PTUNING: 3" TB & SB Exhaust System FAQ
Thread Starter
Senior Member
SL Member
Joined: Dec 2007
Posts: 132
From: Manassas, VA
PTUNING: 3" TB & SB Exhaust System FAQ
We've received quite a few phone calls and emails since we first began testing our prototype exhaust system and now after releasing the final product. Many asked why we did not go with a 2.5" system instead. Btw, for those that purchased our SB or TB Exhaust systems, thank you. You won't be dissappointed.
Now on to the FAQ:
*Note, unless otherwise stated, all comments are directed at turbocharged tCs vs. TRD SC models*
Why did we go with a 3" system and not something larger?
A 3" exhaust system is the largest practical size system you can fit under a street driven tC, a 3.5"would have compromised ground clearance quite a bit.
Wouldn't a 2.5" system offer more better spool-up and thus more low end torque?
Absolutely not. A turbocharged engine's exhaust gases wants to see ZERO (or anything even close to it) backpressure when it leaves the turbine housing and makes its way down the downpipe/s-pipe and through the exhaust. A 3.5" or 4" or even 5" may eventually lead to diminishing returns, but a larger system will NOT have less low-end torque, slower spoolup, less power, etc. than a smaller exhaust system.
Well then, what is the best performing exhaust system for a turbocharged car?
A short conical exhaust pipe right off of turbine outlet would probably offer the best spool-up and power delivery. Ever wonder why guys like Rado and Tran amongst many others don't use strategically sized exhaust systems on their turbocharged race cars? Because the best turbo exhaust system is essentially no exhaust system. Now, given the fact that we all cannot roll around in turbo tCs without exhaust systems, the largest practical size system with muffler will have to suffice.
I'm still not convinced, what about the backpressure and high exhaust gas velocity needed?
Guys, even on a normally aspirated motor, you are not looking to add just the right amount of back pressure for best performance, What you are trying to do is find the right size header and exhaust system that will give you the best Exhaust Gas Scavenging effect and highest exhaust gas velocity. Exhaust backpressure is just an ugly side effect of using smaller tubing or bottleneck (i.e. merge collector shape) to achieve best flow for the given engine. This concept DOES NOT apply to turbocharged engines. When the exhaust gases exit the turbo's turbine housing it wants to see the greatest pressure differential it can get - large exhaust system. The least amount of back pressure present after the turbine outlet results in higher VE, thereby increasing power gains.
Okay here are just a few of many, many, many tech articles written on turbocharger exhaust sizing:
Excerpt from a Garrett Engineer:
"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.
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."
Excerpt from Cobb Tuning "exhaust design" tech page:
"There is more to gain going from 2.5" up to 3" than there is going from 3" to 3.5". A 3" system will not loose torque compared to a 2.5" system if designed properly. In fact if designed properly 3" may be capable of making better low end torque than 2.5". Again, since the way to make the most torque with a turbo exhaust is to get the turbo to spool-up as quickly as possible, it should be the main goal of the entire exhaust system and good flow after the turbo is one way to achieve it. We use 3" as we want our system to flow enough to be capable of coping with a customer's changing goals. Properly designed we can offer it to the big power crowd while still appeasing the low end torque club."
Excerpt from Borla tech/news:
"The objective of a properly-engineered, aftermarket cat-back exhaust system is to provide additional performance while still delivering adequate sound control. Unlike an all-motor exhaust system, a turbo exhaust system suffers no ill effects from going as big as possible. Bigger is better in this case. The bigger or larger diameter exhaust pipes allow the back pressure to be significantly less than the factory exhausts system. As a result, the difference in exhaust pressure before and after the turbocharger is increased. The increase in the magnitude of the pressure difference allows the turbocharger to reach higher shaft speeds at lower engine operating rpms. As a result, boost response increases and boost pressures increase. More boost pressure at the intake manifold results more power at the wheels. Can a turbo exhaust be too big? A turbo exhaust is too big if it drags on the ground or is too large to provide adequate sound suppression. For peak performance, most aftermarket turbo exhaust manufacturers will use the largest diameter tubing that can be properly routed underneath the vehicle. A muffler will be matched that allows an adequate amount of sound suppression."
Given the FACTS stated above, we can say that we designed our 3" systems pretty much by the text book and back it up with dyno testing - lots and lots of dyno testing.
We spared no expense when designing this system. Polished T304 for everything, right down to the very last bolt and even O2 bung and plug.
Our systems utilize a mix of 16 and 18 gauge T304 tubing with gentle bends along with lightweight T304 V-band flanges to create a lightweight, leak-free, system that is also very durable too. Our muffler specs provide for a very nice sound that is amazingly quiet for a 3" system. However, we'll let our customers judge this aspect for themselves.
Lastly, we did not include one, but two extra O2 bungs with our 3" TB Exhaust System. The extra bung on the s-pipe can be used for a wideband gauge. The hidden rear bung can be used by your tuner for more accurate low-load AFR readings when dyno tuning. It can also be used by the DIY tuner looking to use a portable wideband set-up for road tuning. Sure, we could've pocketed the extra $15-$20 bucks and excluded the two extra bungs, but we prefer to offer the best product to our tC customers.
Btw, probably 9 out of 10 cars that go on our dyno are turbocharged or supercharged. Whether it be a turbocharged tC or xB, an EVO, WRX, STI, Supra TT, 350Z TT, S2000 Turbo, STR-4, Mazdaspeed 3, Mazdaspeed 6, RX-7 TT, Civic Turbo, Corolla Turbo, Celica Turbo etc. We've seen them all roll through our dyno facility at one time or another so we can sort or guess
at what mods will work and which won't. We will continue to develop products based off our knowledge of turbocharged cars in general and apply it to the tC and other Scions too for that matter. And you can be assured that all performance-related products will be thoroughly dyno-tested before they ever gets the thumbs-up for production.
Sorry for the long post guys. Just wanted to clear up a few things and add to the wealth of information that this forum provides.
Now on to the FAQ:
*Note, unless otherwise stated, all comments are directed at turbocharged tCs vs. TRD SC models*
Why did we go with a 3" system and not something larger?
A 3" exhaust system is the largest practical size system you can fit under a street driven tC, a 3.5"would have compromised ground clearance quite a bit.
Wouldn't a 2.5" system offer more better spool-up and thus more low end torque?
Absolutely not. A turbocharged engine's exhaust gases wants to see ZERO (or anything even close to it) backpressure when it leaves the turbine housing and makes its way down the downpipe/s-pipe and through the exhaust. A 3.5" or 4" or even 5" may eventually lead to diminishing returns, but a larger system will NOT have less low-end torque, slower spoolup, less power, etc. than a smaller exhaust system.
Well then, what is the best performing exhaust system for a turbocharged car?
A short conical exhaust pipe right off of turbine outlet would probably offer the best spool-up and power delivery. Ever wonder why guys like Rado and Tran amongst many others don't use strategically sized exhaust systems on their turbocharged race cars? Because the best turbo exhaust system is essentially no exhaust system. Now, given the fact that we all cannot roll around in turbo tCs without exhaust systems, the largest practical size system with muffler will have to suffice.
I'm still not convinced, what about the backpressure and high exhaust gas velocity needed?
Guys, even on a normally aspirated motor, you are not looking to add just the right amount of back pressure for best performance, What you are trying to do is find the right size header and exhaust system that will give you the best Exhaust Gas Scavenging effect and highest exhaust gas velocity. Exhaust backpressure is just an ugly side effect of using smaller tubing or bottleneck (i.e. merge collector shape) to achieve best flow for the given engine. This concept DOES NOT apply to turbocharged engines. When the exhaust gases exit the turbo's turbine housing it wants to see the greatest pressure differential it can get - large exhaust system. The least amount of back pressure present after the turbine outlet results in higher VE, thereby increasing power gains.
Okay here are just a few of many, many, many tech articles written on turbocharger exhaust sizing:
Excerpt from a Garrett Engineer:
"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.
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."
Excerpt from Cobb Tuning "exhaust design" tech page:
"There is more to gain going from 2.5" up to 3" than there is going from 3" to 3.5". A 3" system will not loose torque compared to a 2.5" system if designed properly. In fact if designed properly 3" may be capable of making better low end torque than 2.5". Again, since the way to make the most torque with a turbo exhaust is to get the turbo to spool-up as quickly as possible, it should be the main goal of the entire exhaust system and good flow after the turbo is one way to achieve it. We use 3" as we want our system to flow enough to be capable of coping with a customer's changing goals. Properly designed we can offer it to the big power crowd while still appeasing the low end torque club."
Excerpt from Borla tech/news:
"The objective of a properly-engineered, aftermarket cat-back exhaust system is to provide additional performance while still delivering adequate sound control. Unlike an all-motor exhaust system, a turbo exhaust system suffers no ill effects from going as big as possible. Bigger is better in this case. The bigger or larger diameter exhaust pipes allow the back pressure to be significantly less than the factory exhausts system. As a result, the difference in exhaust pressure before and after the turbocharger is increased. The increase in the magnitude of the pressure difference allows the turbocharger to reach higher shaft speeds at lower engine operating rpms. As a result, boost response increases and boost pressures increase. More boost pressure at the intake manifold results more power at the wheels. Can a turbo exhaust be too big? A turbo exhaust is too big if it drags on the ground or is too large to provide adequate sound suppression. For peak performance, most aftermarket turbo exhaust manufacturers will use the largest diameter tubing that can be properly routed underneath the vehicle. A muffler will be matched that allows an adequate amount of sound suppression."
Given the FACTS stated above, we can say that we designed our 3" systems pretty much by the text book and back it up with dyno testing - lots and lots of dyno testing.
We spared no expense when designing this system. Polished T304 for everything, right down to the very last bolt and even O2 bung and plug.
Our systems utilize a mix of 16 and 18 gauge T304 tubing with gentle bends along with lightweight T304 V-band flanges to create a lightweight, leak-free, system that is also very durable too. Our muffler specs provide for a very nice sound that is amazingly quiet for a 3" system. However, we'll let our customers judge this aspect for themselves.
Lastly, we did not include one, but two extra O2 bungs with our 3" TB Exhaust System. The extra bung on the s-pipe can be used for a wideband gauge. The hidden rear bung can be used by your tuner for more accurate low-load AFR readings when dyno tuning. It can also be used by the DIY tuner looking to use a portable wideband set-up for road tuning. Sure, we could've pocketed the extra $15-$20 bucks and excluded the two extra bungs, but we prefer to offer the best product to our tC customers.
Btw, probably 9 out of 10 cars that go on our dyno are turbocharged or supercharged. Whether it be a turbocharged tC or xB, an EVO, WRX, STI, Supra TT, 350Z TT, S2000 Turbo, STR-4, Mazdaspeed 3, Mazdaspeed 6, RX-7 TT, Civic Turbo, Corolla Turbo, Celica Turbo etc. We've seen them all roll through our dyno facility at one time or another so we can sort or guess
at what mods will work and which won't. We will continue to develop products based off our knowledge of turbocharged cars in general and apply it to the tC and other Scions too for that matter. And you can be assured that all performance-related products will be thoroughly dyno-tested before they ever gets the thumbs-up for production.Sorry for the long post guys. Just wanted to clear up a few things and add to the wealth of information that this forum provides.
Thread Starter
Senior Member
SL Member
Joined: Dec 2007
Posts: 132
From: Manassas, VA
Wow, I thought this debate would have been over a long time ago with so many owners running 3" exhaust systems and realizing the true power potential over a smaller exhaust system. Anyhow, I thought I would go ahead and revive my thread to join in on the exhaust debate. I know it's a bit wordy, but that's the only way to get the facts straight.
Remember guys, our products are proven over and over again on the dyno and the track. For those that don't already know, the 3" PTUNING S-Back used on our 2008 Redline Championship winning tC is an "off the shelf" system. It is IDENTICAL to the ones we are currently selling.
Going back to the dyno testing bit. Our tC has about 4,500 miles on it and has already seen 300+ pulls on the dyno and that's not including the countless hours spent on steady state tuning too. We can assure you that when we say a 3" turboback system is superior to a smaller system, we are not making this stuff up.
With that said, I can assure you that our, very soon to be released, turbo kit will see lots of dyno time and track testing too. We will try our best to release pics and info by the end of the week. Don't be too suprised when you find out that many of the key components of the turbo kit are very similar to the turbosystem on our TA tC.
Remember guys, our products are proven over and over again on the dyno and the track. For those that don't already know, the 3" PTUNING S-Back used on our 2008 Redline Championship winning tC is an "off the shelf" system. It is IDENTICAL to the ones we are currently selling.
Going back to the dyno testing bit. Our tC has about 4,500 miles on it and has already seen 300+ pulls on the dyno and that's not including the countless hours spent on steady state tuning too. We can assure you that when we say a 3" turboback system is superior to a smaller system, we are not making this stuff up.
With that said, I can assure you that our, very soon to be released, turbo kit will see lots of dyno time and track testing too. We will try our best to release pics and info by the end of the week. Don't be too suprised when you find out that many of the key components of the turbo kit are very similar to the turbosystem on our TA tC.
im happy with the ptuning exhaust except its less ricer look compared to my old N1 style.. lol but i definitely did not notice a slower spool on the low end and it pulls harder on the high end.. I've been looking for this post for awhile.. thanks for the bump
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