Our engines are air pumps!
Apr 8, 2009, 08:29 AM
#1
Our engines are air pumps!
I thought I would share something I was thinking about the other day. I was helping a friend and we had 3" downpipe (Invidia I think) next to a stock downpipe. The internal diameter of the 3" downpipe was about 2.8". The internal diameter of the stock downpipe was if I recall correctly, about 2.3". But also it appears the IX downpipe squeezes down to smaller. When you put the two pieces next to each other, the difference is astounding!
So I got to thinking about the benefits of the two pieces. Bigger is better right? But then I thought, what happens to velocity? If you think of the exhaust system as a straw, the smaller the straw, the more velocity. So what is good about velocity? And I realized of course, that the greater the velocity, the sooner the turbo will spool. So the large downpipe reduces back pressure, but hurts spool.
I then got to thinking about what size is appropriate for a 450 hp (at the flywheel) Evo. If you think of Buschur's test where he went from a 3" exhaust to 3.5" exhaust and the 3.5" had no benefits, he then went back to a 3" exhaust. So this tells us 3" is sufficient for Buschur's RS which probably makes about 800 hp (at the crank). Then if we look at what size Mitsubishi decided for the Evo:
Afermarket downpipe 3" - 800 hp
Stock downpipe 2.3" (necking down smaller than that) - 300 hp
If the above is true, we can conclude a 450 hp Evo would more likely prefer a ownpipe closer to stock than 3"!
So this is the thinking I wanted to share with you guys. Let me know your thoughts.
So I got to thinking about the benefits of the two pieces. Bigger is better right? But then I thought, what happens to velocity? If you think of the exhaust system as a straw, the smaller the straw, the more velocity. So what is good about velocity? And I realized of course, that the greater the velocity, the sooner the turbo will spool. So the large downpipe reduces back pressure, but hurts spool.
I then got to thinking about what size is appropriate for a 450 hp (at the flywheel) Evo. If you think of Buschur's test where he went from a 3" exhaust to 3.5" exhaust and the 3.5" had no benefits, he then went back to a 3" exhaust. So this tells us 3" is sufficient for Buschur's RS which probably makes about 800 hp (at the crank). Then if we look at what size Mitsubishi decided for the Evo:
Afermarket downpipe 3" - 800 hp
Stock downpipe 2.3" (necking down smaller than that) - 300 hp
If the above is true, we can conclude a 450 hp Evo would more likely prefer a ownpipe closer to stock than 3"!
So this is the thinking I wanted to share with you guys. Let me know your thoughts.
Apr 8, 2009, 08:35 AM
#2
The other interesting thing is that I remember when years ago I used to take my exhaust from the downpipe back off for drag racing. The car would feel really lazy on the low end with just a downpipe in place. I mean very lazy. It would however run 3 tenths better in the 1/4 mile so it did make more top end with the exhaust off. This is when I had just a flashed ecu, ic pipes, exhaust and would run 12.4 with exhaust on and 12.1 with exhaust off.
But for how lazy it felt in the low end, compared to the gains up top, I wouldn't want to make that compromise for daily driving for the 2-3 tenths it offered.
But for how lazy it felt in the low end, compared to the gains up top, I wouldn't want to make that compromise for daily driving for the 2-3 tenths it offered.
Apr 8, 2009, 09:15 AM
#4
I'd love to see a comparison of a bone stock Evo IX with and without a 3" exhaust. I wonder if the difference in spool could be as much 500 rpm or more? I think it might be given how gutless in the low end my car used to feel years ago when I would drop the pipe for racing.
Apr 8, 2009, 09:31 AM
#5
The velocity thing doesn't matter as much in a turbo car because the turbo is in fact a giant restriction in the exhaust pipe. So essentially you want the most open pipes after a turbo to minimize further energy losses.
The good velocity happens pre turbo turbine in the manifold and much discussion has occurred on those fronts
.
The good velocity happens pre turbo turbine in the manifold and much discussion has occurred on those fronts
.
Apr 8, 2009, 09:45 AM
#7
The velocity thing doesn't matter as much in a turbo car because the turbo is in fact a giant restriction in the exhaust pipe. So essentially you want the most open pipes after a turbo to minimize further energy losses.
The good velocity happens pre turbo turbine in the manifold and much discussion has occurred on those fronts.
The good velocity happens pre turbo turbine in the manifold and much discussion has occurred on those fronts
.If you think of the exhaust like a straw, and the straw has a change in diameter half way through, and the turbine is right before the change in diameter, I might be inclined to think that the loss of velocity which occurs at the change in diameter might act negatively on the velocity of exhaust "up wind" of the change in diameter.
But I do understand your point and to me it makes sense, except that, why did Mitsu design the stock downpipe to be so tiny, and why is it that when I dropped my exhaust and my friends have dropped their exhausts for racing we all feel like the low end was compromised dramatically?
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Apr 8, 2009, 10:01 AM
#8
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Because off-boost power will suffer slightly from larger/no exhaust. Once you're under boost, the more free flowing the better. This (and noise level requirements) is why the stock exhaust is small. It's not restrictive at the stock power level, which is all they care about, it's quiet, and it aids in part throttle driving "feel" when you're out of boost.
Apr 8, 2009, 10:05 AM
#9
Because off-boost power will suffer slightly from larger/no exhaust. Once you're under boost, the more free flowing the better. This (and noise level requirements) is why the stock exhaust is small. It's not restrictive at the stock power level, which is all they care about, it's quiet, and it aids in part throttle driving "feel" when you're out of boost.
Apr 8, 2009, 10:18 AM
#10
Oldie but goodie:
The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on Impreza.net regarding exhaust design and exhaust theory:
“Howdy,
This thread was brought to my attention by a friend of mine in hopes of shedding some light on the issue of exhaust size selection for turbocharged vehicles. Most of the facts have been covered already. FWIW I'm an turbocharger development engineer for Garrett Engine Boosting Systems.
N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.
For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.
Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.
Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.
As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”
"As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.
A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.
If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow.
Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.
Also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems.
Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.
Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc.”
"Comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however.
There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.
As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.”
"Here's a worked example (simplified) of how larger exhausts help turbo cars:
Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:
(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure
o here, the turbine contributed 19.6 psig of backpressure to the total.
Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).
So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.
This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.
As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would.
“Howdy,
This thread was brought to my attention by a friend of mine in hopes of shedding some light on the issue of exhaust size selection for turbocharged vehicles. Most of the facts have been covered already. FWIW I'm an turbocharger development engineer for Garrett Engine Boosting Systems.
N/A cars: As most of you know, the design of turbo exhaust systems runs counter to exhaust design for n/a vehicles. N/A cars utilize exhaust velocity (not backpressure) in the collector to aid in scavenging other cylinders during the blowdown process. It just so happens that to get the appropriate velocity, you have to squeeze down the diameter of the discharge of the collector (aka the exhaust), which also induces backpressure. The backpressure is an undesirable byproduct of the desire to have a certain degree of exhaust velocity. Go too big, and you lose velocity and its associated beneficial scavenging effect. Too small and the backpressure skyrockets, more than offsetting any gain made by scavenging. There is a happy medium here.
For turbo cars, you throw all that out the window. You want the exhaust velocity to be high upstream of the turbine (i.e. in the header). You'll notice that primaries of turbo headers are smaller diameter than those of an n/a car of two-thirds the horsepower. The idea is to get the exhaust velocity up quickly, to get the turbo spooling as early as possible. Here, getting the boost up early is a much more effective way to torque than playing with tuned primary lengths and scavenging. The scavenging effects are small compared to what you'd get if you just got boost sooner instead. You have a turbo; you want boost. Just don't go so small on the header's primary diameter that you choke off the high end.
Downstream of the turbine (aka the turboback exhaust), you want the least backpressure possible. No ifs, ands, or buts. Stick a Hoover on the tailpipe if you can. The general rule of "larger is better" (to the point of diminishing returns) of turboback exhausts is valid. Here, the idea is to minimize the pressure downstream of the turbine in order to make the most effective use of the pressure that is being generated upstream of the turbine. Remember, a turbine operates via a pressure ratio. For a given turbine inlet pressure, you will get the highest pressure ratio across the turbine when you have the lowest possible discharge pressure. This means the turbine is able to do the most amount of work possible (i.e. drive the compressor and make boost) with the available inlet pressure.
Again, less pressure downstream of the turbine is goodness. This approach minimizes the time-to-boost (maximizes boost response) and will improve engine VE throughout the rev range.
As for 2.5" vs. 3.0", the "best" turboback exhaust depends on the amount of flow, or horsepower. At 250 hp, 2.5" is fine. Going to 3" at this power level won't get you much, if anything, other than a louder exhaust note. 300 hp and you're definitely suboptimal with 2.5". For 400-450 hp, even 3" is on the small side.”
"As for the geometry of the exhaust at the turbine discharge, the most optimal configuration would be a gradual increase in diameter from the turbine's exducer to the desired exhaust diameter-- via a straight conical diffuser of 7-12° included angle (to minimize flow separation and skin friction losses) mounted right at the turbine discharge. Many turbochargers found in diesels have this diffuser section cast right into the turbine housing. A hyperbolic increase in diameter (like a trumpet snorkus) is theoretically ideal but I've never seen one in use (and doubt it would be measurably superior to a straight diffuser). The wastegate flow would be via a completely divorced (separated from the main turbine discharge flow) dumptube. Due the realities of packaging, cost, and emissions compliance this config is rarely possible on street cars. You will, however, see this type of layout on dedicated race vehicles.
A large "bellmouth" config which combines the turbine discharge and wastegate flow (without a divider between the two) is certainly better than the compromised stock routing, but not as effective as the above.
If an integrated exhaust (non-divorced wastegate flow) is required, keep the wastegate flow separate from the main turbine discharge flow for ~12-18" before reintroducing it. This will minimize the impact on turbine efficiency-- the introduction of the wastegate flow disrupts the flow field of the main turbine discharge flow.
Necking the exhaust down to a suboptimal diameter is never a good idea, but if it is necessary, doing it further downstream is better than doing it close to the turbine discharge since it will minimize the exhaust's contribution to backpressure. Better yet: don't neck down the exhaust at all.
Also, the temperature of the exhaust coming out of a cat is higher than the inlet temperature, due to the exothermic oxidation of unburned hydrocarbons in the cat. So the total heat loss (and density increase) of the gases as it travels down the exhaust is not as prominent as it seems.
Another thing to keep in mind is that cylinder scavenging takes place where the flows from separate cylinders merge (i.e. in the collector). There is no such thing as cylinder scavenging downstream of the turbine, and hence, no reason to desire high exhaust velocity here. You will only introduce unwanted backpressure.
Other things you can do (in addition to choosing an appropriate diameter) to minimize exhaust backpressure in a turboback exhaust are: avoid crush-bent tubes (use mandrel bends); avoid tight-radius turns (keep it as straight as possible); avoid step changes in diameter; avoid "cheated" radii (cuts that are non-perpendicular); use a high flow cat; use a straight-thru perforated core muffler... etc.”
"Comparing the two bellmouth designs, I've never seen either one so I can only speculate. But based on your description, and assuming neither of them have a divider wall/tongue between the turbine discharge and wg dump, I'd venture that you'd be hard pressed to measure a difference between the two. The more gradual taper intuitively appears more desirable, but it's likely that it's beyond the point of diminishing returns. Either one sounds like it will improve the wastegate's discharge coefficient over the stock config, which will constitute the single biggest difference. This will allow more control over boost creep. Neither is as optimal as the divorced wastegate flow arrangement, however.
There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.
As for primary lengths on turbo headers, it is advantageous to use equal-length primaries to time the arrival of the pulses at the turbine equally and to keep cylinder reversion balanced across all cylinders. This will improve boost response and the engine's VE. Equal-length is often difficult to achieve due to tight packaging, fabrication difficulty, and the desire to have runners of the shortest possible length.”
"Here's a worked example (simplified) of how larger exhausts help turbo cars:
Say you have a turbo operating at a turbine pressure ratio (aka expansion ratio) of 1.8:1. You have a small turboback exhaust that contributes, say, 10 psig backpressure at the turbine discharge at redline. The total backpressure seen by the engine (upstream of the turbine) in this case is:
(14.5 +10)*1.8 = 44.1 psia = 29.6 psig total backpressure
o here, the turbine contributed 19.6 psig of backpressure to the total.
Now you slap on a proper low-backpressure, big turboback exhaust. Same turbo, same boost, etc. You measure 3 psig backpressure at the turbine discharge. In this case the engine sees just 17 psig total backpressure! And the turbine's contribution to the total backpressure is reduced to 14 psig (note: this is 5.6 psig lower than its contribution in the "small turboback" case).
So in the end, the engine saw a reduction in backpressure of 12.6 psig when you swapped turbobacks in this example. This reduction in backpressure is where all the engine's VE gains come from.
This is why larger exhausts make such big gains on nearly all stock turbo cars-- the turbine compounds the downstream backpressure via its expansion ratio. This is also why bigger turbos make more power at a given boost level-- they improve engine VE by operating at lower turbine expansion ratios for a given boost level.
As you can see, the backpressure penalty of running a too-small exhaust (like 2.5" for 350 hp) will vary depending on the match. At a given power level, a smaller turbo will generally be operating at a higher turbine pressure ratio and so will actually make the engine more sensitive to the backpressure downstream of the turbine than a larger turbine/turbo would.
Apr 8, 2009, 10:31 AM
#11
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^ Exactly right, all theory on velocity and flow go out the window after the turbo. I can agree that some amount of backpressure will help the turbo spool faster, but after it hits full boost no restriction is best.
Apr 8, 2009, 10:44 AM
#12
Interesting posts you found there Zeus. But I think the OP of those posts has some serious contradictions with real world evidence. He said 3" is too small for 450 hp? Buschur tested going from 3" to 3.5" at 800 hp and saw no gains. So the OP was very wrong in that statement. Which is a pretty big issue!
Secondly, the OP said that if you don't taper the enlarging of the exhaust correctly, that it could have a detrimental effect on spooling the turbo. For example, someone bolting a 3" downpipe onto a stock turbo elbow (o2 housing) which probably has a diameter of less than 2.5". There is no taper there. None of us use any taper in our exhaust. So to take from the above quote that bigger is better after the turbo is not correct.
Secondly, the OP said that if you don't taper the enlarging of the exhaust correctly, that it could have a detrimental effect on spooling the turbo. For example, someone bolting a 3" downpipe onto a stock turbo elbow (o2 housing) which probably has a diameter of less than 2.5". There is no taper there. None of us use any taper in our exhaust. So to take from the above quote that bigger is better after the turbo is not correct.
Apr 8, 2009, 10:46 AM
#13
Edit: Edited out quote because it wasn't relevant to the post
Back pressure is NEVER good for a car so lets just throw that out the window. If it was good people would weld the exhaust shut or make it so tiny only a pin ***** could exit. In my experience putting a bigger exhaust on a turbo always helped it spool sooner, not touching the headers. There is a reason a turbo drag car has essentially an open down pipe of 4+ inch diameter running right to the outside.
What is good is exhaust scavenging which means less than 0 back pressure or slight vacuum in the chamber to help draw in fresh air. Fresh air is good and exhaust fumes stuck in the chamber hurts performance.
The big problem with a turbo ironically is back pressure its unavoidable because of the turbine. Now far as off boost performance goes I really am not sure I know even at very small throttle inputs the turbo is going to hurt performance until it starts spinning fast enough to allow air past it effectively.
So this leaves me with the conclusion if you are seeing better performance with a smaller exhaust on this can only mean that the tune you are running is more effective at lower loads than higher load cells.
Back pressure is NEVER good for a car so lets just throw that out the window. If it was good people would weld the exhaust shut or make it so tiny only a pin ***** could exit. In my experience putting a bigger exhaust on a turbo always helped it spool sooner, not touching the headers. There is a reason a turbo drag car has essentially an open down pipe of 4+ inch diameter running right to the outside.
What is good is exhaust scavenging which means less than 0 back pressure or slight vacuum in the chamber to help draw in fresh air. Fresh air is good and exhaust fumes stuck in the chamber hurts performance.
The big problem with a turbo ironically is back pressure its unavoidable because of the turbine. Now far as off boost performance goes I really am not sure I know even at very small throttle inputs the turbo is going to hurt performance until it starts spinning fast enough to allow air past it effectively.
So this leaves me with the conclusion if you are seeing better performance with a smaller exhaust on this can only mean that the tune you are running is more effective at lower loads than higher load cells.
Last edited by RoadSpike; Apr 8, 2009 at 06:02 PM.
Apr 8, 2009, 10:57 AM
#14
There's more to it, though-- if a larger bellmouth is excessively large right at the turbine discharge (a large step diameter increase), there will be an unrecoverable dump loss that will contribute to backpressure. This is why a gradual increase in diameter, like the conical diffuser mentioned earlier, is desirable at the turbine discharge.
Also, in the example used by the OP of the quote, you decrease back pressure at the turbine discharge, which then decreases back pressure at the cylinder head. I really wonder how these two variables effect each other. At some point going too big, so soon after the turbine discharge must effect spool, and it may only decrease back pressure at the turbine discharge nominally, and even less so at the cylinder head.
Apr 8, 2009, 10:59 AM
#15
Do not confuse the concepts of NA exhaust tuning with turbo exhaust requirements.
An NA engine depends upon exhaust gas velocity in the system to create a negative pressure wave that scavenges the next exhausting cylinder. This is how highly tuned NA engines generate >100% VE, and why those systems require very specific primary and collector designs to be efficient.
With a turbo engine, there is only one place where this principle can possibly be applied, and that is in the turbo manifold/turbine housing area. On the exit side of the turbine, the best possible situation is to have an indicated pressure of "0" (or as close to it as possible), which means using a system of sufficient design and size such that it accumulates no pressure.
JKav is correct.
An NA engine depends upon exhaust gas velocity in the system to create a negative pressure wave that scavenges the next exhausting cylinder. This is how highly tuned NA engines generate >100% VE, and why those systems require very specific primary and collector designs to be efficient.
With a turbo engine, there is only one place where this principle can possibly be applied, and that is in the turbo manifold/turbine housing area. On the exit side of the turbine, the best possible situation is to have an indicated pressure of "0" (or as close to it as possible), which means using a system of sufficient design and size such that it accumulates no pressure.
JKav is correct.