Debate: Is too little backpressure REALLY an issue!?
03-28-2002, 07:19 AM
#1
Senior Member
Thread Starter
Join Date: Jan 2001
Posts: 1,816
Debate: Is too little backpressure REALLY an issue!?
According to SCC and Mike Kojima:
It's an "old hot rodder's tale, that engines need some back pressure to work properly and make torque"
So is it true? i've always been told that was true and you needed backpressure or you would lose a lot of low-end torque. But now that i think about it, the LAST thing an engine would need to run efficiently would be exhaust gases backing up in the cylinder, working against the valves and pistons.. If anything this would be BAD. I've never actually seen a dyno that shows any loss of torque from removing an exhaust or installing a big 3" system. Even on a N/a car. Anyway, I realize that SCC may be full of it, but to me it just makes sense.. If it's true, I see no reason not to put a 3" system on our cars seeing as how we have much bigger displacement than all those little 4 bangers.
Let the flames begin!
It's an "old hot rodder's tale, that engines need some back pressure to work properly and make torque"
So is it true? i've always been told that was true and you needed backpressure or you would lose a lot of low-end torque. But now that i think about it, the LAST thing an engine would need to run efficiently would be exhaust gases backing up in the cylinder, working against the valves and pistons.. If anything this would be BAD. I've never actually seen a dyno that shows any loss of torque from removing an exhaust or installing a big 3" system. Even on a N/a car. Anyway, I realize that SCC may be full of it, but to me it just makes sense.. If it's true, I see no reason not to put a 3" system on our cars seeing as how we have much bigger displacement than all those little 4 bangers.
Let the flames begin!
03-28-2002, 07:27 AM
#2
Re: Debate: Is too little backpressure REALLY an issue!?
It's an "old hot rodder's tale, that engines need some back pressure to work properly and make torque"
the LAST thing an engine would need to run efficiently would be exhaust gases backing up in the cylinder, working against the valves and pistons..
03-28-2002, 07:43 AM
#3
Senior Member
Join Date: Sep 2001
Location: Detroit Metro Area
Posts: 1,617
NA engines need some backpressure to make low end torque. Turbo'd cars are different. Supercharged cars are going to be somewhere in the middle. If you're only worried about top end speed then backpressure isn't really an issue. If you're concerned about low end torque backpressure is an issue.
SCC speaks of honduhs. No amount of backpressure is going to give them any torque. So maybe it is true in that case.
Stereodude
SCC speaks of honduhs. No amount of backpressure is going to give them any torque. So maybe it is true in that case.
Stereodude
03-28-2002, 08:02 AM
#4
Donating Maxima.org Member
Join Date: Jan 2002
Posts: 818
Exhaust Scavenging basics:
An exhaust is usually the LAST thing designed on the car so it is made cheap to conform to all the necessary bends and twists on the car. Putting on a low restriction (bigger pipe diameter) exhaust relieves some backpressure which frees up some hp created by the engine haveing to pump the exhaust gas out of the cylinders.
The exhaust leaves the engine from various cylinders, travelling not in a continuous stream, but in individual "plugs" down the exhaust system. As a plug moves, it cools off, slows down and contracts. Hopefully, the exhaust has low enough backpressure that, given the speed of the plug, it can a larger section of piping where it can expand due to increased volume. At this point the exhaust has slowed down and contracted considerably, but the increased pipe diameter keeps it from contracting so much that it reverses its direction. What does this mean? Well, at the end of the exhaust system, this contracting effect begins to create a bit of a vaccum on the next plug. This vacuum pulls on the plug to keep it from slowing down too much. This small effect (once the proper engine speed is reached) continues to the cylinder and at that point is called scavenging since the engine is at or past TDC and is no longer pumping exhaust out. If the exhaust system is too open, the exhaust gases run through too quickly to create this scavenging effect. This is known and the point of diminishing returns. At higher RPM, airspeeds are much higher and valve overlap helps with the scavenging effect by using the intake charge to dispel the exhaust gases and start the cooling process. So yes, having too little backpressure can hurt at low RPM. That is why Nissan created the variable capacity muffler, to maintain some restriction at low engine speeds.
If any experts see anything wrong here please PM me.
An exhaust is usually the LAST thing designed on the car so it is made cheap to conform to all the necessary bends and twists on the car. Putting on a low restriction (bigger pipe diameter) exhaust relieves some backpressure which frees up some hp created by the engine haveing to pump the exhaust gas out of the cylinders.
The exhaust leaves the engine from various cylinders, travelling not in a continuous stream, but in individual "plugs" down the exhaust system. As a plug moves, it cools off, slows down and contracts. Hopefully, the exhaust has low enough backpressure that, given the speed of the plug, it can a larger section of piping where it can expand due to increased volume. At this point the exhaust has slowed down and contracted considerably, but the increased pipe diameter keeps it from contracting so much that it reverses its direction. What does this mean? Well, at the end of the exhaust system, this contracting effect begins to create a bit of a vaccum on the next plug. This vacuum pulls on the plug to keep it from slowing down too much. This small effect (once the proper engine speed is reached) continues to the cylinder and at that point is called scavenging since the engine is at or past TDC and is no longer pumping exhaust out. If the exhaust system is too open, the exhaust gases run through too quickly to create this scavenging effect. This is known and the point of diminishing returns. At higher RPM, airspeeds are much higher and valve overlap helps with the scavenging effect by using the intake charge to dispel the exhaust gases and start the cooling process. So yes, having too little backpressure can hurt at low RPM. That is why Nissan created the variable capacity muffler, to maintain some restriction at low engine speeds.
If any experts see anything wrong here please PM me.
03-28-2002, 08:39 AM
#5
Guest
Posts: n/a
Proper scavenging of exhaust gases is critical in an exhaust system. Scavenging improves with increased velocity throughout that entire system. The only way to increase velocity is to 1) decrease piping size, and 2) reduce flow restrictions aka restrictive mufflers, etc.
Exhaust backpressure itself does NOT improve performance in any circumstances. However, 0 backpressure is not good BECAUSE of the factr that you would need oversized exhaust piping to achieve that. It's the large size piping that ultimately would reduce engine performance, not due to lack of backpressure, but due to lack of velocity.
However, this does not imply that a restrictive muffler improves performance at any rpm. A restrictive muffler reduces flow velocity and ultimately reduces the ability for the exhaust gases to rapidly leave the system (ie scavenge).
In the case of the 2000+ Maxima variable capacity muffler, the low-rpm setting is setup such that it gives a similar flowpath as the pre-2000 non-variable muffler. This position provides sufficient velocity to provide good scavenging at low rpms. The high-rpm setting adds a second flowpath that more or less doubles the cross sectional flow area, providing better scavenging at higher rpms. The generalization that the muffler operates on backpressure is a little misleading. The muffler operates on SUPPLY pressure...ie when the volumetric flow rate of the exhaust gases reaches a certain point, there will be enough pressure buildup due to the restrictive low-rpm flowpath that the valve opens up to relieve the pressure and add a second flowpath.
Ignore the term backpressure. It's meaningless and misleading. The term velocity means everything with exhaust systems. You want high velocities. That means appropriate-sized exhaust piping with minimal bends and crimps, manifolds, Y, and/or X sections with smooth convergences, and unrestrictive mufflers and catalytic converters. These are what will increase the performance of an engine at all rpms.
Exhaust backpressure itself does NOT improve performance in any circumstances. However, 0 backpressure is not good BECAUSE of the factr that you would need oversized exhaust piping to achieve that. It's the large size piping that ultimately would reduce engine performance, not due to lack of backpressure, but due to lack of velocity.
However, this does not imply that a restrictive muffler improves performance at any rpm. A restrictive muffler reduces flow velocity and ultimately reduces the ability for the exhaust gases to rapidly leave the system (ie scavenge).
In the case of the 2000+ Maxima variable capacity muffler, the low-rpm setting is setup such that it gives a similar flowpath as the pre-2000 non-variable muffler. This position provides sufficient velocity to provide good scavenging at low rpms. The high-rpm setting adds a second flowpath that more or less doubles the cross sectional flow area, providing better scavenging at higher rpms. The generalization that the muffler operates on backpressure is a little misleading. The muffler operates on SUPPLY pressure...ie when the volumetric flow rate of the exhaust gases reaches a certain point, there will be enough pressure buildup due to the restrictive low-rpm flowpath that the valve opens up to relieve the pressure and add a second flowpath.
Ignore the term backpressure. It's meaningless and misleading. The term velocity means everything with exhaust systems. You want high velocities. That means appropriate-sized exhaust piping with minimal bends and crimps, manifolds, Y, and/or X sections with smooth convergences, and unrestrictive mufflers and catalytic converters. These are what will increase the performance of an engine at all rpms.
03-28-2002, 08:50 AM
#6
Senior Member
Thread Starter
Join Date: Jan 2001
Posts: 1,816
Hrmmm, Thank you for replying with the PHYSICS of how this is supposed to work.
Although i don't think the discussion is quite dead yet, is it that it will actually HARM the engine's performance? or just will provide diminishing returns in terms of TQ. Also, isn't the engine expelling exhaust gases at a sufficient velocity that it will greatly overcome the vacuum effect of these gases cooling near the end of the muffler? Also wouldn't a less restrictive system facilitate quicker expulsion of these gases and therefore less time in which the air stays in the exhaust system to cool down and create vacuum. Also how does this vacuum affect the movement of the pistons? on the combustion stroke, the exhaust valves are closed, on the compression stroke they are closed as well. doesn't vacuum also expediate the expulsion of exhaust gases?
I may be completely off kilter here, but i'm jsut playing devils advocate trying to find out as much as i can.
Anyway, anyone have dynos that show this phenomenon?
Keven, so could you sum up what you said in a few sentences? I kinda don't get it sorry... is oversized piping bad and why. Lack of backpressure is not an issue? And, a good high flowing exhaust system will provide good power at all rpms?
Does anyone know what the optimal size is then for 3.0 liter or 3.5 liter engines? SCC published a letter from the president of isuzu performance that said engines 2.6L and larger should have piping larger than 2.5 inches...
thanks in advance
Although i don't think the discussion is quite dead yet, is it that it will actually HARM the engine's performance? or just will provide diminishing returns in terms of TQ. Also, isn't the engine expelling exhaust gases at a sufficient velocity that it will greatly overcome the vacuum effect of these gases cooling near the end of the muffler? Also wouldn't a less restrictive system facilitate quicker expulsion of these gases and therefore less time in which the air stays in the exhaust system to cool down and create vacuum. Also how does this vacuum affect the movement of the pistons? on the combustion stroke, the exhaust valves are closed, on the compression stroke they are closed as well. doesn't vacuum also expediate the expulsion of exhaust gases?
I may be completely off kilter here, but i'm jsut playing devils advocate trying to find out as much as i can.
Anyway, anyone have dynos that show this phenomenon?
Keven, so could you sum up what you said in a few sentences? I kinda don't get it sorry... is oversized piping bad and why. Lack of backpressure is not an issue? And, a good high flowing exhaust system will provide good power at all rpms?
Does anyone know what the optimal size is then for 3.0 liter or 3.5 liter engines? SCC published a letter from the president of isuzu performance that said engines 2.6L and larger should have piping larger than 2.5 inches...
thanks in advance
03-28-2002, 08:54 AM
#7
Member
Join Date: Jan 2002
Posts: 56
Well, I think maybe we confuse wanting backpressure and wanting smaller pipe. Back pressure is needed in 2 stroke engines after the fuel/air mixture is in the cylinder to keep it in there and also compress it somewhat. On a 4 cycle engine, the valves are only open after the power stroke so theoretically you want a vacuum waiting for them to suck the exhaust out(like Badaxxima said). Large pipe creates slower moving, higher pressure exhaust(Bernoulii's principle(sp?)). A smaller pipe creates faster moving, lower pressure exhaust. We want faster moving exhaust so that it will create the vacuum. But of course, it's harder to get a certain volume though a smaller pipe than through a larger pipe which is where we get backpressure. Backpressure is not needed. The smaller pipe is needed to keep the speed up. Too large of a pipe will cause too slow of speeds and the air will reverse direction. So you have to tune for the optimum of the two.
To sum it up, small pipe creates backpressure which you don't want, but large pipe will give reverse direction which you don't want. It has to be somewhere in between.
Gene
To sum it up, small pipe creates backpressure which you don't want, but large pipe will give reverse direction which you don't want. It has to be somewhere in between.
Gene
03-28-2002, 09:00 AM
#9
Guest
Posts: n/a
Cool, we basically said the same thing! *Pat on back*
Originally posted by rcrdps
Well, I think maybe we confuse wanting backpressure and wanting smaller pipe. Back pressure is needed in 2 stroke engines after the fuel/air mixture is in the cylinder to keep it in there and also compress it somewhat. On a 4 cycle engine, the valves are only open after the power stroke so theoretically you want a vacuum waiting for them to suck the exhaust out(like Badaxxima said). Large pipe creates slower moving, higher pressure exhaust(Bernoulii's principle(sp?)). A smaller pipe creates faster moving, lower pressure exhaust. We want faster moving exhaust so that it will create the vacuum. But of course, it's harder to get a certain volume though a smaller pipe than through a larger pipe which is where we get backpressure. Backpressure is not needed. The smaller pipe is needed to keep the speed up. Too large of a pipe will cause too slow of speeds and the air will reverse direction. So you have to tune for the optimum of the two.
To sum it up, small pipe creates backpressure which you don't want, but large pipe will give reverse direction which you don't want. It has to be somewhere in between.
Gene
Well, I think maybe we confuse wanting backpressure and wanting smaller pipe. Back pressure is needed in 2 stroke engines after the fuel/air mixture is in the cylinder to keep it in there and also compress it somewhat. On a 4 cycle engine, the valves are only open after the power stroke so theoretically you want a vacuum waiting for them to suck the exhaust out(like Badaxxima said). Large pipe creates slower moving, higher pressure exhaust(Bernoulii's principle(sp?)). A smaller pipe creates faster moving, lower pressure exhaust. We want faster moving exhaust so that it will create the vacuum. But of course, it's harder to get a certain volume though a smaller pipe than through a larger pipe which is where we get backpressure. Backpressure is not needed. The smaller pipe is needed to keep the speed up. Too large of a pipe will cause too slow of speeds and the air will reverse direction. So you have to tune for the optimum of the two.
To sum it up, small pipe creates backpressure which you don't want, but large pipe will give reverse direction which you don't want. It has to be somewhere in between.
Gene
03-28-2002, 09:23 AM
#10
Senior Member
Thread Starter
Join Date: Jan 2001
Posts: 1,816
it's ok!
Originally posted by rcrdps
sorry about posting the same thing. I was writing at the same time I guess.
Gene
sorry about posting the same thing. I was writing at the same time I guess.
Gene
4 stroke combustion engines "needing backpressure" is a myth then!
So I guess now the question remains, what is the ideal sized piping for the exhaust?
03-28-2002, 10:13 AM
#12
Senior Member
Join Date: Sep 2000
Posts: 2,117
Can someone more versed confirm or correct me? >>
Originally posted by deezo
...our cars wouldn't have EGR(Exhaust Gas Recirculation) Valves.
...our cars wouldn't have EGR(Exhaust Gas Recirculation) Valves.
03-28-2002, 10:35 AM
#13
Senior Member
Join Date: Apr 2001
Posts: 689
Here is a really well written article that will answer the majority of the questions people have about exhaust, back pressure, etc... I know it may be lengthy, but read the entire thing, you'll be happy you did.
http://www.ssheaders.com/header.htm
http://www.ssheaders.com/header.htm
03-28-2002, 12:07 PM
#15
Newbie - Just Registered
Join Date: Mar 2002
Posts: 11
Here’s my two (or more) cents worth. It’s kinda long, and you are welcome to disagree.
BACKGROUND
Your engine breathes in air. Air consists mainly of oxygen and nitrogen. Combustion requires oxygen. A typical automotive engine needs to match the correct amount of fuel to oxygen. To get more power from an engine, it needs more fuel. Before you can put in more fuel, you need more oxygen. This can be accomplish by either forcing more air into the engine (supercharging, turbocharging, etc.), or by nitrous, or by better breathing (CAI, valve timing, exhaust system, etc.)
ENGINE BREATHING
Engine breathing consists of the intake air system, the intake manifold, intake ports, intake valve timing, exhaust valve timing, exhaust port, exhaust manifold, exhaust system, etc. For a given engine speed, intake air flow rate, intake valve timing, exhaust valve timing, etc., there is an ideal exhaust back pressure to get the most air (oxygen) into the cylinder. Change anyone of these, and you will change the characteristics of the engine. For example, changing valve timing will effect how the cylinder gets filled with air, and how the exhaust back pressure interacts with the filling of air and scavenging of exhaust. Change intake runner length or exhaust length will change the filling of air and scavenging of exhaust. You can tune an engine for best performance at any one (or a few) condition, but not at all conditions. Typically, a larger exhaust system will slightly decrease low end torque but increase high end horsepower. What do you want for an exhaust system? It depends on where you want your torque curve to look like. (Of course a severely restrictive exhaust will benefit everywhere with a less restrictive exhaust.)
In the old days, burnt exhaust valves were a problem. This is where the no exhaust back pressure could be bad thought comes from. Exhaust temperatures are hot enough to burn the valves. A fast flow rate past the valve increase the chances of valve burning. A less restrictive exhaust will increase flow rates past the exhaust valve, thereby increase the chances of a burnt valve.
BERNOULLI'S EQUATION
Bernoulli’s equation applies to changing velocity within a system. For example, if you have a large pipe that converges into a smaller pipe, the velocity of the fluid increases, and the static pressure decreases, but total pressure does not change so that energy is conserved (this ignores flow losses). Even though static pressure decreases in the smaller pipe, the total pressure is still the same, and the pressure upstream to push the fluid through the pipe does not change (ignoring flow losses). If you include flow losses, putting the smaller pipe increases flow losses, which will require a high pressure upstream the push the same amount of fluid through the pipe, or a lower flow rate if the upstream pressure remains the same. Changing pipe diameter may change the static pressure, but it does not change the total pressure, which is what drives the flow (ignoring flow losses). It is the flow loss that you should be concerned about and not the static pressure in the exhaust.
EGR
EGR is used to reduce NOx emissions and improve part load efficiency. The nitrogen in the air will adsorb energy from the combustion process (you do not completely loose this energy, it is just stored in the nitrogen, which is turned into useful work at the crankshaft). The nitrogen lowers combustion chamber temperatures, which will lower NOx emissions and improve efficiency. It lowers NOx emissions since the formation of these emissions is mainly dependant on temperature. The higher the temperature, the more NOx emissions you will make. The EGR also provides a method to adsorb energy, thereby lowering temperatures and lowering NOx emissions. It will improve efficiency in two ways. The first is that the lower combustion chamber temperatures means less energy is lost to the coolant, which means more of the energy is available to the crankshaft. The second is that the EGR displaces air in the cylinder, which means a larger throttle opening is needed to get the amount of oxygen needed for the driving situation, which means lower pumping losses (the throttle is open more for the given amount of power, which reduces the flow losses across the throttle).
James
BACKGROUND
Your engine breathes in air. Air consists mainly of oxygen and nitrogen. Combustion requires oxygen. A typical automotive engine needs to match the correct amount of fuel to oxygen. To get more power from an engine, it needs more fuel. Before you can put in more fuel, you need more oxygen. This can be accomplish by either forcing more air into the engine (supercharging, turbocharging, etc.), or by nitrous, or by better breathing (CAI, valve timing, exhaust system, etc.)
ENGINE BREATHING
Engine breathing consists of the intake air system, the intake manifold, intake ports, intake valve timing, exhaust valve timing, exhaust port, exhaust manifold, exhaust system, etc. For a given engine speed, intake air flow rate, intake valve timing, exhaust valve timing, etc., there is an ideal exhaust back pressure to get the most air (oxygen) into the cylinder. Change anyone of these, and you will change the characteristics of the engine. For example, changing valve timing will effect how the cylinder gets filled with air, and how the exhaust back pressure interacts with the filling of air and scavenging of exhaust. Change intake runner length or exhaust length will change the filling of air and scavenging of exhaust. You can tune an engine for best performance at any one (or a few) condition, but not at all conditions. Typically, a larger exhaust system will slightly decrease low end torque but increase high end horsepower. What do you want for an exhaust system? It depends on where you want your torque curve to look like. (Of course a severely restrictive exhaust will benefit everywhere with a less restrictive exhaust.)
In the old days, burnt exhaust valves were a problem. This is where the no exhaust back pressure could be bad thought comes from. Exhaust temperatures are hot enough to burn the valves. A fast flow rate past the valve increase the chances of valve burning. A less restrictive exhaust will increase flow rates past the exhaust valve, thereby increase the chances of a burnt valve.
BERNOULLI'S EQUATION
Bernoulli’s equation applies to changing velocity within a system. For example, if you have a large pipe that converges into a smaller pipe, the velocity of the fluid increases, and the static pressure decreases, but total pressure does not change so that energy is conserved (this ignores flow losses). Even though static pressure decreases in the smaller pipe, the total pressure is still the same, and the pressure upstream to push the fluid through the pipe does not change (ignoring flow losses). If you include flow losses, putting the smaller pipe increases flow losses, which will require a high pressure upstream the push the same amount of fluid through the pipe, or a lower flow rate if the upstream pressure remains the same. Changing pipe diameter may change the static pressure, but it does not change the total pressure, which is what drives the flow (ignoring flow losses). It is the flow loss that you should be concerned about and not the static pressure in the exhaust.
EGR
EGR is used to reduce NOx emissions and improve part load efficiency. The nitrogen in the air will adsorb energy from the combustion process (you do not completely loose this energy, it is just stored in the nitrogen, which is turned into useful work at the crankshaft). The nitrogen lowers combustion chamber temperatures, which will lower NOx emissions and improve efficiency. It lowers NOx emissions since the formation of these emissions is mainly dependant on temperature. The higher the temperature, the more NOx emissions you will make. The EGR also provides a method to adsorb energy, thereby lowering temperatures and lowering NOx emissions. It will improve efficiency in two ways. The first is that the lower combustion chamber temperatures means less energy is lost to the coolant, which means more of the energy is available to the crankshaft. The second is that the EGR displaces air in the cylinder, which means a larger throttle opening is needed to get the amount of oxygen needed for the driving situation, which means lower pumping losses (the throttle is open more for the given amount of power, which reduces the flow losses across the throttle).
James
03-28-2002, 12:10 PM
#16
Better my young Jedi. But you still not strong enough to fight the dark side. 
Originally posted by Craig Mack
Jeff92SE:
BTW it's not "backpressure", it's called exhaust velocity..and it's considered a science finding the right amount of it.
How's that for bandwidth Jeffrey
Jeff92SE:
BTW it's not "backpressure", it's called exhaust velocity..and it's considered a science finding the right amount of it.
How's that for bandwidth Jeffrey
03-28-2002, 12:11 PM
#17
Re: Can someone more versed confirm or correct me? >>
Originally posted by Albertt
But I always thought the EGR was part of the emissions system. It allows the use of exhaust gas (hot) to heat the intake air during cold start running to speed up the warming up process thus producing less air contaminants.
But I always thought the EGR was part of the emissions system. It allows the use of exhaust gas (hot) to heat the intake air during cold start running to speed up the warming up process thus producing less air contaminants.
03-28-2002, 01:26 PM
#18
Senior Member
Thread Starter
Join Date: Jan 2001
Posts: 1,816
Re: Re: Can someone more versed confirm or correct me? >>
Originally posted by deezo
It is part of emissions to allow cleaner exhaust exiting the vehicle. It recirculates gases from the exhaust manifold back into the cylinders to ensure the gas vapors are burned thoroughly.
It is part of emissions to allow cleaner exhaust exiting the vehicle. It recirculates gases from the exhaust manifold back into the cylinders to ensure the gas vapors are burned thoroughly.
03-28-2002, 01:59 PM
#19
Re: Re: Re: Can someone more versed confirm or correct me? >>
Originally posted by Chinkzilla
That sounds pretty evil in regards to horsepower
That sounds pretty evil in regards to horsepower
03-28-2002, 03:28 PM
#20
All YOUR grammer belong to me
Join Date: Jul 2001
Posts: 7,400
Originally posted by Jeff92se
Better my young Jedi. But you still not strong enough to fight the dark side.
Better my young Jedi. But you still not strong enough to fight the dark side.
(Oh yea can't "***** anymore)
Hmm...well carbon that builds up in the cylinders and pistons is not good and hinders performance and engine life. A good run on the highway will clean them out. It is also ideal to get your service checks on time to keep your engine in top condition. Refer to your owners manual for more info.
03-28-2002, 04:12 PM
#21
The point of EGR is to reduce emissions, correct. But it does not do so by burning the exhaust gas again, just to be sure.
Adding some inert (non-combustible) gas to the cylinder keeps combustion temperatures down. This is good because when temps get too high, emissions of oxides of nitrogen (NOx) go way up. Keeping the temp down keeps NOx down, we're all happy.
But, EGR only functions under certain conditions. It only functions at low RPM and under very low load conditions. It never functions at WOT or high RPM. This is why disabling the EGR will get you NO HORSEPOWER.
The downside of EGR is fouling of the intake system. Carbon build up in the intake plenum to be specific. But the advantages clearly outweigh this disadvantage for a street car.
This is why EGR is a good thing.
Adding some inert (non-combustible) gas to the cylinder keeps combustion temperatures down. This is good because when temps get too high, emissions of oxides of nitrogen (NOx) go way up. Keeping the temp down keeps NOx down, we're all happy.
But, EGR only functions under certain conditions. It only functions at low RPM and under very low load conditions. It never functions at WOT or high RPM. This is why disabling the EGR will get you NO HORSEPOWER.
The downside of EGR is fouling of the intake system. Carbon build up in the intake plenum to be specific. But the advantages clearly outweigh this disadvantage for a street car.
This is why EGR is a good thing.
03-29-2002, 06:51 AM
#23
Originally posted by Craig Mack
Jeff92SE:
BTW it's not "backpressure", it's called exhaust velocity..and it's considered a science finding the right amount of it.
How's that for bandwidth Jeffrey
Jeff92SE:
BTW it's not "backpressure", it's called exhaust velocity..and it's considered a science finding the right amount of it.
How's that for bandwidth Jeffrey
the two terms are not interchangeable.
03-29-2002, 06:55 AM
#24
Originally posted by Keven97SE
Proper scavenging of exhaust gases is critical in an exhaust system. Scavenging improves with increased velocity throughout that entire system. The only way to increase velocity is to 1) decrease piping size, and 2) reduce flow restrictions aka restrictive mufflers, etc
Proper scavenging of exhaust gases is critical in an exhaust system. Scavenging improves with increased velocity throughout that entire system. The only way to increase velocity is to 1) decrease piping size, and 2) reduce flow restrictions aka restrictive mufflers, etc
03-29-2002, 07:51 AM
#25
Guest
Posts: n/a
(sigh) there's always one in the crowd... 
You are correct, I forgot that one. Dagnabbit.
You are correct, I forgot that one. Dagnabbit.
Originally posted by got rice?
3) exhaust pipe wrap. keeping the pipes hot(ter) keeps the exhaust gas hotter; hotter gas = higher exhaust gas velocity. Ever notice how aftermarket exhausts are louder after 10-15 minutes of driving.. ?
3) exhaust pipe wrap. keeping the pipes hot(ter) keeps the exhaust gas hotter; hotter gas = higher exhaust gas velocity. Ever notice how aftermarket exhausts are louder after 10-15 minutes of driving.. ?
03-29-2002, 07:58 AM
#26
Senior Member
Join Date: Dec 2001
Posts: 2,084
All this hi-tech talk is making my head hurt. Anyway, a few of us went to a local dyno last night. One of our guys was running a Y pipe, test pipe and B pipe on an apex muffler (basically you could walk up into the exhaust valves from the back of the car. His numbers, according to him, were pretty much the same on the HP side, but he did state that he lost on the torque side from a previous run late last year. Now, I know a bunch of people are going to flame me and say that there was time lapse and the dyno was not done at the same time. I know I know, but this may give you tech guys something to discuss. Remember, in this case it was the same car and the test pipe and b pipe were the only two mods done between dyno runs. Flame on fellows....
03-30-2002, 10:14 AM
#27
you do lose a slight amout of low end torque when you go with a larger piping and you're not able to fully utilize the extra flow (more prevalent on naturally aspirated engines). Once the engine is pulling 3000+ rpms, there's sufficient exhaust flow to utilize the exhaust gas pulses and not have that low end loss.
the torque loss is very slight but the gains you see at mid and high rpms from a more efficient exhaust design (ie 2.5" mandrel bent pipes) is worth it.
the torque loss is very slight but the gains you see at mid and high rpms from a more efficient exhaust design (ie 2.5" mandrel bent pipes) is worth it.
Thread
Thread Starter
Forum
Replies
Last Post
hez8813
5th Generation Maxima (2000-2003)
11
03-12-2020 12:06 AM
BrandonCause
4th Generation Maxima (1995-1999)
4
09-14-2015 10:41 PM