Exhaust back-pressure theory
04-11-2002, 03:06 PM
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Exhaust back-pressure theory
I was thinking about above topic today because I thought that my car is creating too much back pressure due to the bad-designed glasspack, which I will soon change to perfortrated core one.
In my opinion, if a car's exhaust has too much back pressure, then the exhaust would create an effect which makes some air coming from a header points backward to direction at certain angle that would block some of the new air coming from the header, making air hard get out of the exhaust properly at lower RPM, thus creating some lag to the throttle and engine response. At higher RPM, this wouldn't matter too much because by then, the ratio of air velocity to back pressure is far too great that back pressure would not matter that much.
Do you think that I have a right idea about this back pressure theory?
[ April 11, 2002, 10:07 PM: Message edited by: SilverTibby ]
In my opinion, if a car's exhaust has too much back pressure, then the exhaust would create an effect which makes some air coming from a header points backward to direction at certain angle that would block some of the new air coming from the header, making air hard get out of the exhaust properly at lower RPM, thus creating some lag to the throttle and engine response. At higher RPM, this wouldn't matter too much because by then, the ratio of air velocity to back pressure is far too great that back pressure would not matter that much.
Do you think that I have a right idea about this back pressure theory?
[ April 11, 2002, 10:07 PM: Message edited by: SilverTibby ]
04-11-2002, 03:19 PM
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Vehicle: 2010 Honda VFR1200
Here This guy knows what he is talking about take a look. EXHAUST It is all true what he says... plus its easier to link you that for me to type it out
04-12-2002, 04:53 AM
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Someone I know used to own a Sunbird (lol) and the catalytic converter became blocked, which in effect created WAY too much backpressure.
The result was that the engine kept overheating and consequently stalling out, so I think that in extreme cases, that's what would happen in about any case where backpressure is too high.
I'm not the expert Brendan is (nice web site, man
) but I think that too much backpressure would rob the car of power, as it would create a heat buildup in the engine, forcing the ECU to retard the ignition timing.
I may be completely off base with that, so please correct me if I'm wrong.
The result was that the engine kept overheating and consequently stalling out, so I think that in extreme cases, that's what would happen in about any case where backpressure is too high.
I'm not the expert Brendan is (nice web site, man
) but I think that too much backpressure would rob the car of power, as it would create a heat buildup in the engine, forcing the ECU to retard the ignition timing.I may be completely off base with that, so please correct me if I'm wrong.
04-12-2002, 05:16 AM
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You can look at backpressure from a number of viewpoints:
Pressure: the increased pressure causes more parasitic losses on the exhaust stroke, as more power is wasted trying to push out the exhaust gasses.
Temperature: the longer "loiter" time of the hot gasses causes the manifold and head to retain their heat longer. This can make for inefficient combustion as well as other overheating problems.
Now too low of back pressure can also cause problems in how 4 bangers run. During the overlap phase of the valves you have a little leftover exhaust gas in the chamber and a fuel/air mixture waiting to get into the chamber. If the backpressure is too low, a suction can actually pull some of the fuel/air of the intake side out through the exhaust side. This is not good (for fuel economy, your catalytic converter). Usually, more air is pulled through than fuel. This makes the mixture a bit richer, which robs power, especially at low end. If you're wondering how the high pressure, hot exhaust gasses can cause suction on the intake side, remember that the exhaust gas not only has temperature and pressure, but also momentum. As the slug of exhaust gas leaves, it creates a lower pressure at the back, which is what the cylinder head is seeing during overlap. Most people think about gasses in steady terms, but when there are pulses and waves involved, the dynamics get complicated quickly.
Now, if the backpressure is too high, some of the spent gasses stay in the chamber, keep the temps higher, can cause detonation, and don't allow for a full "filling" stroke on the intake side, since some exhaust gas can actually blow into the intake manifold.
So the goal is to get a tuned amount of back pressure: enough to get allow almost all of the exhaust to get out, but not low enough to upset the fuel/air mixture of the intake side.
Hopefully I haven't confused anyone, and backpressure is a hotly debated subject with respect to I4 engines; so everyone else, feel free to chip in.
ac
[ April 12, 2002, 12:18 PM: Message edited by: Curtas ]
Pressure: the increased pressure causes more parasitic losses on the exhaust stroke, as more power is wasted trying to push out the exhaust gasses.
Temperature: the longer "loiter" time of the hot gasses causes the manifold and head to retain their heat longer. This can make for inefficient combustion as well as other overheating problems.
Now too low of back pressure can also cause problems in how 4 bangers run. During the overlap phase of the valves you have a little leftover exhaust gas in the chamber and a fuel/air mixture waiting to get into the chamber. If the backpressure is too low, a suction can actually pull some of the fuel/air of the intake side out through the exhaust side. This is not good (for fuel economy, your catalytic converter). Usually, more air is pulled through than fuel. This makes the mixture a bit richer, which robs power, especially at low end. If you're wondering how the high pressure, hot exhaust gasses can cause suction on the intake side, remember that the exhaust gas not only has temperature and pressure, but also momentum. As the slug of exhaust gas leaves, it creates a lower pressure at the back, which is what the cylinder head is seeing during overlap. Most people think about gasses in steady terms, but when there are pulses and waves involved, the dynamics get complicated quickly.
Now, if the backpressure is too high, some of the spent gasses stay in the chamber, keep the temps higher, can cause detonation, and don't allow for a full "filling" stroke on the intake side, since some exhaust gas can actually blow into the intake manifold.
So the goal is to get a tuned amount of back pressure: enough to get allow almost all of the exhaust to get out, but not low enough to upset the fuel/air mixture of the intake side.
Hopefully I haven't confused anyone, and backpressure is a hotly debated subject with respect to I4 engines; so everyone else, feel free to chip in.
ac
[ April 12, 2002, 12:18 PM: Message edited by: Curtas ]
05-09-2002, 11:58 AM
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According to this theory though, in the powerband of any header, the suction effect would pull some of the charge right out into the exhaust, meaning there would be no powerband at all, because there would be less fuel to burn...
Do every car have an overlap? Dont most cars wait until the exhaust valve is closed before opening the intake valve?
-Steve
Do every car have an overlap? Dont most cars wait until the exhaust valve is closed before opening the intake valve?
-Steve
05-09-2002, 04:07 PM
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nope, most cars have some overlap (minus turbo cars).
Its not really the back pressure that matters, its how well everything flows (air velocity). If you have a louvered core glasspack, the exhuast stream will become turbulent, slow down, and lose heat i.e. lose energy (conservation of energy basically). Because of the lower heat and more turbulence, there is less energy to make the exhaust get outta dodge, causing it to back up and create pressure (i.e. backpressure, lol).
In other words, you want the exhuast stream velocity to be as high as possible for good cylinder scavaging, yet not high enough to suck the intake charge out with it. This is wear variable valve timing comes in handy, but thats a whole 'nother topic. smile.gif
Its not really the back pressure that matters, its how well everything flows (air velocity). If you have a louvered core glasspack, the exhuast stream will become turbulent, slow down, and lose heat i.e. lose energy (conservation of energy basically). Because of the lower heat and more turbulence, there is less energy to make the exhaust get outta dodge, causing it to back up and create pressure (i.e. backpressure, lol).
In other words, you want the exhuast stream velocity to be as high as possible for good cylinder scavaging, yet not high enough to suck the intake charge out with it. This is wear variable valve timing comes in handy, but thats a whole 'nother topic. smile.gif