exhaust pressure versus boost pressure
12-05-2015, 10:01 AM
#1
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exhaust pressure versus boost pressure I am brand new to turbos and have a question. I am working on my first
turbo set up. I have just purchased wastegates and have been thinking
about boost--now you always hear about wastegate spring pressure--my question is how does boost correlate to exhaust pressure? For instance say you have 6 pound wastegate springs--am I correct in assuming that this
actually means nothing as far as what boost these springs will actually provide in this case or any other case as the wastegate spring pressure is only one of many variables and in no way actually means that 6 pound wastegate springs will provide 6 pounds boost. Can some of you experienced guys expound on this subject?
turbo set up. I have just purchased wastegates and have been thinking
about boost--now you always hear about wastegate spring pressure--my question is how does boost correlate to exhaust pressure? For instance say you have 6 pound wastegate springs--am I correct in assuming that this
actually means nothing as far as what boost these springs will actually provide in this case or any other case as the wastegate spring pressure is only one of many variables and in no way actually means that 6 pound wastegate springs will provide 6 pounds boost. Can some of you experienced guys expound on this subject?
12-05-2015, 10:24 AM
#2
If you have a good WG that is properly sized it will make pretty close to what the spring is set to regulate in terms of positive manifold pressure. One variance is taking the port reading off of the compressor housing vs the intake. Whatever the differential pressure between an IC is, if so equipped, it will Impact the gates response to that pressure differential.
12-05-2015, 04:49 PM
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Excessive back pressure before or after the turbo can effect the wastegate operation, but generally if you reference the boost feed for the gate at the manifold, therefore negating boost drop across IC and piping, spring pressure will be close to manifold boost.
12-05-2015, 05:47 PM
#4
Old statement, incorrect: The wastegate does not care what the exhaust pressure is (technically) it only references boost.
New statement, fixed: "The first time buyer of a wastegate will usually not care about exhaust gas pressure (when selecting their gate, especially the first one)"
You decide how much boost, for example if you use a boost controller, it fools the wastegate into seeing less boost (so it stays shut longer). So all the wastegate really does is open when YOU tell it to, which you decide by choosing a spring or boost controller setting. You can also change the reference, for example the pressure AT the turbo is 9psi but at the engine it might only be 7psi. So by referencing the boost from the engine (instead of the compressor, or near the turbo) you will actually have more boost pressure total.
Exhaust pressure is unwanted. Larger turbines make more power peak possible because they operate with less exhaust pressure. There is always some pressure, even in a complete laboratory vacuum on earth there is still some pressure. If you could stick a vacuum cleaner on the exhaust and use a lower pressure to assist the cylinder then the engine would not need to spend as much energy to push the exhaust gas out with a piston. OEM exhaust systems are typically designed with this in mind, they use a specific shape or size that lends exhaust gas velocity and acoustics (sound pressure waves) to cylinder scavenging, increasing power output by applying a suction to the exhaust system through careful design. This is why we hear people say that "their engine lost low end torque when they cut the exhaust system off". Simply put, a well designed exhaust system will lower the pressure at the exhaust valve when it opens through a specific RPM range. Cam timing and port sizes, including exhaust manifold runner sizes, may all be fine tuned for specific ranges of engine operation to give the lowest pressure at the exhaust valves when they open, and the highest pressure at the intake valves when open. The OEM manufacturer does the same thing to the intake manifold- notice the ports of a specific length/shape allow pressure waves to arrive at the intake valves through a specific rpm range to improve VE.
In the case of turbochargers all of the same rules apply- you can find equal length or tuned length turbo manifolds, there are plenty of fancy collector designs, twin scroll, twin wastegate, these are all attempts to lower exhaust pressure at the exhaust valve when it opens (think of the vacuum cleaner and OEM exhaust shape tuning). It is our job to do the best we can with these things, but there is a diminishing returns to cost when it comes to fabrication. If the application is budget minded, and the individual does not have a fabrication shop, the first thing to go is the fancy manifolds and collectors in the exhaust. The advantage of having a turbo is not to generate the lower exhaust gas pressure- in fact turbochargers generally produce higher exhaust gas pressures just by being in the way. The real reason they improve performance is because of their actions at the intake valve- the increased intake valve pressure. All of this talk of adjusting and fabricating for lower exhaust valve pressures is for the sponsor race teams, and the OEM manufacturers, for they have the capital to spend on the designs and testing and engineers with machines for fabrication. As budget racers we often need to use whatever we can get- sometimes a heavy cast log manifold for example. Or a factory header turned around. The reason these still work very well for turbocharger applications is because the net result is a higher pressure at the intake valve, and the cylinder pressure/piston can be used to drive exhaust gas out, especially if the camshaft timing is adjusted for the turbochargers higher exhaust pressure. For example when using a log manifold, which has none of the advantages of a tuned length manifold, the pressure is likely going to be much higher. If we time the camshaft such that the piston is able to push the exhaust gas out, and then the exhaust valve shuts right away before the intake valve begins to open (no overlap or minimal overlap) then we will prevent reverse flow exhaust contamination of the intake charge. You often see features like this in OEM camshafts for OEM turbo engines which use small cast iron manifolds for longevity. It will still lower performance compared to the tuned-appropriate length manifolds since the piston driving the exhaust gas out and the lack of overlap induced VE improvements means less airflow total mass, which means less horsepower output. If we can lower the exhaust pressure by using a nicely designed manifold (instead of the "log" type) we can then open our camshaft profile up to allow some overlap, use a larger turbine to reduce exhaust gas pressure, this will all in turn lower EGT and improve VE (volumetric efficiency) and allow us to make more power, but on the other hand that would cost more money, whereas instead we can also turn up the boost (free) and keep the "log" manifold. Do you see what I am trying to show you? Both ends of attack give improvements (exhaust and intake side) but the intake manifold side has a dial (boost controller) and the exhaust manifold side does not. Yes improvements to both sides improve power but the exhaust side is much more expensive to fix. Since there is a diminishing returns to boost pressure differential (as intake side pressure increases due to turbine RPM exhaust pressure may also climb) sizing the turbine and exhaust system properly will affect how much pressure you make as boost rises. Although I believe there are turbocharged applications that operate with a much higher exhaust pressure than intake pressure, that is far from ideal. This is possible because of the separation of the valve events. Temperature of the intake charge is another issue to rising boost pressure (when using pump gas this is a critical feature) but it can be defeated through creative means, water/methanol/ethanol injection, and proper intercooling, as these will reduce the temperatures associated with compressing air to be more compatible with pump gas. Furthermore, coating and wraps/blankets will keep the temperature inside the exhaust system where it will help expand exhaust gasses and increase exhaust velocity which drive the turbine. Blankets/shields/wraps should always be used to keep the temperature in the exhaust from leaking into the engine bay, and every attempt should be made to isolate the compressor side from the turbine to keep it as cool as possible.
I found this with a google search from a classic book I have seen before (never read it though) and it helps repeat alot of what I just wrote
https://books.google.com/books?id=pe...ake%3F&f=false
New statement, fixed: "The first time buyer of a wastegate will usually not care about exhaust gas pressure (when selecting their gate, especially the first one)"
You decide how much boost, for example if you use a boost controller, it fools the wastegate into seeing less boost (so it stays shut longer). So all the wastegate really does is open when YOU tell it to, which you decide by choosing a spring or boost controller setting. You can also change the reference, for example the pressure AT the turbo is 9psi but at the engine it might only be 7psi. So by referencing the boost from the engine (instead of the compressor, or near the turbo) you will actually have more boost pressure total.
Exhaust pressure is unwanted. Larger turbines make more power peak possible because they operate with less exhaust pressure. There is always some pressure, even in a complete laboratory vacuum on earth there is still some pressure. If you could stick a vacuum cleaner on the exhaust and use a lower pressure to assist the cylinder then the engine would not need to spend as much energy to push the exhaust gas out with a piston. OEM exhaust systems are typically designed with this in mind, they use a specific shape or size that lends exhaust gas velocity and acoustics (sound pressure waves) to cylinder scavenging, increasing power output by applying a suction to the exhaust system through careful design. This is why we hear people say that "their engine lost low end torque when they cut the exhaust system off". Simply put, a well designed exhaust system will lower the pressure at the exhaust valve when it opens through a specific RPM range. Cam timing and port sizes, including exhaust manifold runner sizes, may all be fine tuned for specific ranges of engine operation to give the lowest pressure at the exhaust valves when they open, and the highest pressure at the intake valves when open. The OEM manufacturer does the same thing to the intake manifold- notice the ports of a specific length/shape allow pressure waves to arrive at the intake valves through a specific rpm range to improve VE.
In the case of turbochargers all of the same rules apply- you can find equal length or tuned length turbo manifolds, there are plenty of fancy collector designs, twin scroll, twin wastegate, these are all attempts to lower exhaust pressure at the exhaust valve when it opens (think of the vacuum cleaner and OEM exhaust shape tuning). It is our job to do the best we can with these things, but there is a diminishing returns to cost when it comes to fabrication. If the application is budget minded, and the individual does not have a fabrication shop, the first thing to go is the fancy manifolds and collectors in the exhaust. The advantage of having a turbo is not to generate the lower exhaust gas pressure- in fact turbochargers generally produce higher exhaust gas pressures just by being in the way. The real reason they improve performance is because of their actions at the intake valve- the increased intake valve pressure. All of this talk of adjusting and fabricating for lower exhaust valve pressures is for the sponsor race teams, and the OEM manufacturers, for they have the capital to spend on the designs and testing and engineers with machines for fabrication. As budget racers we often need to use whatever we can get- sometimes a heavy cast log manifold for example. Or a factory header turned around. The reason these still work very well for turbocharger applications is because the net result is a higher pressure at the intake valve, and the cylinder pressure/piston can be used to drive exhaust gas out, especially if the camshaft timing is adjusted for the turbochargers higher exhaust pressure. For example when using a log manifold, which has none of the advantages of a tuned length manifold, the pressure is likely going to be much higher. If we time the camshaft such that the piston is able to push the exhaust gas out, and then the exhaust valve shuts right away before the intake valve begins to open (no overlap or minimal overlap) then we will prevent reverse flow exhaust contamination of the intake charge. You often see features like this in OEM camshafts for OEM turbo engines which use small cast iron manifolds for longevity. It will still lower performance compared to the tuned-appropriate length manifolds since the piston driving the exhaust gas out and the lack of overlap induced VE improvements means less airflow total mass, which means less horsepower output. If we can lower the exhaust pressure by using a nicely designed manifold (instead of the "log" type) we can then open our camshaft profile up to allow some overlap, use a larger turbine to reduce exhaust gas pressure, this will all in turn lower EGT and improve VE (volumetric efficiency) and allow us to make more power, but on the other hand that would cost more money, whereas instead we can also turn up the boost (free) and keep the "log" manifold. Do you see what I am trying to show you? Both ends of attack give improvements (exhaust and intake side) but the intake manifold side has a dial (boost controller) and the exhaust manifold side does not. Yes improvements to both sides improve power but the exhaust side is much more expensive to fix. Since there is a diminishing returns to boost pressure differential (as intake side pressure increases due to turbine RPM exhaust pressure may also climb) sizing the turbine and exhaust system properly will affect how much pressure you make as boost rises. Although I believe there are turbocharged applications that operate with a much higher exhaust pressure than intake pressure, that is far from ideal. This is possible because of the separation of the valve events. Temperature of the intake charge is another issue to rising boost pressure (when using pump gas this is a critical feature) but it can be defeated through creative means, water/methanol/ethanol injection, and proper intercooling, as these will reduce the temperatures associated with compressing air to be more compatible with pump gas. Furthermore, coating and wraps/blankets will keep the temperature inside the exhaust system where it will help expand exhaust gasses and increase exhaust velocity which drive the turbine. Blankets/shields/wraps should always be used to keep the temperature in the exhaust from leaking into the engine bay, and every attempt should be made to isolate the compressor side from the turbine to keep it as cool as possible.
I found this with a google search from a classic book I have seen before (never read it though) and it helps repeat alot of what I just wrote
https://books.google.com/books?id=pe...ake%3F&f=false
Last edited by kingtal0n; 12-14-2015 at 07:48 PM.
12-05-2015, 06:08 PM
#5
Omg durp
Quit posting stupid stories of just durp
"The wastegate doesn't care what exhaust pressure is"
WG operation has everything to do with exhaust pressure
Quit posting stupid stories of just durp
"The wastegate doesn't care what exhaust pressure is"
WG operation has everything to do with exhaust pressure
Last edited by LASTLS1; 12-05-2015 at 10:54 PM. Reason: Ease up killer.
12-05-2015, 06:17 PM
#6
I am brand new to turbos and have a question. I am working on my first
turbo set up. I have just purchased wastegates and have been thinking
about boost--now you always hear about wastegate spring pressure--my question is how does boost correlate to exhaust pressure? For instance say you have 6 pound wastegate springs--am I correct in assuming that this
actually means nothing as far as what boost these springs will actually provide in this case or any other case as the wastegate spring pressure is only one of many variables and in no way actually means that 6 pound wastegate springs will provide 6 pounds boost. Can some of you experienced guys expound on this subject?
turbo set up. I have just purchased wastegates and have been thinking
about boost--now you always hear about wastegate spring pressure--my question is how does boost correlate to exhaust pressure? For instance say you have 6 pound wastegate springs--am I correct in assuming that this
actually means nothing as far as what boost these springs will actually provide in this case or any other case as the wastegate spring pressure is only one of many variables and in no way actually means that 6 pound wastegate springs will provide 6 pounds boost. Can some of you experienced guys expound on this subject?
Yes actual boost delivered will be affected greatly by this ratio
12-05-2015, 07:15 PM
#7
If you used the whole sentence:"The wastegate does not care what the exhaust pressure is (technically) it only references boost." wouldn't that imply he is specifically talking about using a boost controller? The references surrounding that sentence refer to a controller. Not trying to get into the middle here, I could be wrong. I do not use a boost controller but am thinking about it.
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12-05-2015, 07:54 PM
#8
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I am just trying to understand better about wastegate operation--I am still reading and studying. I figured I would get some good info here. When you set up a wastegate can you count on what the spring pressure says to a fair degree or do you just run it on an engine and see what boost it will make just on the spring or does anyone do a bench test with a set pressure from an external source? I would imagine friction on the valve could play a part and then you would have the pressure which is applied to the bottom of the wastegate valve by the exhaust pressure itself also figuring in. I guess what I'm getting at is what procedure is generally used to set up a wastegate when used for the first time?
12-05-2015, 08:09 PM
#9
If you used the whole sentence:"The wastegate does not care what the exhaust pressure is (technically) it only references boost." wouldn't that imply he is specifically talking about using a boost controller? The references surrounding that sentence refer to a controller. Not trying to get into the middle here, I could be wrong. I do not use a boost controller but am thinking about it.
The gate has direct reference to exhaust pressure
You could remove all lines going to the WG and it would still operate on exhaust pressure alone
12-05-2015, 08:16 PM
#10
Conversely, when I said what I said earlier, I meant it: if you have a 5psi spring, the wastegate opens at 5psi, even if the engine is off and there is no exhaust pressure. It also opens at 5psi if the engine is on, and there are several extra psi of exhaust pressure. It doesnt care if there is any exhaust or not. You can operate a wastegate with it in your hands.
12-05-2015, 08:28 PM
#13
I'm not sure what this guy is talking about. Wastegates I am familiar with will overboost and never open if you remove all the lines. None of them reference exhaust pressure, that is unheard of to me. Although there may be a very expensive engine which does, out there somewhere, for other reasons.
Conversely, when I said what I said earlier, I meant it: if you have a 5psi spring, the wastegate opens at 5psi, even if the engine is off and there is no exhaust pressure. It also opens at 5psi if the engine is on, and there are several extra psi of exhaust pressure. It doesnt care if there is any exhaust or not. You can operate a wastegate with it in your hands.
Conversely, when I said what I said earlier, I meant it: if you have a 5psi spring, the wastegate opens at 5psi, even if the engine is off and there is no exhaust pressure. It also opens at 5psi if the engine is on, and there are several extra psi of exhaust pressure. It doesnt care if there is any exhaust or not. You can operate a wastegate with it in your hands.
12-05-2015, 09:04 PM
#17
For the special guy that makes 2000 word special posts, so maybe he learns something
Take a typical LS combo running 15lbs of boost... With 2:1 drive ratio.... Or 30psi of drive pressure
They are using a 60mm gate
So the valve has an area of ~4.3 square inches.
Sooooooooo.... There is ~130lbs of force applied to the valve trying to open the wastegate....
Take a typical LS combo running 15lbs of boost... With 2:1 drive ratio.... Or 30psi of drive pressure
They are using a 60mm gate
So the valve has an area of ~4.3 square inches.
Sooooooooo.... There is ~130lbs of force applied to the valve trying to open the wastegate....
12-05-2015, 09:11 PM
#18
Originally Posted by kingtal0n
I'm not sure what this guy is talking about. Wastegates I am familiar with will overboost and never open if you remove all the lines. None of them reference exhaust pressure, that is unheard of to me. Although there may be a very expensive engine which does, out there somewhere, for other reasons.
Conversely, when I said what I said earlier, I meant it: if you have a 5psi spring, the wastegate opens at 5psi, even if the engine is off and there is no exhaust pressure. It also opens at 5psi if the engine is on, and there are several extra psi of exhaust pressure. It doesnt care if there is any exhaust or not. You can operate a wastegate with it in your hands.
Conversely, when I said what I said earlier, I meant it: if you have a 5psi spring, the wastegate opens at 5psi, even if the engine is off and there is no exhaust pressure. It also opens at 5psi if the engine is on, and there are several extra psi of exhaust pressure. It doesnt care if there is any exhaust or not. You can operate a wastegate with it in your hands.
12-05-2015, 10:49 PM
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I am brand new to turbos and have a question. I am working on my first
turbo set up. I have just purchased wastegates and have been thinking
about boost--now you always hear about wastegate spring pressure--my question is how does boost correlate to exhaust pressure? For instance say you have 6 pound wastegate springs--am I correct in assuming that this
actually means nothing as far as what boost these springs will actually provide in this case or any other case as the wastegate spring pressure is only one of many variables and in no way actually means that 6 pound wastegate springs will provide 6 pounds boost. Can some of you experienced guys expound on this subject?
turbo set up. I have just purchased wastegates and have been thinking
about boost--now you always hear about wastegate spring pressure--my question is how does boost correlate to exhaust pressure? For instance say you have 6 pound wastegate springs--am I correct in assuming that this
actually means nothing as far as what boost these springs will actually provide in this case or any other case as the wastegate spring pressure is only one of many variables and in no way actually means that 6 pound wastegate springs will provide 6 pounds boost. Can some of you experienced guys expound on this subject?
