Alright... This is something Ive been wanting to know for a while, if you think it should be in the engineering forum ill move it.....
This question is regarding exhaust and back pressure. As we all know, when you get headers or some other exhaust system, it reduces your low end power... Using my abstract knowledge of science, less back pressure = less force against the engine, which should= more power throughout the powerband.....
However, this has been a proven fact that increasing flow reduces low end power...
So i simply ask..... Why?
Serious tech question.... Exhaust...
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AZhitman
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Wes - To answer your question, it has to do with the scavenging effect in the exhaust manifold and system.
There's a great writeup that explains it pretty well (I'll look for it).
Keep in mind that you don't ALWAYS lose TQ as a result of a freer-flowing exhaust. Oftentimes, there's an increase, so that's not necessarily a given.
There's a great writeup that explains it pretty well (I'll look for it).
Keep in mind that you don't ALWAYS lose TQ as a result of a freer-flowing exhaust. Oftentimes, there's an increase, so that's not necessarily a given.
What happens when there isn't enough back pressure (not enough restriction) is the engine actually contaminates it's own air supply. If you have too large of an exhaust, or just not enough restriction, it pulls already used exhaust gases back through the exhaust valve while it's still open at the very beginning of the intake stroke.
Of course, having a larger exhaust (up to an extent) decreases restriction which is more valuable as the RPM's increase and more air is being pushed through, thus why higher RPM power is increased and lower RPM power is usually decreased.
Corey
Of course, having a larger exhaust (up to an extent) decreases restriction which is more valuable as the RPM's increase and more air is being pushed through, thus why higher RPM power is increased and lower RPM power is usually decreased.
Corey
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C-Kwik
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Re: Serious tech question.... Exhaust... (elwesso)
http://www.nissanperformancema...2.php
http://www.nissanperformancema...nerds/
http://www.nissanperformancema...nerds/
Or the summarized version:Two principles are described in the articles above. Exhaust pulse tuning for headers and exhaust velocity.
Pulse tuning involves using the negative pressure wave that immediately follows a positive pressure wave in the header primaries. Tuned correctly, it will actually draw air out of the cylinder. Timed with overlap, it may even go as far as sucking intake air into the cylinder for a short range of RPM.
Exhaust velocity just has to do with momentum. Air has mass. Anything with mass that is moving has momentum/inertia. An opposing force will need to act on it to change it's speed or direction. The higher the speed the higher the force required. High velocity exhaust gas will want to stay at that speed. This aids the scavangeing effect mentioned above by allowing the column of high velocity air to pull additional gasses out of the cylinders and header primaries. With a pipe that is too big, the exhaust gas velocity will be low and you will lose some of the scavenging effect.
Read the articles though, they explain it better than I do.
http://www.nissanperformancema...nerds/
http://www.nissanperformancema...nerds/
Or the summarized version:Two principles are described in the articles above. Exhaust pulse tuning for headers and exhaust velocity.
Pulse tuning involves using the negative pressure wave that immediately follows a positive pressure wave in the header primaries. Tuned correctly, it will actually draw air out of the cylinder. Timed with overlap, it may even go as far as sucking intake air into the cylinder for a short range of RPM.
Exhaust velocity just has to do with momentum. Air has mass. Anything with mass that is moving has momentum/inertia. An opposing force will need to act on it to change it's speed or direction. The higher the speed the higher the force required. High velocity exhaust gas will want to stay at that speed. This aids the scavangeing effect mentioned above by allowing the column of high velocity air to pull additional gasses out of the cylinders and header primaries. With a pipe that is too big, the exhaust gas velocity will be low and you will lose some of the scavenging effect.
Read the articles though, they explain it better than I do.
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elwesso
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Re: Serious tech question.... Exhaust... (C-Kwik)
Oh ok..
So basically, the higher restriction at lower RPMs keeps the exhaust flowing, to sacrifice at higher RPM...
With the lower restriction, the exhaust I guess is stagnant for a while, before the next pulse comes through.. This would mean it gets cooled somewhat, and thus it would tend to move backwards......
Got it! its all making sense now!
So basically, the higher restriction at lower RPMs keeps the exhaust flowing, to sacrifice at higher RPM...
With the lower restriction, the exhaust I guess is stagnant for a while, before the next pulse comes through.. This would mean it gets cooled somewhat, and thus it would tend to move backwards......
Got it! its all making sense now!
Re: Serious tech question.... Exhaust... (elwesso)
Pretty close Wes.elwesso wrote:Oh ok..
So basically, the higher restriction at lower RPMs keeps the exhaust flowing, to sacrifice at higher RPM...
With the lower restriction, the exhaust I guess is stagnant for a while, before the next pulse comes through.. This would mean it gets cooled somewhat, and thus it would tend to move backwards......
Got it! its all making sense now!
The lower restriction makes up for the increased amount of air flowing through it at higher RPMs. The sacrifice being at lower RPMs the engine sucks air into the cylinder any way it can (not because it's cooled).
Imagine you're holding your hand over the end of a vacuum. Well, the vacuum is trying to move air into itself right? Does it really matter which side of your hand you lift up to let that air flow in? Left or right, it can't tell the difference... it'll take whatever it can get! Same deal with an engine.
Corey
you see, its not the restriction, its the ability of the exhaust pulse to create a low pressure area behind it, and suck all the exhaust out of the cylinder [scavenging]
its not the restriction that causes the low end torque, its the tuned pipe. restriction is a function of low rpm tuning.
another words, its low end tuning that causes the restriction, not the restriction that causes good low end power.
with low end power, you want the scavenging. there is small volume flow, and you want to line up the pulses so that the complement each other. they all push and pull each other along in sync.
at high rpm, you want the maximum flow. tuning doesnt matter as much, because you want to move a large volume of gas as quickly as possible.
its a comprimise, just like everything else relating to engines.
its not the restriction that causes the low end torque, its the tuned pipe. restriction is a function of low rpm tuning.
another words, its low end tuning that causes the restriction, not the restriction that causes good low end power.
with low end power, you want the scavenging. there is small volume flow, and you want to line up the pulses so that the complement each other. they all push and pull each other along in sync.
at high rpm, you want the maximum flow. tuning doesnt matter as much, because you want to move a large volume of gas as quickly as possible.
its a comprimise, just like everything else relating to engines.
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Re: (IvoryJ30t)
That is why there is vast differences in headers.
Some companies spend $$$ in research, to get the best scavenging, and others just make them look cool.
Some companies spend $$$ in research, to get the best scavenging, and others just make them look cool.
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C-Kwik
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Re: Serious tech question.... Exhaust... (elwesso)
It's not restriction that you are trying to use for scavenging, but rather volume(less of it for lower RPM). The bigger the exhaust, the more air is required to fill it. At low RPM's , you are pushing less air out of the exhaust. So it may end up with low exhaust velocity with too big a pipe.
Think of it this way. If you were to blow air though a pipe at a high rate of speed, and then suddenly close a throttle valve located at the front of the pipe, the air already in the pipe wil for a moment continue in the direction it was travelling. The column of moving air will then create a low pressure area just behind the throttle. This is the effect you want to see occur behind an exhaust valve. This low pressure area helps to suck out gasses from inside the combustion chamber. The volume of air at a given RPM and WOT, will be very similar with different sized pipes, with the only differences being the change in VE of the motor that results. For the purposes of my example, we will assume they remain constant. Since the airflow is the same, the mass will be the same. Momentum consists of mass and speed. Since mass is, for the most part, fixed here, the only way to increase momentum is to increase the speed of the gasses. And the higher the velocity of the gasses, the lower the pressure will be behind the valve when the valve shuts. Of course if you size the pipe too small you start to increase the overall pressure in the system to the point that the low pressure area(which is relative to the pressure in the pipes) is higher than it would be with a bigger pipe. Most factory exhaust systems are sized quite small to keep noise levels down so they tend to have plenty of room to upgrade to a bigger pipe without affecting low RPM scavenging or at least a minimal sacrifice to it.
And Tuning at high RPM is just as important. The same scavenging effects still exists. Higher velocity gasses are much easier to acheive at high RPM's though so increasing flow becomes a prioroty there when looking at an OEM exhaust.
Think of it this way. If you were to blow air though a pipe at a high rate of speed, and then suddenly close a throttle valve located at the front of the pipe, the air already in the pipe wil for a moment continue in the direction it was travelling. The column of moving air will then create a low pressure area just behind the throttle. This is the effect you want to see occur behind an exhaust valve. This low pressure area helps to suck out gasses from inside the combustion chamber. The volume of air at a given RPM and WOT, will be very similar with different sized pipes, with the only differences being the change in VE of the motor that results. For the purposes of my example, we will assume they remain constant. Since the airflow is the same, the mass will be the same. Momentum consists of mass and speed. Since mass is, for the most part, fixed here, the only way to increase momentum is to increase the speed of the gasses. And the higher the velocity of the gasses, the lower the pressure will be behind the valve when the valve shuts. Of course if you size the pipe too small you start to increase the overall pressure in the system to the point that the low pressure area(which is relative to the pressure in the pipes) is higher than it would be with a bigger pipe. Most factory exhaust systems are sized quite small to keep noise levels down so they tend to have plenty of room to upgrade to a bigger pipe without affecting low RPM scavenging or at least a minimal sacrifice to it.
And Tuning at high RPM is just as important. The same scavenging effects still exists. Higher velocity gasses are much easier to acheive at high RPM's though so increasing flow becomes a prioroty there when looking at an OEM exhaust.