exhaust design in theory
Jan 7, 2004, 05:52 AM
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exhaust design in theory
The following excerpts are from Jay Kavanaugh, a turbosystems engineer at Garret, responding to a thread on http://www.impreza.net :
“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
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."
“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
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."
Jan 7, 2004, 07:12 AM
#3
Excellent tech info.. Its comforting to know that my current understanding is similar to what the the "big boys" indicate.. Makes me feel more comfortable that the stuff I'm working with and engineering is going to work my intended application.
Thanks for the great info!
Thanks for the great info!
Jan 7, 2004, 07:44 AM
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Damn! This is excellent insight! I just wish it came here firsthand - I guess this board needs to work a little harder to attract more people of Mr. Kavanaugh's caliber.
I have a question. I do not quite understand what Jay means by "divorced wastegate flow". Are these the two gas streams coming into the O2 housing? And if so, do OEM and AM designs run them "separately" for the recommended 12-18"? Thanks.
I have a question. I do not quite understand what Jay means by "divorced wastegate flow". Are these the two gas streams coming into the O2 housing? And if so, do OEM and AM designs run them "separately" for the recommended 12-18"? Thanks.
Jan 8, 2004, 08:04 AM
#7
I have done lots of exhaust work on many different turbo cars. Some of this theory is not completely correct.
Eliminating all the backpressure in the exhaust is not necesarily always a good thing. Here are some back to back test that kinda through me for a loop.
When I first built my AWD mirage the car had a 2.3stroker with 57trim/stage3 turbo. For about 6 months I tooled around with just a downpipe. It was 2-3/4 in diameter and it ended just after the oil pan. running 16psi the engine made 5psi at 3200rpm, and 16psi at 4200 rpm. After being totally annoyed with the exhaust noise I finally built the rest of the exhaust. From 3" tubing. When I took the car for its first spin I was amazed at how much faster the car built boost. It had 10psi by 3200 and 16psi by 3800rpm. These numbers are not fudged. The technical explanation I got from my local turbo shop was. "the turbo spooled too fast and the compressor was unable to grab air. When the exhaust was added it slowed the turbine down allowing the compressor to grab more air"
With a 300-325 whp car(16g turbo) the switch from a 2.5 o 3.0 exhast also left me completely surprised at the loss of torque. On the highway in 5th gear at 4000rpm, the before and after was huge difference. The car had way more torque with the 2.5. With the 3.0 you had to downshift to 4th gear and it still did not pull quite as hard as 5th with the 2.5 exhaust. The 2.5 exhaust had way more torque in all the gears below 4000rpm. After 6 months or so I felt the added noise level of the three wasn't worth it so i built a 2-3/4 sytem for that car. The torque characteristics were right in the middle.
On a friends car. We went to the dyno to replace his 3.0dp/2.5turboback with a full 3.0 exhaust.(both were burshur exhausts) We did it on the dyno cause we wanted to see the before and after gain of the 3.0 exhaust. There was a bunch of people around and we were all wagering our geusses as to how much new power he was going to find. The range was 20-50whp. The first pull produced silence from everyone. The car lost 6whp. The frist thing I did was look at the air fuel ratios to see if the added flow was the culprit. 11.5 on both. This car had a 20g and was making 375 whp. the car later recieved a 60trim turbo and went on to make 475whp with the 3.0 exhaust.
In short the theories are great but sometimes the theories dont tell the whole story. Backpressure and scavanging play a role in turbo spool and torque. The statement "build the exhaust as loud as your ears can handle" applies well to a drag car that stays at high rpms through the 1/4.
Eliminating all the backpressure in the exhaust is not necesarily always a good thing. Here are some back to back test that kinda through me for a loop.
When I first built my AWD mirage the car had a 2.3stroker with 57trim/stage3 turbo. For about 6 months I tooled around with just a downpipe. It was 2-3/4 in diameter and it ended just after the oil pan. running 16psi the engine made 5psi at 3200rpm, and 16psi at 4200 rpm. After being totally annoyed with the exhaust noise I finally built the rest of the exhaust. From 3" tubing. When I took the car for its first spin I was amazed at how much faster the car built boost. It had 10psi by 3200 and 16psi by 3800rpm. These numbers are not fudged. The technical explanation I got from my local turbo shop was. "the turbo spooled too fast and the compressor was unable to grab air. When the exhaust was added it slowed the turbine down allowing the compressor to grab more air"
With a 300-325 whp car(16g turbo) the switch from a 2.5 o 3.0 exhast also left me completely surprised at the loss of torque. On the highway in 5th gear at 4000rpm, the before and after was huge difference. The car had way more torque with the 2.5. With the 3.0 you had to downshift to 4th gear and it still did not pull quite as hard as 5th with the 2.5 exhaust. The 2.5 exhaust had way more torque in all the gears below 4000rpm. After 6 months or so I felt the added noise level of the three wasn't worth it so i built a 2-3/4 sytem for that car. The torque characteristics were right in the middle.
On a friends car. We went to the dyno to replace his 3.0dp/2.5turboback with a full 3.0 exhaust.(both were burshur exhausts) We did it on the dyno cause we wanted to see the before and after gain of the 3.0 exhaust. There was a bunch of people around and we were all wagering our geusses as to how much new power he was going to find. The range was 20-50whp. The first pull produced silence from everyone. The car lost 6whp. The frist thing I did was look at the air fuel ratios to see if the added flow was the culprit. 11.5 on both. This car had a 20g and was making 375 whp. the car later recieved a 60trim turbo and went on to make 475whp with the 3.0 exhaust.
In short the theories are great but sometimes the theories dont tell the whole story. Backpressure and scavanging play a role in turbo spool and torque. The statement "build the exhaust as loud as your ears can handle" applies well to a drag car that stays at high rpms through the 1/4.
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Jan 8, 2004, 08:27 AM
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Great point by 94AWDcoupe. All the info from the Garret engineer is great, but it's hard to apply that to everyday driving where torque is more important. I wish I had the luxury of revving high all the time.
Jan 8, 2004, 11:58 AM
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would it be optimal, then, to run a 2.5" downpipe with a 3" catback? (if you aren't planning on heavy modification)
i still need to run the stock cat to. california isn't friendly with emissions.
i still need to run the stock cat to. california isn't friendly with emissions.
Jan 8, 2004, 12:12 PM
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Originally posted by 94AWDcoupe
...In short the theories are great but sometimes the theories dont tell the whole story. Backpressure and scavanging play a role in turbo spool and torque. The statement "build the exhaust as loud as your ears can handle" applies well to a drag car that stays at high rpms through the 1/4...
...In short the theories are great but sometimes the theories dont tell the whole story. Backpressure and scavanging play a role in turbo spool and torque. The statement "build the exhaust as loud as your ears can handle" applies well to a drag car that stays at high rpms through the 1/4...
Jan 9, 2004, 06:52 AM
#11
Hugh Mcinnes wrote a book on turbos 30 years ago that states some theory that differ from some of jays text above.
He stated it was best to make the 02 housing and first 40 inches of the exhaust as big as possible then it could be necked down in size after the gasses have cooled a bit. His theory stated the gasses are swirling out of the turbine. The swirling he says hurts flow and is best to break up the swirl as soon as possible. So by his text you might think a 3.0 downpipe with 2.5 exaust would work better than a 2.5 downpipe and 3.0 exhaust.
Which works better? I doubt you will find anyone who wold take the time to show what works better in a back to back test.
He stated it was best to make the 02 housing and first 40 inches of the exhaust as big as possible then it could be necked down in size after the gasses have cooled a bit. His theory stated the gasses are swirling out of the turbine. The swirling he says hurts flow and is best to break up the swirl as soon as possible. So by his text you might think a 3.0 downpipe with 2.5 exaust would work better than a 2.5 downpipe and 3.0 exhaust.
Which works better? I doubt you will find anyone who wold take the time to show what works better in a back to back test.
Jan 9, 2004, 07:31 AM
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Originally posted by 94AWDcoupe
Hugh Mcinnes wrote a book on turbos 30 years ago that states some theory that differ from some of jays text above.
Hugh Mcinnes wrote a book on turbos 30 years ago that states some theory that differ from some of jays text above.
94AWDcoupe, on the example you gave above about your buddy who lost torque going from a 2.5" to a full 3" did he have a ported or high flow exhaust manifold? If the flow was too high BEFORE the turbo the switch in exhaust diameter could cause a drop in torque. You are trying to achieve the proper pressure RATIO before and after the turbine to have the best spool-up characteristics, high velocity in…high volume out
Jan 9, 2004, 07:52 AM
#13
Originally posted by 94AWDcoupe
I have done lots of exhaust work on many different turbo cars. Some of this theory is not completely correct.
Eliminating all the backpressure in the exhaust is not necesarily always a good thing. Here are some back to back test that kinda through me for a loop.
When I first built my AWD mirage the car had a 2.3stroker with 57trim/stage3 turbo. For about 6 months I tooled around with just a downpipe. It was 2-3/4 in diameter and it ended just after the oil pan. running 16psi the engine made 5psi at 3200rpm, and 16psi at 4200 rpm. After being totally annoyed with the exhaust noise I finally built the rest of the exhaust. From 3" tubing. When I took the car for its first spin I was amazed at how much faster the car built boost. It had 10psi by 3200 and 16psi by 3800rpm. These numbers are not fudged. The technical explanation I got from my local turbo shop was. "the turbo spooled too fast and the compressor was unable to grab air. When the exhaust was added it slowed the turbine down allowing the compressor to grab more air"
With a 300-325 whp car(16g turbo) the switch from a 2.5 o 3.0 exhast also left me completely surprised at the loss of torque. On the highway in 5th gear at 4000rpm, the before and after was huge difference. The car had way more torque with the 2.5. With the 3.0 you had to downshift to 4th gear and it still did not pull quite as hard as 5th with the 2.5 exhaust. The 2.5 exhaust had way more torque in all the gears below 4000rpm. After 6 months or so I felt the added noise level of the three wasn't worth it so i built a 2-3/4 sytem for that car. The torque characteristics were right in the middle.
On a friends car. We went to the dyno to replace his 3.0dp/2.5turboback with a full 3.0 exhaust.(both were burshur exhausts) We did it on the dyno cause we wanted to see the before and after gain of the 3.0 exhaust. There was a bunch of people around and we were all wagering our geusses as to how much new power he was going to find. The range was 20-50whp. The first pull produced silence from everyone. The car lost 6whp. The frist thing I did was look at the air fuel ratios to see if the added flow was the culprit. 11.5 on both. This car had a 20g and was making 375 whp. the car later recieved a 60trim turbo and went on to make 475whp with the 3.0 exhaust.
In short the theories are great but sometimes the theories dont tell the whole story. Backpressure and scavanging play a role in turbo spool and torque. The statement "build the exhaust as loud as your ears can handle" applies well to a drag car that stays at high rpms through the 1/4.
I have done lots of exhaust work on many different turbo cars. Some of this theory is not completely correct.
Eliminating all the backpressure in the exhaust is not necesarily always a good thing. Here are some back to back test that kinda through me for a loop.
When I first built my AWD mirage the car had a 2.3stroker with 57trim/stage3 turbo. For about 6 months I tooled around with just a downpipe. It was 2-3/4 in diameter and it ended just after the oil pan. running 16psi the engine made 5psi at 3200rpm, and 16psi at 4200 rpm. After being totally annoyed with the exhaust noise I finally built the rest of the exhaust. From 3" tubing. When I took the car for its first spin I was amazed at how much faster the car built boost. It had 10psi by 3200 and 16psi by 3800rpm. These numbers are not fudged. The technical explanation I got from my local turbo shop was. "the turbo spooled too fast and the compressor was unable to grab air. When the exhaust was added it slowed the turbine down allowing the compressor to grab more air"
With a 300-325 whp car(16g turbo) the switch from a 2.5 o 3.0 exhast also left me completely surprised at the loss of torque. On the highway in 5th gear at 4000rpm, the before and after was huge difference. The car had way more torque with the 2.5. With the 3.0 you had to downshift to 4th gear and it still did not pull quite as hard as 5th with the 2.5 exhaust. The 2.5 exhaust had way more torque in all the gears below 4000rpm. After 6 months or so I felt the added noise level of the three wasn't worth it so i built a 2-3/4 sytem for that car. The torque characteristics were right in the middle.
On a friends car. We went to the dyno to replace his 3.0dp/2.5turboback with a full 3.0 exhaust.(both were burshur exhausts) We did it on the dyno cause we wanted to see the before and after gain of the 3.0 exhaust. There was a bunch of people around and we were all wagering our geusses as to how much new power he was going to find. The range was 20-50whp. The first pull produced silence from everyone. The car lost 6whp. The frist thing I did was look at the air fuel ratios to see if the added flow was the culprit. 11.5 on both. This car had a 20g and was making 375 whp. the car later recieved a 60trim turbo and went on to make 475whp with the 3.0 exhaust.
In short the theories are great but sometimes the theories dont tell the whole story. Backpressure and scavanging play a role in turbo spool and torque. The statement "build the exhaust as loud as your ears can handle" applies well to a drag car that stays at high rpms through the 1/4.
Not to metion Chrono's point on the ratio...
Last edited by Zeus; Jan 9, 2004 at 07:54 AM.
Jan 9, 2004, 08:20 AM
#14
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Originally posted by chronohunter
Thirty years ago this stuff was theory. These days people like Garret can do complex 3d modeling of gasses traveling through and out of the turbine. So I would trust those guys, it's not in their best interest to give out bad or false information.
94AWDcoupe, on the example you gave above about your buddy who lost torque going from a 2.5" to a full 3" did he have a ported or high flow exhaust manifold? If the flow was too high BEFORE the turbo the switch in exhaust diameter could cause a drop in torque. You are trying to achieve the proper pressure RATIO before and after the turbine to have the best spool-up characteristics, high velocity in…high volume out
Thirty years ago this stuff was theory. These days people like Garret can do complex 3d modeling of gasses traveling through and out of the turbine. So I would trust those guys, it's not in their best interest to give out bad or false information.
94AWDcoupe, on the example you gave above about your buddy who lost torque going from a 2.5" to a full 3" did he have a ported or high flow exhaust manifold? If the flow was too high BEFORE the turbo the switch in exhaust diameter could cause a drop in torque. You are trying to achieve the proper pressure RATIO before and after the turbine to have the best spool-up characteristics, high velocity in…high volume out
So, sounds like 3" with OEM O2 housing is the way to go from the exhaust side. But is the OEM exhaust manifold optimized for the proper pressure differential ratio and should be the next thing on the upgrade path? I am wondering if taking care of exhaust flow of things before getting into the engine, like switching cams, is the way to go...
Jan 9, 2004, 09:14 AM
#15
Thirty years ago this stuff was theory. These days people like Garret can do complex 3d modeling of gasses traveling through and out of the turbine. So I would trust those guys, it's not in their best interest to give out bad or false information.
I personnally rely heavily on real world results.
Al I am saying with exhausts is there are trade offs in performance from different combos. A 2.5 exhaust performs quite nicely up to 300-350whp . The 3.0 isn't the clear winner till you go above 350-400whp. But this is opinion of mine based on testing not theory.