Reducing lag and making torque peak lower rpm
01-06-2006, 10:36 PM
#31
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Jamie,
I must get to PET and have a look! Memory is a wonderful thing !
Why not fit a flap as the N/A does to overcome the low end issue ?
The 2005 Boxster divided primary resonance is Porsche starting to think at last . The Varioram manifold was a poorly thought out solution. Even on turbo motors the idea of splitting the inlet into two 3 cylinder halves at less than 4000rpm does good things for low down torque and boost response if the runner lenghts are right .
Could really help with the lag from big turbos.
Be good to know if anyone has tried this on a 993 motor.
All the best
Geoff
I must get to PET and have a look! Memory is a wonderful thing !
Why not fit a flap as the N/A does to overcome the low end issue ?
The 2005 Boxster divided primary resonance is Porsche starting to think at last . The Varioram manifold was a poorly thought out solution. Even on turbo motors the idea of splitting the inlet into two 3 cylinder halves at less than 4000rpm does good things for low down torque and boost response if the runner lenghts are right .
Could really help with the lag from big turbos.
Be good to know if anyone has tried this on a 993 motor.
All the best
Geoff
01-07-2006, 03:17 AM
#32
Three Wheelin'
01-07-2006, 04:25 AM
#33
Pro
Thread Starter
Join Date: Nov 2005
Location: on the road..
Posts: 578
Likes: 0
Received 0 Likes
on
0 Posts
This seems to be very interesting topic. I started that because I wondered if NA car's exhaust system where exhaust are driven by cycles and pulses, are possbily related to improving turbo exhaust systems.
It seems that not, because turbo requires low back pressure and big piping but.. how about that exhaust gas speed, like ttAmerica wrote, good velocity is also needed. There must be also some mathematic equation for these optimization?
It seems that not, because turbo requires low back pressure and big piping but.. how about that exhaust gas speed, like ttAmerica wrote, good velocity is also needed. There must be also some mathematic equation for these optimization?
01-07-2006, 11:12 AM
#34
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Good to get some background and know that this path has been trodden before !
The flat six presents all kinds of problems for intake manifold design .The resonance chamber is there to provide another resonance path and to increase the area of the link pipes so raising their resonance frequency.
At low ,<4000 rpm, the six cylinders feeding from a common plenum is not the best arrangement for cylinder filling . An improvement is to seperate the two banks , making two ,3 cylinder motors ,joined together with a long intake pipe that resonates at these rpm.
At over 4000rpm the long pipes are a problem and need to be switched out.
The 2005 Boxster arrangement is a neat solution for very little cost !!
Makes the Varioram look silly!!
As you may tell ,this subject is a bit of a passion for me . I just love the idea of "free" torque and response.
I hope all this may get one of our turbo tuners to take a look and see what can be done.
All the best
Geoff
The flat six presents all kinds of problems for intake manifold design .The resonance chamber is there to provide another resonance path and to increase the area of the link pipes so raising their resonance frequency.
At low ,<4000 rpm, the six cylinders feeding from a common plenum is not the best arrangement for cylinder filling . An improvement is to seperate the two banks , making two ,3 cylinder motors ,joined together with a long intake pipe that resonates at these rpm.
At over 4000rpm the long pipes are a problem and need to be switched out.
The 2005 Boxster arrangement is a neat solution for very little cost !!
Makes the Varioram look silly!!
As you may tell ,this subject is a bit of a passion for me . I just love the idea of "free" torque and response.
I hope all this may get one of our turbo tuners to take a look and see what can be done.
All the best
Geoff
01-07-2006, 03:20 PM
#35
Pro
Thread Starter
Join Date: Nov 2005
Location: on the road..
Posts: 578
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by pstoppani
Where exactly are you reading this? I went to the "mods" page and it says this:
You are talking about bypass ones, they are pipes without cats. Not legal and very loud.
But when I read shiv's great writing again,
I can now make my summary, so when changing good shaped, equally length headers they gives better exhaust gas velocity which is needed for quick spoolup.
Also Kevin's ZC machining must improve quick spoolups because all air that goes to turbine will rotate the wheel because there are no clearance.
Also as big as possible and free flowing cats and mufflers will help spool because they reduce back pressure which should be as near zero as possible without been too loud at same time.
Is this summary right?
Here is a good picture from Paul Frere's book about turbos system:
Last edited by Jussi; 01-07-2006 at 05:44 PM.
01-07-2006, 06:40 PM
#36
Addict
Rennlist Member
Rennlist Member
I assumed it was clear that if "no cats is good" then you'd understand that "sports cats is good" too since properly designed sports cats do the same thing as no cats, reduce back pressure, in a legal manner.
Sport cats are louder than stock cats.
Does anyone make headers that are better than stock?
I believe that ZC turbos are ZC only on the compressor side so I'd expect that you'd get the same spool up but you'd get more power for the same turbine speed because the compressor is more efficient (compared to the equivalent non-ZC turbo). The turbine side is the same (right?).
Sport cats are louder than stock cats.
Does anyone make headers that are better than stock?
I believe that ZC turbos are ZC only on the compressor side so I'd expect that you'd get the same spool up but you'd get more power for the same turbine speed because the compressor is more efficient (compared to the equivalent non-ZC turbo). The turbine side is the same (right?).
01-08-2006, 02:41 AM
#37
Pro
Thread Starter
Join Date: Nov 2005
Location: on the road..
Posts: 578
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by pstoppani
I believe that ZC turbos are ZC only on the compressor side so I'd expect that you'd get the same spool up but you'd get more power for the same turbine speed because the compressor is more efficient (compared to the equivalent non-ZC turbo). The turbine side is the same (right?).
So that's why I think ZC modified turbos with K24/K26 wheels must be best choice for 3.8TT (when target is very powerful street usage car).
We are trying to modify old original K16 turbos like that. It should be possible?
TTP company sells Turboheaders which at least looks very nice:
http://www.t-t-p.de/english/headers.php
01-08-2006, 03:10 AM
#38
Addict
Rennlist Member
Rennlist Member
Very interesting (or odd) that TTP does not quote a power increase for the 993 headers. I've heard that the factory headers are pretty much as good as it gets.
01-08-2006, 09:51 AM
#39
Nordschleife Master
I would not activate the factory resonance flap in a turbocharged environment. From my experience with a 964 N/A plastic manifold on a single turbo 3.45l in the area of high load naturally aspirated, but before turbo boost pressure, and above 5000 rpm the resonance flap helped the engine to produce about 10-12 additional rwhp in steady state tuning tests. However, the flap assembly is weak and I know of several that have broken under boost pressure, sending parts through the engine which is bad. If you designed the flap to me more solid to stand up to the boost pressure, then it might be a worthwhile addition, at least on paper. Although, the ranges that we are talking about are not usually reached in a turbocharged environment, so the value add in the real world would probably be not noticed.
Opening the secondary chamber without the flap does in crease power in the higher RPM ranges over a blocked off manifold.
Opening the secondary chamber without the flap does in crease power in the higher RPM ranges over a blocked off manifold.
Last edited by Geoffrey; 01-08-2006 at 10:28 AM.
01-08-2006, 01:54 PM
#40
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Geoffrey,
Thanks for that.Voice of experience again! You are right that the resonance flap disk on the N/A throttle housing I have just looked at is made of plastic .A aluminium replacement should fix that problem .The shaft is steel.
I am still a little puzzled why there is so little thought going in to inlet systems.
My simple thinking says that if a motor makes more torque off boost it will make more on boost ? and because of the better cylinder filling, wind the turbo up quicker ?
I noticed that on the Protomotive site the 993 N/A conversions state that the Varioram motor gives more bhp/torque than the 95 system .I know there is a valve size difference but the cams are very similar .
Is this an area that has been ignored because ever bigger turbos are more exciting !!
Geoff
Thanks for that.Voice of experience again! You are right that the resonance flap disk on the N/A throttle housing I have just looked at is made of plastic .A aluminium replacement should fix that problem .The shaft is steel.
I am still a little puzzled why there is so little thought going in to inlet systems.
My simple thinking says that if a motor makes more torque off boost it will make more on boost ? and because of the better cylinder filling, wind the turbo up quicker ?
I noticed that on the Protomotive site the 993 N/A conversions state that the Varioram motor gives more bhp/torque than the 95 system .I know there is a valve size difference but the cams are very similar .
Is this an area that has been ignored because ever bigger turbos are more exciting !!
Geoff
01-08-2006, 02:29 PM
#42
Three Wheelin'
Join Date: Dec 2004
Location: Halifax, Nova Scotia , Canada
Posts: 1,779
Likes: 0
Received 0 Likes
on
0 Posts
Geoffrey,
Thats suprising . The pressure on the disc is the pressure difference between the two sides of the manifold and that breaks a 1/4 inch rod ! Guess I had better start doing summs on disc area/pressure and forces !!!!
Any clues as to where the shaft breaks ? At the root or center ?
Thanks
Geoff
Thats suprising . The pressure on the disc is the pressure difference between the two sides of the manifold and that breaks a 1/4 inch rod ! Guess I had better start doing summs on disc area/pressure and forces !!!!
Any clues as to where the shaft breaks ? At the root or center ?
Thanks
Geoff
11-04-2007, 08:27 PM
#43
Addict
Rennlist Member
Rennlist Member
Regarding Optimal Exhaust System Design
Here's some good info from a friend of mine. Keep in mind that we were talking about 4 cylinder engines with a single exhaust system. For Porsches and other cars with two separate exhaust systems, pipe diameter requirements will be reduced by 50% to support the same power.
This thread was brought to my attention by a friend of mine [shiv@vishnu] 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
So 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. As for output temperatures, I'm not sure I understand the question. Are you referring to compressor outlet temperatures?
The advantage to the bellmouth setup from the wg's perspective is that it allows a less torturous path for the bypassed gases to escape. This makes it more effective in bypassing gases for a given pressure differential and wg valve position. Think of it as improving the VE of the wastegate. If you have a very compromised wg discharge routing, under some conditions the wg may not be able bypass enough flow to control boost, even when wide open. So the gases go through the turbine instead of the wg, and boost creeps up.
The downside to a bellmouth is that the wg flow still dumps right into the turbine discharge. A divider wall would be beneficial here. And, as mentioned earlier, if you go too big on the bellmouth and the turbine discharge flow sees a rapid area change (regardless of whether the wg flow is being introduced there or not), you will incur a backpressure penalty right at the site of the step. This is why you want gradual area changes in your exhaust.
Here's some good info from a friend of mine. Keep in mind that we were talking about 4 cylinder engines with a single exhaust system. For Porsches and other cars with two separate exhaust systems, pipe diameter requirements will be reduced by 50% to support the same power.
This thread was brought to my attention by a friend of mine [shiv@vishnu] 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
So 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. As for output temperatures, I'm not sure I understand the question. Are you referring to compressor outlet temperatures?
The advantage to the bellmouth setup from the wg's perspective is that it allows a less torturous path for the bypassed gases to escape. This makes it more effective in bypassing gases for a given pressure differential and wg valve position. Think of it as improving the VE of the wastegate. If you have a very compromised wg discharge routing, under some conditions the wg may not be able bypass enough flow to control boost, even when wide open. So the gases go through the turbine instead of the wg, and boost creeps up.
The downside to a bellmouth is that the wg flow still dumps right into the turbine discharge. A divider wall would be beneficial here. And, as mentioned earlier, if you go too big on the bellmouth and the turbine discharge flow sees a rapid area change (regardless of whether the wg flow is being introduced there or not), you will incur a backpressure penalty right at the site of the step. This is why you want gradual area changes in your exhaust.
