I was in YellowBullet forum reading a few things and came across this post. A guy posted on the forum about billet S484 and its ability to make power. Another guy (smokeshow) chimed in with some douchebag comments about how a turbine doesn't care about the size of engine and how much exhaust flowing through any turbine size. Then there was this post...I learned a lot no matter what kind of fuel exhaust is pushing the turbine (the point is in the long run to making efficient power is 1:1 boost/drive ratio with any given compressor map/turbine combination):
For those that want to know how this really works,
If you have a given sized orifice, you can increase flow through that orifice with more pressure (fairly obvious, since we all know that in general more boost will flow more air). That being said, there is a limit to the increase because at some point the orifice will “choke”. Which means it doesn’t matter how much more pressure you put on it… it’s not going to flow any additional air. This is nicely shown by Mr. Engineer’s (smokeshow) graph. You can see that this graph has pressure ratio on the bottom, and mass flow on the left. (keep in mind this is NOT a backpressure ratio. This is a ratio comparing pressure pre and post turbine (so hot side vs. downpipe). As you move from left to right (increasing pressure), all of the curves go “up”… meaning they flow more air at higher pressures. But what you will notice is around 2:1 pressure ratio, the curves flat line. As you go to 3:1 ratio and up, they don’t actually flow more air. This is because the exhaust housing outlet at the turbine is “choked”.
Looking at this another way, no matter how hard you push a 4808R compressor… it’s not flowing over 45lbs/min (in this chart anyway).
This is an important thing to understand. Understand that high pressures mean more flow to a given point, but at some point the flow flatlines… (why this happens is for a deeper discussion, but basically what happens is that the “signal” to flow more air can only travel at the speed of sound. Once you reach this (or get close), the pressure signal between both sides of the orfice can’t move fast enough to flow any more air. It’s the same principal of having an intake port too small for a lot of RPM.)
That being said…Why is this point important? The real answer to why small motors can get more out of a given turbine/housing lies in this detail.
The biggest important point that captain know it all (smokeshow) is missing, is that the engine does not flow air in a steady state way. Steady state is a term engineers use to mean “not changing”. A river flows in steady state (for the most part). The rate that water passing by does not change greatly with time. However in an engine this is not true… this is because exhaust flow is only happening when the exhaust valve is open and the blow down or piston movement is pushing it out! Since the exhaust valve is not always open, we end up with flow “pulses”. Large swings of high and low pressure as each exhaust valve opens to release it’s combusted gasses into the hotside.
If you put a gauge on your hotside, you will not see this “pulsing”. This is because the pulsing happens so quickly that your pressure gauge can’t react. To put it in perspective, a 300 degree duration cam only has each exhaust valve open for .007 seconds at 7,000rpm.
While the gauge doesn’t care about the pulses (because it can’t react that fast, and will only show you the AVERAGE PRESSURE), the turbine orifice does because the amount of air it flows is dependent on the pressure (as we talked about above). The turbine doesn’t care about average pressure… it cares about the actual pressure of the exhaust pulses.
So lets make an easy example (with fictitious easy to understand numbers) to show why peak pulsing pressures are not all the same. Engine one is twice as big as engine number two… but runs at half the RPM.
Engine #1 - 10 pulses of 1lb each in 1 second… 10pulses x 1lb / 1 second = 10lbs/sec.
Engine #2 - 20 pulses of 0.5lb each in 1 second… 20 pulses x 0.5lbs / 1 second = 10lbs/sec.
So both of them flow 10lbs/sec… but what’s actually happening inside the engine is much different.
If you looked at the pulses… they would look something like this (not exactly… but you get the idea).
graph_20140623_204428
Few big tall pulses for the big/low RPM engine… and many small short pulses for the small/high RPM engine.
But as was said before (and depicted in the flow charts previously in this post), there is a mass flow “choke point” to every turbine housing/wheel combination. What happens when that big pulse of the V8 “chokes” the turbine?... Then the flow will actually look like this…
Untitled
Anything above the choke point (shown in grey hatching) will not actually flow as hoped… which means the pressure will continue to go up in the cylinder, but the flow will not increase.
What does this mean for the turbine? As outlined above, the flow through the turbine is maxed for a given pressure/flow once choked. The V8/Large Engine tries to force feed the turbo a huge volume of air on each exhaust stroke. This huge volume of air gets choked off, and does NOT flow through the turbine. This is what’s actually happening when you max out the hot side of a turbo.
This is also exactly why a small motor (with high RPM and many small exhaust pulses) can get more out of a given turbine then a big motor (with low RPM and few large exhaust pulses).
Both have the same overall flow rate, but they do it in different ways.
If you have a small motor, it will “pulse” many times (High RPM) and at lower air mass flow (small motor) for each exhaust stroke. These small pulses never go over the “choke point”, so they never loose any overall flow. The large V8 motor will do the opposite… it will pulse fewer times (low RPM) and at a higher mass flow (large motor) for each exhaust stroke. Each time the large engine pulse goes over the choke point, the piston is pushing harder (robbing horsepower), but getting no additional flow for the work it’s doing.
Since you can only shove in, what you can shove out… the small motor at high RPM can use more compressor for a given turbine/exhaust housing then the large V8 motor can.
Smokeshow,
I am an engineer... so I can COMPLETELY relate with the need to understand the science behind everything. When you understand how things work, you can really get the most out of a combination.
But my Dad taught me something a long time ago that apparently no one has ever taught you...
When someone does something in real life time and time again that doesn't agree with the proven scientific law or theory you know... you are likely applying the theory wrong.
Now that you've probably skipped over what I just wrote because you are an arrogant ****... let me try to explain what's really going on here.
First of all... based on your responses I can tell that you are associated with every other "Book smart, but I can't tie my own shoes" engineer that I know (and let me admit... that is a LARGE portion of the engineering community, so you shouldn't feel too bad...). I can also tell that everything you learned about fluid dynamics came from a book focused on steady state turbines, probably based on nuclear powerplant steam driven mechanics and examples. (Just like most engineers...)
The part you are TOTALLY ignoring that blows your "theory" out of the water, is that an engine is NOT EVEN CLOSE to a steady state machine, nor does it ACT like a steady state machine, NOR CAN STEADY STATE EQUATIONS FULLY EXPLAIN WHAT IS HAPPENING IN THE ENGINE.
The part I find most hilarious is your blinding arrogance. You think that just because the guys out there racing day in/day out don't fully understand the science behind things... that it makes them wrong in doing it somehow.
You couldn't be further from the truth... these are the guys that equations are written about. They are doing things that science (or in my case, home grown engineers) are trying to understand. They aren't out there trying to only do things they read in a book...
Don’t get me wrong… you can bet your ass the REALLY fast guys out there understand both what works, and why it works. They have a much better chance of bolting the right part on the first time, and getting to their goals with a lot less trial and error.
But if you only get to pick one… I’ll take knowing what works, over why it should work every day of the week.
And for the record... the other thing my Dad taught me that obviously no one has ever taught you is... there are ALWAYS people out there smarter then you are.
So be careful when you go out of your way to make someone else look stupid by trying to be a "know it all"...
In conclusion…go **** yourself.
Sent from my SM-N910V using Tapatalk
For those that want to know how this really works,
If you have a given sized orifice, you can increase flow through that orifice with more pressure (fairly obvious, since we all know that in general more boost will flow more air). That being said, there is a limit to the increase because at some point the orifice will “choke”. Which means it doesn’t matter how much more pressure you put on it… it’s not going to flow any additional air. This is nicely shown by Mr. Engineer’s (smokeshow) graph. You can see that this graph has pressure ratio on the bottom, and mass flow on the left. (keep in mind this is NOT a backpressure ratio. This is a ratio comparing pressure pre and post turbine (so hot side vs. downpipe). As you move from left to right (increasing pressure), all of the curves go “up”… meaning they flow more air at higher pressures. But what you will notice is around 2:1 pressure ratio, the curves flat line. As you go to 3:1 ratio and up, they don’t actually flow more air. This is because the exhaust housing outlet at the turbine is “choked”.
Looking at this another way, no matter how hard you push a 4808R compressor… it’s not flowing over 45lbs/min (in this chart anyway).
This is an important thing to understand. Understand that high pressures mean more flow to a given point, but at some point the flow flatlines… (why this happens is for a deeper discussion, but basically what happens is that the “signal” to flow more air can only travel at the speed of sound. Once you reach this (or get close), the pressure signal between both sides of the orfice can’t move fast enough to flow any more air. It’s the same principal of having an intake port too small for a lot of RPM.)
That being said…Why is this point important? The real answer to why small motors can get more out of a given turbine/housing lies in this detail.
The biggest important point that captain know it all (smokeshow) is missing, is that the engine does not flow air in a steady state way. Steady state is a term engineers use to mean “not changing”. A river flows in steady state (for the most part). The rate that water passing by does not change greatly with time. However in an engine this is not true… this is because exhaust flow is only happening when the exhaust valve is open and the blow down or piston movement is pushing it out! Since the exhaust valve is not always open, we end up with flow “pulses”. Large swings of high and low pressure as each exhaust valve opens to release it’s combusted gasses into the hotside.
If you put a gauge on your hotside, you will not see this “pulsing”. This is because the pulsing happens so quickly that your pressure gauge can’t react. To put it in perspective, a 300 degree duration cam only has each exhaust valve open for .007 seconds at 7,000rpm.
While the gauge doesn’t care about the pulses (because it can’t react that fast, and will only show you the AVERAGE PRESSURE), the turbine orifice does because the amount of air it flows is dependent on the pressure (as we talked about above). The turbine doesn’t care about average pressure… it cares about the actual pressure of the exhaust pulses.
So lets make an easy example (with fictitious easy to understand numbers) to show why peak pulsing pressures are not all the same. Engine one is twice as big as engine number two… but runs at half the RPM.
Engine #1 - 10 pulses of 1lb each in 1 second… 10pulses x 1lb / 1 second = 10lbs/sec.
Engine #2 - 20 pulses of 0.5lb each in 1 second… 20 pulses x 0.5lbs / 1 second = 10lbs/sec.
So both of them flow 10lbs/sec… but what’s actually happening inside the engine is much different.
If you looked at the pulses… they would look something like this (not exactly… but you get the idea).
graph_20140623_204428
Few big tall pulses for the big/low RPM engine… and many small short pulses for the small/high RPM engine.
But as was said before (and depicted in the flow charts previously in this post), there is a mass flow “choke point” to every turbine housing/wheel combination. What happens when that big pulse of the V8 “chokes” the turbine?... Then the flow will actually look like this…
Untitled
Anything above the choke point (shown in grey hatching) will not actually flow as hoped… which means the pressure will continue to go up in the cylinder, but the flow will not increase.
What does this mean for the turbine? As outlined above, the flow through the turbine is maxed for a given pressure/flow once choked. The V8/Large Engine tries to force feed the turbo a huge volume of air on each exhaust stroke. This huge volume of air gets choked off, and does NOT flow through the turbine. This is what’s actually happening when you max out the hot side of a turbo.
This is also exactly why a small motor (with high RPM and many small exhaust pulses) can get more out of a given turbine then a big motor (with low RPM and few large exhaust pulses).
Both have the same overall flow rate, but they do it in different ways.
If you have a small motor, it will “pulse” many times (High RPM) and at lower air mass flow (small motor) for each exhaust stroke. These small pulses never go over the “choke point”, so they never loose any overall flow. The large V8 motor will do the opposite… it will pulse fewer times (low RPM) and at a higher mass flow (large motor) for each exhaust stroke. Each time the large engine pulse goes over the choke point, the piston is pushing harder (robbing horsepower), but getting no additional flow for the work it’s doing.
Since you can only shove in, what you can shove out… the small motor at high RPM can use more compressor for a given turbine/exhaust housing then the large V8 motor can.
Smokeshow,
I am an engineer... so I can COMPLETELY relate with the need to understand the science behind everything. When you understand how things work, you can really get the most out of a combination.
But my Dad taught me something a long time ago that apparently no one has ever taught you...
When someone does something in real life time and time again that doesn't agree with the proven scientific law or theory you know... you are likely applying the theory wrong.
Now that you've probably skipped over what I just wrote because you are an arrogant ****... let me try to explain what's really going on here.
First of all... based on your responses I can tell that you are associated with every other "Book smart, but I can't tie my own shoes" engineer that I know (and let me admit... that is a LARGE portion of the engineering community, so you shouldn't feel too bad...). I can also tell that everything you learned about fluid dynamics came from a book focused on steady state turbines, probably based on nuclear powerplant steam driven mechanics and examples. (Just like most engineers...)
The part you are TOTALLY ignoring that blows your "theory" out of the water, is that an engine is NOT EVEN CLOSE to a steady state machine, nor does it ACT like a steady state machine, NOR CAN STEADY STATE EQUATIONS FULLY EXPLAIN WHAT IS HAPPENING IN THE ENGINE.
The part I find most hilarious is your blinding arrogance. You think that just because the guys out there racing day in/day out don't fully understand the science behind things... that it makes them wrong in doing it somehow.
You couldn't be further from the truth... these are the guys that equations are written about. They are doing things that science (or in my case, home grown engineers) are trying to understand. They aren't out there trying to only do things they read in a book...
Don’t get me wrong… you can bet your ass the REALLY fast guys out there understand both what works, and why it works. They have a much better chance of bolting the right part on the first time, and getting to their goals with a lot less trial and error.
But if you only get to pick one… I’ll take knowing what works, over why it should work every day of the week.
And for the record... the other thing my Dad taught me that obviously no one has ever taught you is... there are ALWAYS people out there smarter then you are.
So be careful when you go out of your way to make someone else look stupid by trying to be a "know it all"...
In conclusion…go **** yourself.
Sent from my SM-N910V using Tapatalk
