How are you going to show its more power when you dont even have the same engine as before. There are obviously variances in engines even if they were made one after the other. By the ET's at the track, he's meaning who can not only put it down better, but also how the general powerband "performs" (at least thats what I took out of it).PurpleCherokee said:
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How free flowing should the XJ exhaust be?
- Thread starter Quills
- Start date
So back to the backpressure thing, go put a 5" exhaust and see what happens with the flow velocity. And on top of anything, my point in arguing is more along the lines of you removing the cat which is illegal. Especially with such a lower power engine I doubt even more that you'll see a gain if any compared to something with 600hp (would prolly see a gain).PurpleCherokee said:
On a side note:
"Modifying the exhaust system, (i.e. modifying the exhaust gas velocity by changing exhaust tube diameters) can detract from the "ideal" scavenging effects, and reduce fuel efficiency and power."
And another one for ya:
"Whatever you do, do not remove or gut out the catalytic converter on your street car. The monolithic, straight through design of modern 3-way catalytic converters is usually quite free flowing on most modern sport compact cars, producing at the most, only a pound or two of extra backpressure. A gutted cat can actually hurt power as the empty box can cause flow stagnation, which effectively shortens the length of the moving gas column in the exhaust pipe. The empty box can also reduce important flow velocity. This can be felt as a loss in bottom end power.
Because of these factors, some of our project cars have actually gained power with the addition of a cat. As the number of vehicles on our highways grows every year, we must all do our part to manage pollution. If every last bit of power must be extracted, as in real, off-the-street sanctioned racing, then the cat can be removed and replaced with a length of pipe, not simply gutted.
When changing a factory cat for a larger high flowing one, consider that Random Technology and Pace Setter make replacement cats with 3-inch ore even larger inlets and outlets."
"Whatever you do, do not remove or gut out the catalytic converter on your street car. The monolithic, straight through design of modern 3-way catalytic converters is usually quite free flowing on most modern sport compact cars, producing at the most, only a pound or two of extra backpressure. A gutted cat can actually hurt power as the empty box can cause flow stagnation, which effectively shortens the length of the moving gas column in the exhaust pipe. The empty box can also reduce important flow velocity. This can be felt as a loss in bottom end power.
Because of these factors, some of our project cars have actually gained power with the addition of a cat. As the number of vehicles on our highways grows every year, we must all do our part to manage pollution. If every last bit of power must be extracted, as in real, off-the-street sanctioned racing, then the cat can be removed and replaced with a length of pipe, not simply gutted.
When changing a factory cat for a larger high flowing one, consider that Random Technology and Pace Setter make replacement cats with 3-inch ore even larger inlets and outlets."
PurpleCherokee
NAXJA Forum User
- Location
- MO
Ya, you're right, "the dyno is not a useful tool for proving anything"... except for knowing how much power your motor's making. Why do you think the factory uses a DYNO to come up w/power numbers instead of taking the car to the track to figure out how many ponies it's puttin out? So by your dumbass logic, if I take a tenth off of my ET because I took about a hundred pounds out of jeep then that means I've gained about 10 horsepower. And I'd like to see this 500hp CAR that you referred to that runs 15's. Either it's pullin a boat, or it's on bicycle tires... or you're just full of a bunch of :bs:wolfpackjeeper said:
Ok, so you obviously don't understand the difference between exhaust pipe diameter, and backpressure...BBeach said:
Ok smart guy, so what is it?BBeach said:
You just admitted that a cat does add backpressure.BBeach said:
Sorry buddy but I didn't "gut" my cat, I don't HAVE cats... period... so much for the "empty case" argument.BBeach said:
Wasn't it you that said "So back to the backpressure thing, go put a 5" exhaust and see what happens with the flow velocity"? So wouldn't my 2 1/4" pipes have a higher velocity than "3-inch ore even larger inlets and outlets"??? Your source just contridicted your argument...BBeach said:
I know the difference between backpressure, exhaust diameter, engine flow rate, etc.
I didnt read all of your other thread ( apparently I have reading problems ) but Id say a bearing.
Hey Chief, did you see anywhere where I said a cat did or didn't add backpressure? Didn't think so...and obviously it adds backpressure because its an obstruction. But like I said, did I ever say it did or didnt? Nope.
I know you said you had a straight pipe, but I just quoted the entire portion of an article on a site on the topic....
And as far as the exhaust velocity, obivously the 2.25" is going to have higher velocity. Again, I never said it doesnt tough guy. And on top of that, they are saying by putting on a larger diameter cat converter, it'll reduce the backpressure compared to a stock one, which is decently flowing in the first place.
Reread my earlier posts, I never said cats dont decrease backpressure, I never said that smaller diameter pipes are slower, etc.
Maybe you have the reading problem...anything else you have to say?
I didnt read all of your other thread ( apparently I have reading problems ) but Id say a bearing.
Hey Chief, did you see anywhere where I said a cat did or didn't add backpressure? Didn't think so...and obviously it adds backpressure because its an obstruction. But like I said, did I ever say it did or didnt? Nope.
I know you said you had a straight pipe, but I just quoted the entire portion of an article on a site on the topic....
And as far as the exhaust velocity, obivously the 2.25" is going to have higher velocity. Again, I never said it doesnt tough guy. And on top of that, they are saying by putting on a larger diameter cat converter, it'll reduce the backpressure compared to a stock one, which is decently flowing in the first place.
Reread my earlier posts, I never said cats dont decrease backpressure, I never said that smaller diameter pipes are slower, etc.
Maybe you have the reading problem...anything else you have to say?
PurpleCherokee
NAXJA Forum User
- Location
- MO
I guess I'm the dummy for assuming that if you quote something, you probably agree with what it says...:dunno: 
I was quoting sections of the article, and yes I'd agree with that even if it doesnt pertain to your straight pipe situation. Itd cause turbulence instead of keeping it less turbulent with a straight pipe. Glad i cleared that up for ya.PurpleCherokee said:I guess I'm the dummy for assuming that if you quote something, you probably agree with what it says...:dunno:
BBeach said:
i would certainly agree with this. gutting a cat is not a very good idea, since what remains will most likely not be designed to flow properly. it was designed with the internals in place to help guide the flow through. definitely go with either a hi-flow or a straight pipe.
as far as the track times, those are useful as well, although it can be even more difficult to keep all other variables constant (driver error, track and weather conditions, etc). A dyno machine will be much more consistent than a human driver, which is why I personally give dyno results a little more weight. dynos also give you more details about exactly where the gains were seen (the HP or TQ gained or lost, in which RPM range, in which gear, etc).
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Quarter miles on the dyno's are pretty cool. Best of both worlds. Im actually looking into getting a high flow cat in the next couple weeks. Seeing as it pertains to freer flowing I'll have to post any results I have as well including mpgs, various temps, and horsepower if I can configure this thing for it.copbait said:
seanyb505
NAXJA Forum User
- Location
- Simpsonville, SC
PurpleCherokee said:
Im advising you not to be an ignorant tool and claim that your test will be conclusive while not taking into account all the aspects of the test. You said it yourself, you arent able to spend a weekend with a dyno to "prove" something. So when you dyno yours, it wouldnt be smart to come back say it proved anything. Your results will be for your Jeep. I wish I would go dyno mine, as Im sure other people do. Im not flaming you for dynoing it.
wolfpackjeeper
NAXJA Forum User
- Location
- 500' and 500KCAS, RAF lakenheath
so what happened to the dyno sheets that would settle this once and for all?
PurpleCherokee
NAXJA Forum User
- Location
- MO
Be patient. And I was being facetious when I said that... hence the ":firedevil" and the "lol". But some people must have missed that. God forbid we use humor here on the forum without some people gettin their panties in a bunch sheesh...
sheesh...
wolfpackjeeper
NAXJA Forum User
- Location
- 500' and 500KCAS, RAF lakenheath
thought this was going down 3 days ago.
PurpleCherokee
NAXJA Forum User
- Location
- MO
I dunno where three days ago came from but it should happen saturday if all goes well. Track time also.
jeepinwi
NAXJA Forum User
- Location
- Eau Claire, WI
PurpleCherokee said:
Apparently you have a thick skull becuase you just can't understand that you have NOTHING to compare your dyno numbers to. What if your engine is making 5 hp less than a stock 4.0? What if it's making 10hp more than a 4.0? Will you just neglect to factor in that 15 hp swing that might be present? How much did your intake add? 2hp? 5hp?? 7hp?? There's another variable. You should realize that a free flowing exhaust may only add 10-15 hp to a 4.0L. Lets be hypothetical and assume your engine is making 10hp less than a stock 4.0L. The exhaust may give you 10 hp and the intake may give you 5 hp. Will you come back here bragging if you're making 5 more hp than a stock 4.0L? You'll get laughed off the site if that happens. Now if you had a baseline graph, you may be respected. Why do you not understand that your results can not be compared to a stock 4.0 graph since it is not an accurate baseline?:dunno:
stephenspann27
NAXJA Forum User
- Location
- College Station, Tx
PurpleCherokee said:
The last time I disproved the "back pressure is good" myth while searching online I found good data from a guy in Japan who set out to make a kind of inline exhuast valve the would create back pressure at low RPM and open up at high RPM. He found that no matter how he configured it, the backpressure would always decrease HP AND torque.
I could probably find it after some time on google..
PurpleCherokee
NAXJA Forum User
- Location
- MO
I know I'm probably gunna come off as arragant for sayin this but it's really just as simple as asking somebody who REALLY knows what they're talking about and they'll tell you the same thing. So really, there's no need to do ANY dyno testing to determine if a cat hurts performance because the answer is obvious. I don't think anyone has argued that cats don't create extra backpressure, but they have argued that that backpressure is a GOOD thing. But since we know that backpressure is a BAD thing, simple logic says that therefore, a cat does hurt performance. I think that the big thing is the debate over how much it hurts performance. But to me, it doesn't matter. If I'm gunna drop several hundred dollars on over-hauling, and completely redesigning my exhaust system, I'm not gunna cut ANY corners. ESPECIALLY when it's cheaper on top of everything else.
stephenspann27
NAXJA Forum User
- Location
- College Station, Tx
" One thing that I get tired of hearing is the concept that engines need back-pressure. Simply, there is no properly tuned engine where increasing exhaust back-pressure causes an improvement - in power, torque or fuel economy. One of the reasons that this idea has gained support is because when people change their exhaust they seldom check the air/fuel ratio or re-map the ignition timing to once again give optimal performance. For example, some MAP sensed cars drop substantially in power with a large exhaust fitted because they are then running lean.
Atmospherically inducted cars that use a tuned length system to improve cylinder scavenging (via extractors, for example) are sensitive to exhaust diameters within the tuned length part of the system. This means that the maximum effect of exhaust pulsing may come from an exhaust system that is small enough that some exhaust back-pressure is developed. However, that is a quite different concept to saying that engines "need" exhaust back-pressure! Turbocharged engines require as big an exhaust as possible, with the same applying for naturally aspirated cars once the tuned length part of the exhaust is passed.
Few tests have been done that clearly show the affect of changing back-pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back-pressure as a culprit difficult. However, Wollongong (Australia) mechanic Kevin Davis is one who has done very extensive testing of varying back-pressure on a number of performance engines. These range from turbocharged Subaru Liberty [Legacy] RS flat fours to full-house traditional pushrod V8's. In not one case has he found any improvement in any engine performance parameter by increasing exhaust back-pressure!
The tests came about because Kevin has developed a patented variable flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back-pressure at low loads "to help torque". However, he soon changed his mind when any increase in back-pressure proved to decrease torque (and therefore power at those revs) on a properly tuned engine! What increasing the back-pressure does do is dramatically quieten the exhaust.
One of the engine dyno tests carried out by Kevin was on warm 351 4V Cleveland V8. Following the extractors, he fitted a huge exhaust that gave a measured zero back-pressure. Torque peaked at 423 ft-lb at 4700 rpm, with power a rousing 441hp at 6300 rpm. He then dialled-in 1.5 psi back-pressure. Note that very few exhausts are capable of delivering such a low back-pressure on a road car. Even with this small amount of back-pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the butterfly position to give 2.5 psi back-pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back-pressure!
And if you still believe that exhaust back-pressure improves performance, simply block off part of your exhaust outlet and see if your car goes any faster!
Source: http://autospeed.drive.com.au/cms/article.html?&A=0046"
Atmospherically inducted cars that use a tuned length system to improve cylinder scavenging (via extractors, for example) are sensitive to exhaust diameters within the tuned length part of the system. This means that the maximum effect of exhaust pulsing may come from an exhaust system that is small enough that some exhaust back-pressure is developed. However, that is a quite different concept to saying that engines "need" exhaust back-pressure! Turbocharged engines require as big an exhaust as possible, with the same applying for naturally aspirated cars once the tuned length part of the exhaust is passed.
Few tests have been done that clearly show the affect of changing back-pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back-pressure as a culprit difficult. However, Wollongong (Australia) mechanic Kevin Davis is one who has done very extensive testing of varying back-pressure on a number of performance engines. These range from turbocharged Subaru Liberty [Legacy] RS flat fours to full-house traditional pushrod V8's. In not one case has he found any improvement in any engine performance parameter by increasing exhaust back-pressure!
The tests came about because Kevin has developed a patented variable flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back-pressure at low loads "to help torque". However, he soon changed his mind when any increase in back-pressure proved to decrease torque (and therefore power at those revs) on a properly tuned engine! What increasing the back-pressure does do is dramatically quieten the exhaust.
One of the engine dyno tests carried out by Kevin was on warm 351 4V Cleveland V8. Following the extractors, he fitted a huge exhaust that gave a measured zero back-pressure. Torque peaked at 423 ft-lb at 4700 rpm, with power a rousing 441hp at 6300 rpm. He then dialled-in 1.5 psi back-pressure. Note that very few exhausts are capable of delivering such a low back-pressure on a road car. Even with this small amount of back-pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the butterfly position to give 2.5 psi back-pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back-pressure!
And if you still believe that exhaust back-pressure improves performance, simply block off part of your exhaust outlet and see if your car goes any faster!
Source: http://autospeed.drive.com.au/cms/article.html?&A=0046"
stephenspann27
NAXJA Forum User
- Location
- College Station, Tx
I.[FONT="] [/FONT]Introduction
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.
II. Some basic exhaust theory
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.
III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.
The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.
Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.
IV. So how did this myth come to be?
I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.
V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).
One of the most misunderstood concepts in exhaust theory is backpressure. People love to talk about backpressure on message boards with no real understanding of what it is and what it's consequences are. I'm sure many of you have heard or read the phrase "Hondas need backpressure" when discussing exhaust upgrades. That phrase is in fact completely inaccurate and a wholly misguided notion.
II. Some basic exhaust theory
Your exhaust system is designed to evacuate gases from the combustion chamber quickly and efficently. Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. A 4 cylinder motor will have 4 distinct pulses per complete engine cycle, a 6 cylinder has 6 pules and so on. The more pulses that are produced, the more continuous the exhaust flow. Backpressure can be loosely defined as the resistance to positive flow - in this case, the resistance to positive flow of the exhaust stream.
III. Backpressure and velocity
Some people operate under the misguided notion that wider pipes are more effective at clearing the combustion chamber than narrower pipes. It's not hard to see how this misconception is appealing - wider pipes have the capability to flow more than narrower pipes. So if they have the ability to flow more, why isn't "wider is better" a good rule of thumb for exhaust upgrading? In a word - VELOCITY. I'm sure that all of you have at one time used a garden hose w/o a spray nozzle on it. If you let the water just run unrestricted out of the house it flows at a rather slow rate. However, if you take your finger and cover part of the opening, the water will flow out at a much much faster rate.
The astute exhaust designer knows that you must balance flow capacity with velocity. You want the exhaust gases to exit the chamber and speed along at the highest velocity possible - you want a FAST exhaust stream. If you have two exhaust pulses of equal volume, one in a 2" pipe and one in a 3" pipe, the pulse in the 2" pipe will be traveling considerably FASTER than the pulse in the 3" pipe. While it is true that the narrower the pipe, the higher the velocity of the exiting gases, you want make sure the pipe is wide enough so that there is as little backpressure as possible while maintaining suitable exhaust gas velocity. Backpressure in it's most extreme form can lead to reversion of the exhaust stream - that is to say the exhaust flows backwards, which is not good. The trick is to have a pipe that that is as narrow as possible while having as close to zero backpressure as possible at the RPM range you want your power band to be located at. Exhaust pipe diameters are best suited to a particular RPM range. A smaller pipe diameter will produce higher exhaust velocities at a lower RPM but create unacceptably high amounts of backpressure at high rpm. Thus if your powerband is located 2-3000 RPM you'd want a narrower pipe than if your powerband is located at 8-9000RPM.
Many engineers try to work around the RPM specific nature of pipe diameters by using setups that are capable of creating a similar effect as a change in pipe diameter on the fly. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb backpressure - since there is greater exhaust volume there is no loss in flow velocity. BMW and Nissan use a simpler and less effective method - there is a single exhaust path to the muffler; the muffler has two paths; one path is closed at low RPM but both are open at high RPM.
IV. So how did this myth come to be?
I often wonder how the myth "Hondas need backpressure" came to be. Mostly I believe it is a misunderstanding of what is going on with the exhaust stream as pipe diameters change. For instance, someone with a civic decides he's going to uprade his exhaust with a 3" diameter piping. Once it's installed the owner notices that he seems to have lost a good bit of power throughout the powerband. He makes the connections in the following manner: "My wider exhaust eliminated all backpressure but I lost power, therefore the motor must need some backpressure in order to make power." What he did not realize is that he killed off all his flow velocity by using such a ridiculously wide pipe. It would have been possible for him to achieve close to zero backpressure with a much narrower pipe - in that way he would not have lost all his flow velocity.
V. So why is exhaust velocity so important?
The faster an exhaust pulse moves, the better it can scavenge out all of the spent gasses during valve overlap. The guiding principles of exhaust pulse scavenging are a bit beyond the scope of this doc but the general idea is a fast moving pulse creates a low pressure area behind it. This low pressure area acts as a vacuum and draws along the air behind it. A similar example would be a vehicle traveling at a high rate of speed on a dusty road. There is a low pressure area immediately behind the moving vehicle - dust particles get sucked into this low pressure area causing it to collect on the back of the vehicle. This effect is most noticeable on vans and hatchbacks which tend to create large trailing low pressure areas - giving rise to the numerous "wash me please" messages written in the thickly collected dust on the rear door(s).
