, wastegates, etc... no matter what platform.
I will admit that I have zero experience with the RCD 75mm... Morgan speaks from experience.
I did not mean to imply that you will see higher EBP with the RCD 75. I only meant to point out that there are no absolutes and that there are some instances in which you could see abnormal results. I have learned there are not very many "absolutes" when it comes to turbos... results may vary
