Heide264
Full Access Member
Good Evening.
I've decided to do an 'armchair warrior writeup' for discussion and reference. While my past 'armchair warrior writeups' on other forums have required a nice day, plenty of good beer, and several cigars... some crappy red wine, my college graphing calculator, and a book will have to do for now. For some reason, all of my nice days seem to go into a large one ton pickup that is still keeping my driveway from blowing away.
This thread spawned out of this thread here: http://www.oilburners.net/forums/showthread.php?71879-turbos-wastegates-and-blow-off-valves
There is some great information in that thread, from several different view points. I don't intend to discredit any of those view points in here, I just would like to 'tie it all together' as best as I can and add on what may be useful.
I learned most of what I know about cars after getting a 2011 Subaru STi years ago. Before that, I didn't even know how to do an oil change. The more I looked around, the more I realized a lot of people had no idea what the heck they were doing. I got a few books, started fixing my own tune, and dumping way more money into it than I should have. Long story short, I learned a LOT in a short period of time. Some of it is applicable to giant non-EFI diesel trucks... some not. However, I figure you guys deal with my stupid questions regarding power steering lines and vacuum pumps, so I may as well share what I can.
It's a long thread. If you don't like reading, sorry. If you do like reading, I'd love some discussion on the material. Hopefully this benefits the community and is a nice reference for those in the future. I'll try to put the important stuff in bold, regardless.
I'm going to cover an overview of some various terms and concepts in here. I'll also reserve a Q&A posting or two to sum up some of the discussion, if any follows.
Turbocharger:
A device that drives more air into an engine. In an engine, power is created by burning fuel in the presence of air. If you have no air, you have no combustion. Turbochargers primarily use the heat from the engines exhaust to spin a turbine wheel, which is connected via a shaft to a compressor wheel. The compressor spins and compresses air in the intake manifold. While the power in either a gas/diesel engine is dependent on fuel burning, both engine types require some form of air to allow that fuel to burn. More air mass => more fuel can be burnt => more power.
It is important to note that a turbocharger has 'specs'. There is no 'one turbocharger to rule them all'. Every compressor/turbine/housing combination is unique. They are sized to be efficient within certain airflow (lbs/min) at certain pressure ratios (psi/psi... it has no unit). If you go outside that ratio, the turbo is less efficient at compressing air and will heat the air up - quickly. If we step back and look, the goal of a turbo is to get more air into our engine - thats a mass... not a volume or a pressure. The air mass your engine can 'injest' at a given time depends on a few things - the time and space the air has to enter the combustion chamber (re: cam profile, valves), the pressure differential within the chamber to the intake manifold ('boost' pressure), and the density of the air entering the chamber. It's important to understand that your engine cares about air MASS when you are talking about power gains. For example, one cylinder of air at 15psi and 100F contains less mass than the same cylinder at 15psi at 120F which contains less mass than the same cylinder at 17psi but 300F (I think... I'm not crunching the numbers now... its the idea that's important here, haha).
The bottom line is that as a turbocharger moves out of its efficiency range, it turns into a much less useful hair dryer sorta thing. There are many situations where increasing your 'boost pressure' will actually HURT power levels due to the high temperatures involved. As the temperatures climb, the air in the intake manifold is less dense (even if under higher pressure) and the bottom line is that the engine will injest less air MASS even under the higher pressure. I can't really think of a good analogy for this.
The good news is that diesel trucks run in a relatively small operating range! If the turbo is selected properly, you should be able to stay in good efficiency ranges most of the time . Gassers... yeah... sorry guys. Those 8k rev bands have their drawbacks .
The Wastegate:
Simply a valve that allows air around your turbo's turbine. This can be built into the turbo (Internal WG - IWG) or otherwise connected to the exhaust manifold (External - EWG). The basic idea is that you can route gas around the turbine instead of through it. This allows you to not build more boost. I worded it like that for a reason. As discussed above, you may not always want more boost pressure... however... the turbo may keep cranking it out whether you want it or not depending on your turbo, engine selection, and several other factors. A good example of why you would want a wastegate is if you wanted a flatter power band for a wider RPM range, or a much higher torque 'peak' and maybe not as much power at redline (pending engine). You could select a turbo that is more efficient at a lower air flow and higher drivepressure... and then allow the wastegate to keep the turbo within its sweet spot as you approach redline.
A wastegate can be a useful safety function for a turbo as well. In some turbo systems, it is possible to drive the turbo faster than it is spec'd to. A properly sized wastegate will help to prevent this.
Boost Controller:
A wastegate is basically a spring tensioned valve that opens whenever it sees a pressure above a certain amount. I do not know what the spring tension is - I'm assuming somewhere around 10psi. In some applications, a separate device - a boost controller - either an adjustable manual one or an electronic solenoid based one - is plumbed in the middle to 'block' the wastegate from seeing the full manifold pressure. This allows you to use different boost pressures than what your wastegate spring is set at. In a gasser, an electronic boost controller mated up to a properly tuned computer is a powerful asset. If you see the acronyms MBC (manual boost controller) or EBCS (electronic boost control solenoid) tossed around, they aren't applicable to our trucks in a factory configuration. A MBC is, in my opinion, not a bad investment if anybody is looking to play around with different boost levels on a wastegated turbo. It's kinda important to note that if you don't have a wastegate... there is nothing for a separate boost controller to control.
Blow off Valves:
So as a disclaimer, I hate BOVs. They annoy the crap out of me, personally. That being said, in terms of an IDI, they are 'not applicable'. A Blow off valve (BOV) or bypass valve (BPV) allows a pressurized intake track (intercooler & plumbing) to 'vent' due to the now closed throttle blade which is sealing shut the intended air path. Instead of applying pressure on the compressor wheel/housing, the air is either vented to the atmosphere or recirculated back to the intake (pre-compressor). In an IDI, we have no throttle blade... and hence no pressure 'backup' in the intake track. In most diesels (some newer diesels have throttle blades for egr/exhaust gas recirculation), the main thing you need to realize is that a BOV is NOT a wastegate. A BOV vents the intake track between the compressor wheel and the throttle. A Wastegate vents the exhaust manifold to prevent the turbine (and hence the compressor) from spinning faster.
Some closing thoughts:
In my opinion, the merit of a wastegated turbo is a pretty hot topic for an IDI. It appears that the big limiting factor in IDI world is the intake pressure. The head gaskets just seem to be the limiting factor if you can get the fuel in the engine. If you are limited by drive pressure, it should be an important priority to make sure you are really giving the engine all the air mass you can give it at a given pressure. I think the best efforts would be spent on a turbo that spools before you are through most of your power band (e.g. smaller than 'ideal') but has the proper supporting mods to keep the air as dense as possible as the turbo leaves its prime - a proper sized intercooler and **** injection, for example. I'm not too familiar with tuning injection pumps, but if the fuel supply can be made to match a boost curve a bit better, I think there is a lot of potential out there for a nice fun truck that is extremely reliable.
I'll reserve the next post for any Q&A that comes up by myself or other people who know more than myself, and I'll reserve a post after that for some numbers that I will run at a later day.
EDIT: I forgot to give credit as needed. A lot of my turbo specific knowledge comes from "Turbo: Real World High-Performance Turbocharger Systems" by Jay K. Miller. It's a great book, and is actually a pretty quick read. If you guys actually read through this thread, I can't recommend enough to just go buy it on amazon for ~$10. A book is so much more coherent than an internet forum, especially one with my rambling in it.
I've decided to do an 'armchair warrior writeup' for discussion and reference. While my past 'armchair warrior writeups' on other forums have required a nice day, plenty of good beer, and several cigars... some crappy red wine, my college graphing calculator, and a book will have to do for now. For some reason, all of my nice days seem to go into a large one ton pickup that is still keeping my driveway from blowing away.
This thread spawned out of this thread here: http://www.oilburners.net/forums/showthread.php?71879-turbos-wastegates-and-blow-off-valves
There is some great information in that thread, from several different view points. I don't intend to discredit any of those view points in here, I just would like to 'tie it all together' as best as I can and add on what may be useful.
I learned most of what I know about cars after getting a 2011 Subaru STi years ago. Before that, I didn't even know how to do an oil change. The more I looked around, the more I realized a lot of people had no idea what the heck they were doing. I got a few books, started fixing my own tune, and dumping way more money into it than I should have. Long story short, I learned a LOT in a short period of time. Some of it is applicable to giant non-EFI diesel trucks... some not. However, I figure you guys deal with my stupid questions regarding power steering lines and vacuum pumps, so I may as well share what I can.
It's a long thread. If you don't like reading, sorry. If you do like reading, I'd love some discussion on the material. Hopefully this benefits the community and is a nice reference for those in the future. I'll try to put the important stuff in bold, regardless.
I'm going to cover an overview of some various terms and concepts in here. I'll also reserve a Q&A posting or two to sum up some of the discussion, if any follows.
Turbocharger:
A device that drives more air into an engine. In an engine, power is created by burning fuel in the presence of air. If you have no air, you have no combustion. Turbochargers primarily use the heat from the engines exhaust to spin a turbine wheel, which is connected via a shaft to a compressor wheel. The compressor spins and compresses air in the intake manifold. While the power in either a gas/diesel engine is dependent on fuel burning, both engine types require some form of air to allow that fuel to burn. More air mass => more fuel can be burnt => more power.
It is important to note that a turbocharger has 'specs'. There is no 'one turbocharger to rule them all'. Every compressor/turbine/housing combination is unique. They are sized to be efficient within certain airflow (lbs/min) at certain pressure ratios (psi/psi... it has no unit). If you go outside that ratio, the turbo is less efficient at compressing air and will heat the air up - quickly. If we step back and look, the goal of a turbo is to get more air into our engine - thats a mass... not a volume or a pressure. The air mass your engine can 'injest' at a given time depends on a few things - the time and space the air has to enter the combustion chamber (re: cam profile, valves), the pressure differential within the chamber to the intake manifold ('boost' pressure), and the density of the air entering the chamber. It's important to understand that your engine cares about air MASS when you are talking about power gains. For example, one cylinder of air at 15psi and 100F contains less mass than the same cylinder at 15psi at 120F which contains less mass than the same cylinder at 17psi but 300F (I think... I'm not crunching the numbers now... its the idea that's important here, haha).
The bottom line is that as a turbocharger moves out of its efficiency range, it turns into a much less useful hair dryer sorta thing. There are many situations where increasing your 'boost pressure' will actually HURT power levels due to the high temperatures involved. As the temperatures climb, the air in the intake manifold is less dense (even if under higher pressure) and the bottom line is that the engine will injest less air MASS even under the higher pressure. I can't really think of a good analogy for this.
The good news is that diesel trucks run in a relatively small operating range! If the turbo is selected properly, you should be able to stay in good efficiency ranges most of the time . Gassers... yeah... sorry guys. Those 8k rev bands have their drawbacks .
The Wastegate:
Simply a valve that allows air around your turbo's turbine. This can be built into the turbo (Internal WG - IWG) or otherwise connected to the exhaust manifold (External - EWG). The basic idea is that you can route gas around the turbine instead of through it. This allows you to not build more boost. I worded it like that for a reason. As discussed above, you may not always want more boost pressure... however... the turbo may keep cranking it out whether you want it or not depending on your turbo, engine selection, and several other factors. A good example of why you would want a wastegate is if you wanted a flatter power band for a wider RPM range, or a much higher torque 'peak' and maybe not as much power at redline (pending engine). You could select a turbo that is more efficient at a lower air flow and higher drivepressure... and then allow the wastegate to keep the turbo within its sweet spot as you approach redline.
A wastegate can be a useful safety function for a turbo as well. In some turbo systems, it is possible to drive the turbo faster than it is spec'd to. A properly sized wastegate will help to prevent this.
Boost Controller:
A wastegate is basically a spring tensioned valve that opens whenever it sees a pressure above a certain amount. I do not know what the spring tension is - I'm assuming somewhere around 10psi. In some applications, a separate device - a boost controller - either an adjustable manual one or an electronic solenoid based one - is plumbed in the middle to 'block' the wastegate from seeing the full manifold pressure. This allows you to use different boost pressures than what your wastegate spring is set at. In a gasser, an electronic boost controller mated up to a properly tuned computer is a powerful asset. If you see the acronyms MBC (manual boost controller) or EBCS (electronic boost control solenoid) tossed around, they aren't applicable to our trucks in a factory configuration. A MBC is, in my opinion, not a bad investment if anybody is looking to play around with different boost levels on a wastegated turbo. It's kinda important to note that if you don't have a wastegate... there is nothing for a separate boost controller to control.
Blow off Valves:
So as a disclaimer, I hate BOVs. They annoy the crap out of me, personally. That being said, in terms of an IDI, they are 'not applicable'. A Blow off valve (BOV) or bypass valve (BPV) allows a pressurized intake track (intercooler & plumbing) to 'vent' due to the now closed throttle blade which is sealing shut the intended air path. Instead of applying pressure on the compressor wheel/housing, the air is either vented to the atmosphere or recirculated back to the intake (pre-compressor). In an IDI, we have no throttle blade... and hence no pressure 'backup' in the intake track. In most diesels (some newer diesels have throttle blades for egr/exhaust gas recirculation), the main thing you need to realize is that a BOV is NOT a wastegate. A BOV vents the intake track between the compressor wheel and the throttle. A Wastegate vents the exhaust manifold to prevent the turbine (and hence the compressor) from spinning faster.
Some closing thoughts:
In my opinion, the merit of a wastegated turbo is a pretty hot topic for an IDI. It appears that the big limiting factor in IDI world is the intake pressure. The head gaskets just seem to be the limiting factor if you can get the fuel in the engine. If you are limited by drive pressure, it should be an important priority to make sure you are really giving the engine all the air mass you can give it at a given pressure. I think the best efforts would be spent on a turbo that spools before you are through most of your power band (e.g. smaller than 'ideal') but has the proper supporting mods to keep the air as dense as possible as the turbo leaves its prime - a proper sized intercooler and **** injection, for example. I'm not too familiar with tuning injection pumps, but if the fuel supply can be made to match a boost curve a bit better, I think there is a lot of potential out there for a nice fun truck that is extremely reliable.
I'll reserve the next post for any Q&A that comes up by myself or other people who know more than myself, and I'll reserve a post after that for some numbers that I will run at a later day.
EDIT: I forgot to give credit as needed. A lot of my turbo specific knowledge comes from "Turbo: Real World High-Performance Turbocharger Systems" by Jay K. Miller. It's a great book, and is actually a pretty quick read. If you guys actually read through this thread, I can't recommend enough to just go buy it on amazon for ~$10. A book is so much more coherent than an internet forum, especially one with my rambling in it.
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