[C2] Sidepipes vs. under the car exhaust power comparison
10-22-2016, 02:24 PM
#21
Team Owner
Member Since: Mar 2003
Location: Greenville, Indiana
Posts: 26,118
Received 1,843 Likes
on
1,398 Posts
Depends on which aftermarket sidepipe mufflers you run or test. Light years difference - much improved options these days. Here's a photo of the inside of an OE chambered sidepipe muffler: 1-5/8" ID - measured it myself. I wouldn't run something that small on a 289 Mustang. No wonder the power losses were so excessive.
I agree but pipe size alone is not the bottom line on restriction whether talking about sidepipes or mufflers. Too big can be a negative as well. Somebody must have flow numbers on these different components?
That picture you show. Is that an OEM 2" pipe or 2 1'2" pipe?
I have no horse in this race. I did put factory 2 1/2" sidepipes on my 327/250 years ago. I removed an under car 2" factory system. SOP, my car felt a little livelier but that was just SOP. I certainly didn't feel any power loss.
10-22-2016, 03:17 PM
#22
Drifting
And how about low end torque? Slightly smaller tubes have sometimes been used in applications where bottom end grunt was the objective. Remember that torque is what you want for acceleration; peak horsepower is more about top speed.
10-22-2016, 04:29 PM
#23
Le Mans Master
As far as the factory muffler internal photo, I have no idea what size the factory head pipes were as the guy had installed long tube headers, so the head pipes were new. The OD of the muffler bodies are 2-3/4", so you can see how far the crimps go in to the core - the cores are very small diameter. He bought the car when it was 2 years old & never changed the sidepipes - it's a California car so the mufflers were not destroyed by rust. The car is a '66 327 as far as I can remember. Also, going too big on the pipes is not good, as it causes the exhaust to lose velocity. For anything under 500 HP, I think 2.5" internals are perfect - 2.25" would be fine on anything under 350 HP. With either of these size cores, you should not lose bottom-end or midrange, & you'll have plenty of flow in the high RPM range. Also, the design of the inner cores relates to flow. If the core is adequate diameter, but has huge louvers in the flow tube, that will definitely affect the flow as well.
10-22-2016, 04:54 PM
#24
Team Owner
Member Since: Mar 2003
Location: Greenville, Indiana
Posts: 26,118
Received 1,843 Likes
on
1,398 Posts
As far as the factory muffler internal photo, I have no idea what size the factory head pipes were as the guy had installed long tube headers, so the head pipes were new. The OD of the muffler bodies are 2-3/4", so you can see how far the crimps go in to the core - the cores are very small diameter. He bought the car when it was 2 years old & never changed the sidepipes - it's a California car so the mufflers were not destroyed by rust. The car is a '66 327 as far as I can remember. Also, going too big on the pipes is not good, as it causes the exhaust to lose velocity. For anything under 500 HP, I think 2.5" internals are perfect - 2.25" would be fine on anything under 350 HP. With either of these size cores, you should not lose bottom-end or midrange, & you'll have plenty of flow in the high RPM range. Also, the design of the inner cores relates to flow. If the core is adequate diameter, but has huge louvers in the flow tube, that will definitely affect the flow as well.
From what little I think I know on the subject, I'd agree with your post. If I add much of anything, I'd be above my pay grade.
Last edited by MikeM; 10-22-2016 at 04:56 PM.
10-22-2016, 07:59 PM
#26
Safety Car
In the following paper, most of your questions should be answered in the first seven paragraphs. The paper should also help you understand why over the counter headers are almost NEVER a one-size-fits-all item. Properly designed and installed headers enhance the power/torque production of all but the mildest engines; those with low overlap camshafts should at least benefit from low restriction even if they don't get the benefit of negative pressure due to the reflected wave during the valve overlap period.
The balance of the article explains why even the best EQUAL LENGTH PRIMARY TUBE headers (which are almost never available OTC), are still not perfect when dealing with American made V8 engines. The exception to this being the latest Boss 302 Mustang and many Italian made engines like Ferrari V8s.........all of which use a "flat" (180 degree) crankshaft.
VVVVVVVVVVVVVVVVVVVVVVVVVVVVV
Headers 101 ( Header Basics - A Guide To Understanding )
You have probably heard words like: back pressure, scavenging, tuned length, merged collector, rotational firing order, compatible combination and many others that meant something, but how they relate to a header may be a little vague. This article should give you a basic understanding of how a header works, what the terminology means, and how it plays a part in the header's performance gains.
The first misconception that needs to be cleared up is that a header relieves backpressure, but a certain amount of backpressure is needed for optimum performance. Just the opposite is true. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "Scavenging" means.
The first consideration is the proper tube diameter. Many people think "Bigger is Better", but this is not the case. The smallest diameter that will flow enough air to handle the engine's c.c. at your desired Red Line R.P.M. should be used. This small diameter will generate the velocity (air speed) needed to "Scavenge" at low R.P.M.s. If too small a diameter is used the engine will pull hard at low R.P.M.s but at some point in the higher R.P.M.s the tube will not be able to flow as much air as the engine is pumping out, and the engine will "sign off" early, not reaching its potential peak R.P.M. This situation would require going one size larger in tube diameter.
The second consideration is the proper tube length. The length directly controls the power band in the R.P.M. range. Longer tube lengths pull the torque down to a lower R.P.M. range. Shorter tubes move the power band up into a higher R.P.M. range. Engines that Red Line at 10,000 R.P.M. would need short tube lengths about 26" long. Engines that are torquers and Red Line at 5,500 R.P.M.s would need a tube length of 36". This is what is meant by the term "Tuned Length". The tube length is tuned to make the engine operate at a desired R.P.M. range.
The third consideration is the collector outlet diameter and extension length. This is where major differences occur between four cylinder engines and V-8 engines. The optimum situation is the four cylinder because of it's firing cycle. Every 180 degree of crankshaft rotation there is one exhaust pulse entering the collector. This is ideal timing because, as one pulse exits the collector, the next exhaust valve is opening and the vacuum created in the system pulls the exhaust from the cylinder. In this ideal 180 degree cycling the collector outlet diameter only needs to be 20% larger than the primary tube diameter. (Example: 1 3/4" primary tubes need a 2" collector outlet diameter.) The rule of thumb here is two tube sizes. This keeps the velocity fast to increase scavenging, especially at lower R.P.M.s. Going to a larger outlet diameter will hurt the midrange and low R.P.M. torque.
The amount of straight in the collector extension can move the engines torque up or down in the R.P.M. range. Longer extension length will pull the torque down into the midrange.
Engines that "Red Line" at 10,000 R.P.M. would only need 2" of straight between the collector and the megaphone. This is just enough length to straighten out the air flow before it enters the megaphone. This creates an orifice action that enhances exhaust velocity.
In the case of V-8 firing order, the five pulses fire alternately back and forth from left to right collector, giving the ideal 180 degree firing cycle. Then it fires two in succession into the left collector, then two in succession into the right collector. If the proper collector outlet diameter is being used (two sizes larger than primaries) the two pulses in succession load up the collector with more air than it can flow. This results in a very strong midrange torque, but causes the engine to "sign off" early, not reaching its potential peek R.P.M. The improper firing order on a V-8 engine results in the need to use large diameter collectors so the engine will perform well at high R.P.M.s. Unfortunately the large diameter collectors cause a tremendous drop in air velocity, resulting in less scavenging through the entire R.P.M. range.
Often cams are used with extended valve timing to help the exhaust cycling. This results in valve timing overlap (Intake and Exhaust valves both open at T.D.C.) which causes a "Reversion"cycle in the exhaust. When this happens, exhaust actually backs up into the cylinder causing intake air to be pushed back out the intake. This reversion causes "Standoff" (fuel blowing out of the Intake) at low R.P.M.s. This whole improper cycling has resulted in a number of "Cure Alls" to help stop this reversion and standoff.
The plentum intake was created to stop the fuel "Standoff". Then came "Anti Reversionary" Cones in the exhaust tubes, and stepped tube diameter in the header, extended collector lengths and even plentums in the exhaust tubes. In this chain of events beginning with improper firing order, a series of cures has developed, each one causing a new problem.
The optimum cure to this whole problem is to correct the exhaust firing cycle. The two cylinders that fire in succession into each collector have to be separated. This can be done partially by a "Tri-Y" header, where the four primary tubes from each bank merge into two secondary tubes (separating the two pulses firing in succession) and finally collect into a single collector. This type of header helps, but the two pulses are still coming back together at the collector.
The second optimum cure is to cross the two center tubes from each bank, across the engine running them into the collector on the opposite side. This makes the firing cycle in each collector 180 degrees apart, the same as a four cylinder engine. Once this firing order is achieved, the small collector outlet diameter can be used and the "High Velocity Scavenging" at low R.P.M.s cures the reversion problems and eliminates the need for extreme cam duration.
This sounds so easy, you are probably asking why wasn't this done from the start?
If you have ever seen a set of 180 degree headers you would understand.
On today's cars, with space virtually nonexistent, crossing four tubes either under the oil pan or around the front or rear of the engine presents major problems. On racing applications where it is possible, there is still the problem of keeping the tube length down to a reasonable 32" long. If that's not enough challenge, then try to arrange the tubes into each collector so they fire in a "Rotational Firing" pattern. Then you have, what has been called "A Bundle of Snakes".
Arranging the tubes to fire rotationally adds to the scavenging capabilities. The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more suction on that tube than it would on a tube on the opposite side of the collector.
The next problem is "Turbulence" in the collector. When four round tubes are grouped together in a square pattern, so a collector can be attached, you notice a gapping hole in the center of the four tubes. The standard method in manufacturing headers is to cap this hole off with a square plate. This plate in the center of the four tubes creates dead air space, or turbulence, disrupting the high velocity in the collector. This problem is solved by using a "Merge Collector". This collector is formed from four tubes, cut at approximately an 8 degree angle on two sides. When the tubes are all fitted together they form a collector with a "Pyramid" in the center. This has eliminated the need for the square plate and has taken up some of the volume inside the collector, speeding up the air velocity.
Other methods of curing this problem are: fabricating a pyramid out of sheet metal and welding it over the hole between the tubes, or squaring the tubes on two sides so they fit together forming a "+" weld in the center eliminating the hole all together.
You can see that there are a great many factors that go into making a good header. When the header, intake system, and cam timing are all designed to operate to their maximum in the same R.P.M. range, then you have a "Compatible Combination". This combination can be tuned to deliver maximum power at any desired R.P.M. range.
These are some of the "Basics" you need to know about building a good high performance header. There are many other adjustments that can be made to fine tune a header, but this should give you a basic understanding of how all the components work together.
The balance of the article explains why even the best EQUAL LENGTH PRIMARY TUBE headers (which are almost never available OTC), are still not perfect when dealing with American made V8 engines. The exception to this being the latest Boss 302 Mustang and many Italian made engines like Ferrari V8s.........all of which use a "flat" (180 degree) crankshaft.
VVVVVVVVVVVVVVVVVVVVVVVVVVVVV
Headers 101 ( Header Basics - A Guide To Understanding )
You have probably heard words like: back pressure, scavenging, tuned length, merged collector, rotational firing order, compatible combination and many others that meant something, but how they relate to a header may be a little vague. This article should give you a basic understanding of how a header works, what the terminology means, and how it plays a part in the header's performance gains.
The first misconception that needs to be cleared up is that a header relieves backpressure, but a certain amount of backpressure is needed for optimum performance. Just the opposite is true. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "Scavenging" means.
The first consideration is the proper tube diameter. Many people think "Bigger is Better", but this is not the case. The smallest diameter that will flow enough air to handle the engine's c.c. at your desired Red Line R.P.M. should be used. This small diameter will generate the velocity (air speed) needed to "Scavenge" at low R.P.M.s. If too small a diameter is used the engine will pull hard at low R.P.M.s but at some point in the higher R.P.M.s the tube will not be able to flow as much air as the engine is pumping out, and the engine will "sign off" early, not reaching its potential peak R.P.M. This situation would require going one size larger in tube diameter.
The second consideration is the proper tube length. The length directly controls the power band in the R.P.M. range. Longer tube lengths pull the torque down to a lower R.P.M. range. Shorter tubes move the power band up into a higher R.P.M. range. Engines that Red Line at 10,000 R.P.M. would need short tube lengths about 26" long. Engines that are torquers and Red Line at 5,500 R.P.M.s would need a tube length of 36". This is what is meant by the term "Tuned Length". The tube length is tuned to make the engine operate at a desired R.P.M. range.
The third consideration is the collector outlet diameter and extension length. This is where major differences occur between four cylinder engines and V-8 engines. The optimum situation is the four cylinder because of it's firing cycle. Every 180 degree of crankshaft rotation there is one exhaust pulse entering the collector. This is ideal timing because, as one pulse exits the collector, the next exhaust valve is opening and the vacuum created in the system pulls the exhaust from the cylinder. In this ideal 180 degree cycling the collector outlet diameter only needs to be 20% larger than the primary tube diameter. (Example: 1 3/4" primary tubes need a 2" collector outlet diameter.) The rule of thumb here is two tube sizes. This keeps the velocity fast to increase scavenging, especially at lower R.P.M.s. Going to a larger outlet diameter will hurt the midrange and low R.P.M. torque.
The amount of straight in the collector extension can move the engines torque up or down in the R.P.M. range. Longer extension length will pull the torque down into the midrange.
Engines that "Red Line" at 10,000 R.P.M. would only need 2" of straight between the collector and the megaphone. This is just enough length to straighten out the air flow before it enters the megaphone. This creates an orifice action that enhances exhaust velocity.
In the case of V-8 firing order, the five pulses fire alternately back and forth from left to right collector, giving the ideal 180 degree firing cycle. Then it fires two in succession into the left collector, then two in succession into the right collector. If the proper collector outlet diameter is being used (two sizes larger than primaries) the two pulses in succession load up the collector with more air than it can flow. This results in a very strong midrange torque, but causes the engine to "sign off" early, not reaching its potential peek R.P.M. The improper firing order on a V-8 engine results in the need to use large diameter collectors so the engine will perform well at high R.P.M.s. Unfortunately the large diameter collectors cause a tremendous drop in air velocity, resulting in less scavenging through the entire R.P.M. range.
Often cams are used with extended valve timing to help the exhaust cycling. This results in valve timing overlap (Intake and Exhaust valves both open at T.D.C.) which causes a "Reversion"cycle in the exhaust. When this happens, exhaust actually backs up into the cylinder causing intake air to be pushed back out the intake. This reversion causes "Standoff" (fuel blowing out of the Intake) at low R.P.M.s. This whole improper cycling has resulted in a number of "Cure Alls" to help stop this reversion and standoff.
The plentum intake was created to stop the fuel "Standoff". Then came "Anti Reversionary" Cones in the exhaust tubes, and stepped tube diameter in the header, extended collector lengths and even plentums in the exhaust tubes. In this chain of events beginning with improper firing order, a series of cures has developed, each one causing a new problem.
The optimum cure to this whole problem is to correct the exhaust firing cycle. The two cylinders that fire in succession into each collector have to be separated. This can be done partially by a "Tri-Y" header, where the four primary tubes from each bank merge into two secondary tubes (separating the two pulses firing in succession) and finally collect into a single collector. This type of header helps, but the two pulses are still coming back together at the collector.
The second optimum cure is to cross the two center tubes from each bank, across the engine running them into the collector on the opposite side. This makes the firing cycle in each collector 180 degrees apart, the same as a four cylinder engine. Once this firing order is achieved, the small collector outlet diameter can be used and the "High Velocity Scavenging" at low R.P.M.s cures the reversion problems and eliminates the need for extreme cam duration.
This sounds so easy, you are probably asking why wasn't this done from the start?
If you have ever seen a set of 180 degree headers you would understand.
On today's cars, with space virtually nonexistent, crossing four tubes either under the oil pan or around the front or rear of the engine presents major problems. On racing applications where it is possible, there is still the problem of keeping the tube length down to a reasonable 32" long. If that's not enough challenge, then try to arrange the tubes into each collector so they fire in a "Rotational Firing" pattern. Then you have, what has been called "A Bundle of Snakes".
Arranging the tubes to fire rotationally adds to the scavenging capabilities. The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more suction on that tube than it would on a tube on the opposite side of the collector.
The next problem is "Turbulence" in the collector. When four round tubes are grouped together in a square pattern, so a collector can be attached, you notice a gapping hole in the center of the four tubes. The standard method in manufacturing headers is to cap this hole off with a square plate. This plate in the center of the four tubes creates dead air space, or turbulence, disrupting the high velocity in the collector. This problem is solved by using a "Merge Collector". This collector is formed from four tubes, cut at approximately an 8 degree angle on two sides. When the tubes are all fitted together they form a collector with a "Pyramid" in the center. This has eliminated the need for the square plate and has taken up some of the volume inside the collector, speeding up the air velocity.
Other methods of curing this problem are: fabricating a pyramid out of sheet metal and welding it over the hole between the tubes, or squaring the tubes on two sides so they fit together forming a "+" weld in the center eliminating the hole all together.
You can see that there are a great many factors that go into making a good header. When the header, intake system, and cam timing are all designed to operate to their maximum in the same R.P.M. range, then you have a "Compatible Combination". This combination can be tuned to deliver maximum power at any desired R.P.M. range.
These are some of the "Basics" you need to know about building a good high performance header. There are many other adjustments that can be made to fine tune a header, but this should give you a basic understanding of how all the components work together.
Last edited by 65tripleblack; 10-22-2016 at 08:15 PM.
The following users liked this post:
63Corvette (10-22-2016)
10-22-2016, 08:38 PM
#27
Le Mans Master
In the following paper, most of your questions should be answered in the first seven paragraphs. The paper should also help you understand why over the counter headers are almost NEVER a one-size-fits-all item. Properly designed and installed headers enhance the power/torque production of all but the mildest engines; those with low overlap camshafts should at least benefit from low restriction even if they don't get the benefit of negative pressure due to the reflected wave during the valve overlap period.
The balance of the article explains why even the best EQUAL LENGTH PRIMARY TUBE headers (which are almost never available OTC), are still not perfect when dealing with American made V8 engines. The exception to this being the latest Boss 302 Mustang and many Italian made engines like Ferrari V8s.........all of which use a "flat" (180 degree) crankshaft.
VVVVVVVVVVVVVVVVVVVVVVVVVVVVV
Headers 101 ( Header Basics - A Guide To Understanding )
You have probably heard words like: back pressure, scavenging, tuned length, merged collector, rotational firing order, compatible combination and many others that meant something, but how they relate to a header may be a little vague. This article should give you a basic understanding of how a header works, what the terminology means, and how it plays a part in the header's performance gains.
The first misconception that needs to be cleared up is that a header relieves backpressure, but a certain amount of backpressure is needed for optimum performance. Just the opposite is true. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "Scavenging" means.
The first consideration is the proper tube diameter. Many people think "Bigger is Better", but this is not the case. The smallest diameter that will flow enough air to handle the engine's c.c. at your desired Red Line R.P.M. should be used. This small diameter will generate the velocity (air speed) needed to "Scavenge" at low R.P.M.s. If too small a diameter is used the engine will pull hard at low R.P.M.s but at some point in the higher R.P.M.s the tube will not be able to flow as much air as the engine is pumping out, and the engine will "sign off" early, not reaching its potential peak R.P.M. This situation would require going one size larger in tube diameter.
The second consideration is the proper tube length. The length directly controls the power band in the R.P.M. range. Longer tube lengths pull the torque down to a lower R.P.M. range. Shorter tubes move the power band up into a higher R.P.M. range. Engines that Red Line at 10,000 R.P.M. would need short tube lengths about 26" long. Engines that are torquers and Red Line at 5,500 R.P.M.s would need a tube length of 36". This is what is meant by the term "Tuned Length". The tube length is tuned to make the engine operate at a desired R.P.M. range.
The third consideration is the collector outlet diameter and extension length. This is where major differences occur between four cylinder engines and V-8 engines. The optimum situation is the four cylinder because of it's firing cycle. Every 180 degree of crankshaft rotation there is one exhaust pulse entering the collector. This is ideal timing because, as one pulse exits the collector, the next exhaust valve is opening and the vacuum created in the system pulls the exhaust from the cylinder. In this ideal 180 degree cycling the collector outlet diameter only needs to be 20% larger than the primary tube diameter. (Example: 1 3/4" primary tubes need a 2" collector outlet diameter.) The rule of thumb here is two tube sizes. This keeps the velocity fast to increase scavenging, especially at lower R.P.M.s. Going to a larger outlet diameter will hurt the midrange and low R.P.M. torque.
The amount of straight in the collector extension can move the engines torque up or down in the R.P.M. range. Longer extension length will pull the torque down into the midrange.
Engines that "Red Line" at 10,000 R.P.M. would only need 2" of straight between the collector and the megaphone. This is just enough length to straighten out the air flow before it enters the megaphone. This creates an orifice action that enhances exhaust velocity.
In the case of V-8 firing order, the five pulses fire alternately back and forth from left to right collector, giving the ideal 180 degree firing cycle. Then it fires two in succession into the left collector, then two in succession into the right collector. If the proper collector outlet diameter is being used (two sizes larger than primaries) the two pulses in succession load up the collector with more air than it can flow. This results in a very strong midrange torque, but causes the engine to "sign off" early, not reaching its potential peek R.P.M. The improper firing order on a V-8 engine results in the need to use large diameter collectors so the engine will perform well at high R.P.M.s. Unfortunately the large diameter collectors cause a tremendous drop in air velocity, resulting in less scavenging through the entire R.P.M. range.
Often cams are used with extended valve timing to help the exhaust cycling. This results in valve timing overlap (Intake and Exhaust valves both open at T.D.C.) which causes a "Reversion"cycle in the exhaust. When this happens, exhaust actually backs up into the cylinder causing intake air to be pushed back out the intake. This reversion causes "Standoff" (fuel blowing out of the Intake) at low R.P.M.s. This whole improper cycling has resulted in a number of "Cure Alls" to help stop this reversion and standoff.
The plentum intake was created to stop the fuel "Standoff". Then came "Anti Reversionary" Cones in the exhaust tubes, and stepped tube diameter in the header, extended collector lengths and even plentums in the exhaust tubes. In this chain of events beginning with improper firing order, a series of cures has developed, each one causing a new problem.
The optimum cure to this whole problem is to correct the exhaust firing cycle. The two cylinders that fire in succession into each collector have to be separated. This can be done partially by a "Tri-Y" header, where the four primary tubes from each bank merge into two secondary tubes (separating the two pulses firing in succession) and finally collect into a single collector. This type of header helps, but the two pulses are still coming back together at the collector.
The second optimum cure is to cross the two center tubes from each bank, across the engine running them into the collector on the opposite side. This makes the firing cycle in each collector 180 degrees apart, the same as a four cylinder engine. Once this firing order is achieved, the small collector outlet diameter can be used and the "High Velocity Scavenging" at low R.P.M.s cures the reversion problems and eliminates the need for extreme cam duration.
This sounds so easy, you are probably asking why wasn't this done from the start?
If you have ever seen a set of 180 degree headers you would understand.
On today's cars, with space virtually nonexistent, crossing four tubes either under the oil pan or around the front or rear of the engine presents major problems. On racing applications where it is possible, there is still the problem of keeping the tube length down to a reasonable 32" long. If that's not enough challenge, then try to arrange the tubes into each collector so they fire in a "Rotational Firing" pattern. Then you have, what has been called "A Bundle of Snakes".
Arranging the tubes to fire rotationally adds to the scavenging capabilities. The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more suction on that tube than it would on a tube on the opposite side of the collector.
The next problem is "Turbulence" in the collector. When four round tubes are grouped together in a square pattern, so a collector can be attached, you notice a gapping hole in the center of the four tubes. The standard method in manufacturing headers is to cap this hole off with a square plate. This plate in the center of the four tubes creates dead air space, or turbulence, disrupting the high velocity in the collector. This problem is solved by using a "Merge Collector". This collector is formed from four tubes, cut at approximately an 8 degree angle on two sides. When the tubes are all fitted together they form a collector with a "Pyramid" in the center. This has eliminated the need for the square plate and has taken up some of the volume inside the collector, speeding up the air velocity.
Other methods of curing this problem are: fabricating a pyramid out of sheet metal and welding it over the hole between the tubes, or squaring the tubes on two sides so they fit together forming a "+" weld in the center eliminating the hole all together.
You can see that there are a great many factors that go into making a good header. When the header, intake system, and cam timing are all designed to operate to their maximum in the same R.P.M. range, then you have a "Compatible Combination". This combination can be tuned to deliver maximum power at any desired R.P.M. range.
These are some of the "Basics" you need to know about building a good high performance header. There are many other adjustments that can be made to fine tune a header, but this should give you a basic understanding of how all the components work together.
The balance of the article explains why even the best EQUAL LENGTH PRIMARY TUBE headers (which are almost never available OTC), are still not perfect when dealing with American made V8 engines. The exception to this being the latest Boss 302 Mustang and many Italian made engines like Ferrari V8s.........all of which use a "flat" (180 degree) crankshaft.
VVVVVVVVVVVVVVVVVVVVVVVVVVVVV
Headers 101 ( Header Basics - A Guide To Understanding )
You have probably heard words like: back pressure, scavenging, tuned length, merged collector, rotational firing order, compatible combination and many others that meant something, but how they relate to a header may be a little vague. This article should give you a basic understanding of how a header works, what the terminology means, and how it plays a part in the header's performance gains.
The first misconception that needs to be cleared up is that a header relieves backpressure, but a certain amount of backpressure is needed for optimum performance. Just the opposite is true. A good header not only relieves the backpressure, but goes one step further and creates a vacuum in the system. When the next cylinder's exhaust valve opens, the vacuum in the system pulls the exhaust out of the cylinder. This is what the term "Scavenging" means.
The first consideration is the proper tube diameter. Many people think "Bigger is Better", but this is not the case. The smallest diameter that will flow enough air to handle the engine's c.c. at your desired Red Line R.P.M. should be used. This small diameter will generate the velocity (air speed) needed to "Scavenge" at low R.P.M.s. If too small a diameter is used the engine will pull hard at low R.P.M.s but at some point in the higher R.P.M.s the tube will not be able to flow as much air as the engine is pumping out, and the engine will "sign off" early, not reaching its potential peak R.P.M. This situation would require going one size larger in tube diameter.
The second consideration is the proper tube length. The length directly controls the power band in the R.P.M. range. Longer tube lengths pull the torque down to a lower R.P.M. range. Shorter tubes move the power band up into a higher R.P.M. range. Engines that Red Line at 10,000 R.P.M. would need short tube lengths about 26" long. Engines that are torquers and Red Line at 5,500 R.P.M.s would need a tube length of 36". This is what is meant by the term "Tuned Length". The tube length is tuned to make the engine operate at a desired R.P.M. range.
The third consideration is the collector outlet diameter and extension length. This is where major differences occur between four cylinder engines and V-8 engines. The optimum situation is the four cylinder because of it's firing cycle. Every 180 degree of crankshaft rotation there is one exhaust pulse entering the collector. This is ideal timing because, as one pulse exits the collector, the next exhaust valve is opening and the vacuum created in the system pulls the exhaust from the cylinder. In this ideal 180 degree cycling the collector outlet diameter only needs to be 20% larger than the primary tube diameter. (Example: 1 3/4" primary tubes need a 2" collector outlet diameter.) The rule of thumb here is two tube sizes. This keeps the velocity fast to increase scavenging, especially at lower R.P.M.s. Going to a larger outlet diameter will hurt the midrange and low R.P.M. torque.
The amount of straight in the collector extension can move the engines torque up or down in the R.P.M. range. Longer extension length will pull the torque down into the midrange.
Engines that "Red Line" at 10,000 R.P.M. would only need 2" of straight between the collector and the megaphone. This is just enough length to straighten out the air flow before it enters the megaphone. This creates an orifice action that enhances exhaust velocity.
In the case of V-8 firing order, the five pulses fire alternately back and forth from left to right collector, giving the ideal 180 degree firing cycle. Then it fires two in succession into the left collector, then two in succession into the right collector. If the proper collector outlet diameter is being used (two sizes larger than primaries) the two pulses in succession load up the collector with more air than it can flow. This results in a very strong midrange torque, but causes the engine to "sign off" early, not reaching its potential peek R.P.M. The improper firing order on a V-8 engine results in the need to use large diameter collectors so the engine will perform well at high R.P.M.s. Unfortunately the large diameter collectors cause a tremendous drop in air velocity, resulting in less scavenging through the entire R.P.M. range.
Often cams are used with extended valve timing to help the exhaust cycling. This results in valve timing overlap (Intake and Exhaust valves both open at T.D.C.) which causes a "Reversion"cycle in the exhaust. When this happens, exhaust actually backs up into the cylinder causing intake air to be pushed back out the intake. This reversion causes "Standoff" (fuel blowing out of the Intake) at low R.P.M.s. This whole improper cycling has resulted in a number of "Cure Alls" to help stop this reversion and standoff.
The plentum intake was created to stop the fuel "Standoff". Then came "Anti Reversionary" Cones in the exhaust tubes, and stepped tube diameter in the header, extended collector lengths and even plentums in the exhaust tubes. In this chain of events beginning with improper firing order, a series of cures has developed, each one causing a new problem.
The optimum cure to this whole problem is to correct the exhaust firing cycle. The two cylinders that fire in succession into each collector have to be separated. This can be done partially by a "Tri-Y" header, where the four primary tubes from each bank merge into two secondary tubes (separating the two pulses firing in succession) and finally collect into a single collector. This type of header helps, but the two pulses are still coming back together at the collector.
The second optimum cure is to cross the two center tubes from each bank, across the engine running them into the collector on the opposite side. This makes the firing cycle in each collector 180 degrees apart, the same as a four cylinder engine. Once this firing order is achieved, the small collector outlet diameter can be used and the "High Velocity Scavenging" at low R.P.M.s cures the reversion problems and eliminates the need for extreme cam duration.
This sounds so easy, you are probably asking why wasn't this done from the start?
If you have ever seen a set of 180 degree headers you would understand.
On today's cars, with space virtually nonexistent, crossing four tubes either under the oil pan or around the front or rear of the engine presents major problems. On racing applications where it is possible, there is still the problem of keeping the tube length down to a reasonable 32" long. If that's not enough challenge, then try to arrange the tubes into each collector so they fire in a "Rotational Firing" pattern. Then you have, what has been called "A Bundle of Snakes".
Arranging the tubes to fire rotationally adds to the scavenging capabilities. The exhaust gas exiting one tube, passing across the opening of the tube directly beside it, creates more suction on that tube than it would on a tube on the opposite side of the collector.
The next problem is "Turbulence" in the collector. When four round tubes are grouped together in a square pattern, so a collector can be attached, you notice a gapping hole in the center of the four tubes. The standard method in manufacturing headers is to cap this hole off with a square plate. This plate in the center of the four tubes creates dead air space, or turbulence, disrupting the high velocity in the collector. This problem is solved by using a "Merge Collector". This collector is formed from four tubes, cut at approximately an 8 degree angle on two sides. When the tubes are all fitted together they form a collector with a "Pyramid" in the center. This has eliminated the need for the square plate and has taken up some of the volume inside the collector, speeding up the air velocity.
Other methods of curing this problem are: fabricating a pyramid out of sheet metal and welding it over the hole between the tubes, or squaring the tubes on two sides so they fit together forming a "+" weld in the center eliminating the hole all together.
You can see that there are a great many factors that go into making a good header. When the header, intake system, and cam timing are all designed to operate to their maximum in the same R.P.M. range, then you have a "Compatible Combination". This combination can be tuned to deliver maximum power at any desired R.P.M. range.
These are some of the "Basics" you need to know about building a good high performance header. There are many other adjustments that can be made to fine tune a header, but this should give you a basic understanding of how all the components work together.
10-22-2016, 10:00 PM
#28
Race Director
Thread Starter
Member Since: May 2000
Location: Redondo Beach USA
Posts: 12,487
Received 1,974 Likes
on
1,188 Posts
...hoping all your threads get highjacked, too...
Duke
Duke
10-22-2016, 10:45 PM
#29
Team Owner
Magazines from back in the day figured out a power loss, except in the lower hp engines by going with the factory side pipes! They kind of were known as an appearance option by those that read magazines!
As far as the Hookers, the quality isn't what it used to be! The flange area is not so greatly done and not real clean in the modern! Hooker at one time had street side tube exhaust system and a race side exhaust system! They kept the street config in their line up! Now you get a choice of two differing side pipe muffler inserts for the tubes! Therefore the Stahls and Doug's more modern units flowed better! To run any of the 4 inch tubes you really should have ported heads, to run them for a benefit! One of the best were the old Hedman 1-7-8/3-1/2 collector units even angle plug clearanced under the car units, but those were discontinued also. I would rather if I had to source new Hookers would be to get the painted side pipes and redo the entire pipes into the head flange! Just bring the 1-7-8 pipes into a matched flange using fresh 1-7/8 sections! I would even essentially thicken the flange to effectively lengthen the exhaust port! Even the machine welding in the modern seems to look like it causes flow issues or restriction on the modern ones that I have seen sold!
As far as the Hookers, the quality isn't what it used to be! The flange area is not so greatly done and not real clean in the modern! Hooker at one time had street side tube exhaust system and a race side exhaust system! They kept the street config in their line up! Now you get a choice of two differing side pipe muffler inserts for the tubes! Therefore the Stahls and Doug's more modern units flowed better! To run any of the 4 inch tubes you really should have ported heads, to run them for a benefit! One of the best were the old Hedman 1-7-8/3-1/2 collector units even angle plug clearanced under the car units, but those were discontinued also. I would rather if I had to source new Hookers would be to get the painted side pipes and redo the entire pipes into the head flange! Just bring the 1-7-8 pipes into a matched flange using fresh 1-7/8 sections! I would even essentially thicken the flange to effectively lengthen the exhaust port! Even the machine welding in the modern seems to look like it causes flow issues or restriction on the modern ones that I have seen sold!
Last edited by TCracingCA; 10-22-2016 at 10:54 PM.
10-23-2016, 10:43 AM
#30
Safety Car
Headers become part of the discussion since you brought them up in post #3, and also stated that they are of no use except for racing. Also in response to others who stated that they are a nuisance and not worth having.
BTW: "Hijacked" is not spelled "highjacked" look it up.
Joe
BTW: "Hijacked" is not spelled "highjacked" look it up.
Joe
..some additional info. The "327 LT-1" tested on the lab dyno that started life as a '65 L-79 was tested with both OE 2.5" manifolds/OE front pipes and 1 5/8" dyno headers, both systems being routed into the 3" dyno exhaust system with no mufflers. All data uses STP correction so it is effectively SAE gross. CR averaged 10.35:1 across the eight cylinders and 100LL avgas was used to ensure no detonation during the tests.
The system engineering and testing was documented here on the CF with extensive discussion back in the early/mid 2000s. Search for "327 LT-1" in the thread titles and threads started by Dave McDufford, Then search for threads by ghostrider 20. That was the 327 LT-1 (started life as a '65 L-76) that was tested on a Dynojet chassis dyno. Both engines were essentially the same configuration.
With manifolds the best lab dyno torque and power from multiple tests was 338 lb-ft @ 3700 and 356 GHP @ 6400, 45 degress total WOT spark advance.
With headers the best torque/power from multiple tests was 358lb-ft @ 4400 and 369 GHP @ 6500, 41 degrees Total WOT advance.
Headers/open exhaust increased peak torque by 5.9 percent and peak power by 3.7 percent compared to manifolds/open exhaust.
Since exhaust back pressure negates some of the wave dynamics effects, I expect in the car with the under the car exhaust the difference between headers and manifolds would be cut in half.
These tests show that the OE 2.5" manifolds and 2.5" under-the-car pipes/mufflers are very efficient - probably the lowest restriction OE exhaust system of the era.
Duke
The system engineering and testing was documented here on the CF with extensive discussion back in the early/mid 2000s. Search for "327 LT-1" in the thread titles and threads started by Dave McDufford, Then search for threads by ghostrider 20. That was the 327 LT-1 (started life as a '65 L-76) that was tested on a Dynojet chassis dyno. Both engines were essentially the same configuration.
With manifolds the best lab dyno torque and power from multiple tests was 338 lb-ft @ 3700 and 356 GHP @ 6400, 45 degress total WOT spark advance.
With headers the best torque/power from multiple tests was 358lb-ft @ 4400 and 369 GHP @ 6500, 41 degrees Total WOT advance.
Headers/open exhaust increased peak torque by 5.9 percent and peak power by 3.7 percent compared to manifolds/open exhaust.
Since exhaust back pressure negates some of the wave dynamics effects, I expect in the car with the under the car exhaust the difference between headers and manifolds would be cut in half.
These tests show that the OE 2.5" manifolds and 2.5" under-the-car pipes/mufflers are very efficient - probably the lowest restriction OE exhaust system of the era.
Duke
Last edited by 65tripleblack; 10-23-2016 at 12:13 PM.
10-23-2016, 03:42 PM
#31
Melting Slicks
Duke, or someone else needs to explain to couple of things to them.
Back Pressure & cause effect.
Exhaust Reversion Wave/Pulse, cause & effect.
Why 3.0" headers open headers out perform 3.0" full length out straight pipes. Main Reason tied to one of the above effects. How on some engines cross over pipe works better, while on others, X-Pipe does to improve flow.
Back Pressure & cause effect.
Exhaust Reversion Wave/Pulse, cause & effect.
Why 3.0" headers open headers out perform 3.0" full length out straight pipes. Main Reason tied to one of the above effects. How on some engines cross over pipe works better, while on others, X-Pipe does to improve flow.
Last edited by Poorhousenext; 10-23-2016 at 03:47 PM.
10-24-2016, 01:51 AM
#32
Team Owner
Member Since: Nov 2005
Location: Beach & High Desert Southern California
Posts: 25,496
Received 2,339 Likes
on
890 Posts
Duke, or someone else needs to explain to couple of things to them.
Back Pressure & cause effect.
Exhaust Reversion Wave/Pulse, cause & effect.
Why 3.0" headers open headers out perform 3.0" full length out straight pipes. Main Reason tied to one of the above effects. How on some engines cross over pipe works better, while on others, X-Pipe does to improve flow.
Back Pressure & cause effect.
Exhaust Reversion Wave/Pulse, cause & effect.
Why 3.0" headers open headers out perform 3.0" full length out straight pipes. Main Reason tied to one of the above effects. How on some engines cross over pipe works better, while on others, X-Pipe does to improve flow.
1. Exhaust backpressure costs power (small or long tube = bad)
2. Exhaust airflow momentum aids power (right diameter or right length = good)
3. Exhaust airflow momentum creates backpressure (TANSTAFL or the 2nd Law of Thermodynamics), so design momentum benefits have to exceed the backpressure costs.
Good momentum is achieved at 230 to 260 feet per second for headpipe exhaust gas airflow, and 120 to 130 feet per second for collected exhaust gas airflow (rule of thumb designing). Smooth exhaust tubes extend the rpm sweet range of momentum effect, compared to rough surface cast manifolds, because they have less surface finish restriction and edge effects that quickly elevate backpressure as flow increases. Size the tube diameter for momentum velocity at peak torque rpm.
Long tubes aid in definition of the contained exhaust airflow mass creating the momentum benefit, but the benefits are quickly exceeded by friction pressure losses once the mass exceeds three to five times the swept volume of the cylinder (more rule of thumb designing). Size tube length for momentum volume.
4. Wave/Pulse dynamic effects that work well on 2-stroke open port engines do not have similar significant benefits when an independently controlled valve is operating in the exhaust port. You can tune tube length and divergence cone size for more power, but the benefits when a valve is also controlling airflow timing are minimal compared to the benefits of momentum and costs of backpressure. KISS.
5. Detailed exhaustive analysis of special cases can bruise rule of thumb designing, so be aware of how you use your thumbs.
10-24-2016, 10:48 AM
#33
Melting Slicks
One can burn hours explaining pressure waves and flow dynamics, and few people will understand. The big take away points however are simple.
1. Exhaust backpressure costs power (small or long tube = bad)
2. Exhaust airflow momentum aids power (right diameter or right length = good)
3. Exhaust airflow momentum creates backpressure (TANSTAFL or the 2nd Law of Thermodynamics), so design momentum benefits have to exceed the backpressure costs.
Good momentum is achieved at 230 to 260 feet per second for headpipe exhaust gas airflow, and 120 to 130 feet per second for collected exhaust gas airflow (rule of thumb designing). Smooth exhaust tubes extend the rpm sweet range of momentum effect, compared to rough surface cast manifolds, because they have less surface finish restriction and edge effects that quickly elevate backpressure as flow increases. Size the tube diameter for momentum velocity at peak torque rpm.
Long tubes aid in definition of the contained exhaust airflow mass creating the momentum benefit, but the benefits are quickly exceeded by friction pressure losses once the mass exceeds three to five times the swept volume of the cylinder (more rule of thumb designing). Size tube length for momentum volume.
4. Wave/Pulse dynamic effects that work well on 2-stroke open port engines do not have similar significant benefits when an independently controlled valve is operating in the exhaust port. You can tune tube length and divergence cone size for more power, but the benefits when a valve is also controlling airflow timing are minimal compared to the benefits of momentum and costs of backpressure. KISS.
5. Detailed exhaustive analysis of special cases can bruise rule of thumb designing, so be aware of how you use your thumbs.
1. Exhaust backpressure costs power (small or long tube = bad)
2. Exhaust airflow momentum aids power (right diameter or right length = good)
3. Exhaust airflow momentum creates backpressure (TANSTAFL or the 2nd Law of Thermodynamics), so design momentum benefits have to exceed the backpressure costs.
Good momentum is achieved at 230 to 260 feet per second for headpipe exhaust gas airflow, and 120 to 130 feet per second for collected exhaust gas airflow (rule of thumb designing). Smooth exhaust tubes extend the rpm sweet range of momentum effect, compared to rough surface cast manifolds, because they have less surface finish restriction and edge effects that quickly elevate backpressure as flow increases. Size the tube diameter for momentum velocity at peak torque rpm.
Long tubes aid in definition of the contained exhaust airflow mass creating the momentum benefit, but the benefits are quickly exceeded by friction pressure losses once the mass exceeds three to five times the swept volume of the cylinder (more rule of thumb designing). Size tube length for momentum volume.
4. Wave/Pulse dynamic effects that work well on 2-stroke open port engines do not have similar significant benefits when an independently controlled valve is operating in the exhaust port. You can tune tube length and divergence cone size for more power, but the benefits when a valve is also controlling airflow timing are minimal compared to the benefits of momentum and costs of backpressure. KISS.
5. Detailed exhaustive analysis of special cases can bruise rule of thumb designing, so be aware of how you use your thumbs.
Reversion is exhaust Pulse wave back up the system cause by exhaust pulse of cylinder firing that effects/slows air flow into engine all the way to where air is first ingested slowing intake air speed due to slowing cylinder fill. It can be tuned out to a point.
Best example of it's effects is on a Harley Davidson V Twin engine.
Same engine, 120 CI S&S engine with CV51 carb. 3 different exhaust systems installed. One has 1 3/4" exhaust tubes and tuneable collector, one has 1 7/8" exhaust tubes non tuneable collector, while the 3rd system is made up of the 1 7/8 head pipes, with the tuneable collector from the other system.
Both systems produced the almost the same peak numbers and power band curves, with the tunable system bringing on start of power band at slightly in slightly lower RPM range. The tunable system was tuned with both open and closed end cap on collector, 10 disc open, 16 disc closed.
The 3rd system was a combination of the best parts of each system. The 1 7/8 head pipes from one system and the tunable collector from the other. The tunable collector was run was with closed end cap and 16 disc. Adding or taking away disc did not improve power band. Open end cap and 10 disc made same numbers. But look what happened power band of engine when reversion wave was tuned out as best as possible. Power came on higher and earlier and peaked higher.
The Borzilla system performed worst on low end. The Supertrapp system with smaller header pipe open or closed allowed engine TQ to rise quicker, and smaller tubes did not hurt it on upper end.
Tuning reversion wave out allowed the larger Borzilla header pipes to provide more intake velocity on low end and make a little more HP on Top end by increasing cylinder fill.
Note: Amount of air the collector can flow is fixed, adding more disc than needed to dissipate/negate reversion wave can hurt power curve, not help it. The carb used limited power output of engine, but ride-ability was greatly improved on low end by it's design. Never ventured into TOP End power range...LOL Yes, I did have a steering damper installed to try and stop a Tank Slap. They work by the way...LOl
Now if I could just figure out how to make a Supertrapp mufflers look good sticking out the back of a C2, and afford the dyno time needed to tune them....LOL
Last edited by Poorhousenext; 10-24-2016 at 10:55 AM.
The following users liked this post:
ctmccloskey (09-23-2021)
10-24-2016, 12:21 PM
#34
Race Director
Thread Starter
Member Since: May 2000
Location: Redondo Beach USA
Posts: 12,487
Received 1,974 Likes
on
1,188 Posts
Duke, or someone else needs to explain to couple of things to them.
Back Pressure & cause effect.
Exhaust Reversion Wave/Pulse, cause & effect.
Why 3.0" headers open headers out perform 3.0" full length out straight pipes. Main Reason tied to one of the above effects. How on some engines cross over pipe works better, while on others, X-Pipe does to improve flow.
Back Pressure & cause effect.
Exhaust Reversion Wave/Pulse, cause & effect.
Why 3.0" headers open headers out perform 3.0" full length out straight pipes. Main Reason tied to one of the above effects. How on some engines cross over pipe works better, while on others, X-Pipe does to improve flow.
Fluid mechanics is not simple to explain to say the least, but there are some basic qualitative ideas that most should be able to understand. Take a single cylinder engine with a straight exhaust pipe. When the exhaust valve begins to open a primary pressure wave begins traveling down the pipe at sonic velocity, which is a function of fluid temperature. Sonic velocity in the hottest part of the exhaust system is about 1500 ft/sec and about 1100 ft/sec in the inlet system.
When that positive pressure wave reaches the end of the pipe, it is reflected back as a rarefaction wave that creates a slight negative pressure when it arrives back at the cylinder, and we want this to arrive during the valve overlap period within a rev range that we want peak torque or power.
There is a simple formula to compute this, sometimes called the "organ pipe" formula that I'm sure can be found on the Web.
In a multicylinder engine things get considerably more complicated, In addition to the primary wave reflection from the end of the primary pipe, there is a weaker reflection from the end of the collector that is reflected back into all the primary pipes, and those waves are then reflected back as pressure waves, albeit, weak. So there are a lot of waves, both positive and negative bouncing back and forth, but a properly designed collected system with proper collector length can extend the rev range where there is negative pressure at the exhaust port during the overlap period on a naturally aspirated engine.
On a boosted engine at high load inlet pressure is nearly always greater than exhaust pressure (even with a well designed road exhaust system), so the most efficient exhaust system has large individual pipes just long enough to safely clear the hot exhaust gas from the chassis and driver, which is what you see on Top Fuel and Funny Cars, and for the same reason, high specific output boosted engines don't need as much overlap as a high output naturally aspirated engine. Supercharged hot rods with headers are typical, but large diameter streamlined manifolds would be better.
If you want to understand basic exhaust system wave dynamics without all the math get a copy of Phillip H. Smith's "The Scientific Design of Intake and Exhaust Systems" He talks about the concept of an "exhaust box" that I doubt any of you have ever heard of, but when you see a Sprint Cup car flying through the air upside down, it's clear that the exhaust system is designed exactly as recommended by Smith, exhaust box and all.
One ofter basic concept in exhaust (and inlet) design is you want to keep maximum bulk fluid velocity at no more than about Mach 0.3 to avoid excessive viscous friction losses in the fluid. The flow is non-steady, but you can usually get away with making sure that average flow velocity does not exceed Mach 0.3, which would be about 400 ft/sec in the primary exhaust pipes and 300 ft/sec on the inlet side.
Duke
Last edited by SWCDuke; 10-24-2016 at 12:34 PM.
10-24-2016, 12:36 PM
#35
Melting Slicks
I spent six years in engineering schools including a Master's thesis modeling what goes on in a carburetor that was basically a fluid mechanics problem. Even after making considerable simplifying assumptions regarding friction and heat transfer I ended up with a set of six simultaneous partial differential equations that were a bitch to solve.
Fluid mechanics is not simple to explain to say the least, but there are some basic qualitative ideas that most should be able to understand. Take a single cylinder engine with a straight exhaust pipe. When the exhaust valve begins to open a primary pressure wave begins traveling down the pipe at sonic velocity, which is a function of fluid temperature. Sonic velocity in the hottest part of the exhaust system is about 1500 ft/sec and about 1100 ft/sec in the inlet system.
When that positive pressure wave reaches the end of the pipe, it is reflected back as a rarefaction wave that creates a slight negative pressure when it arrives back at the cylinder, and we want this to arrive during the valve overlap period within a rev range that we want peak torque or power.
There is a simple formula to compute this, sometimes called the "organ pipe" formula that I'm sure can be found on the Web.
In a multicylinder engine things get considerably more complicated, In addition to the primary wave reflection from the end of the primary pipe, there is a weaker reflection from the end of the collector that is reflected back into all the primary pipes, and those waves are then reflected back as pressure waves, albeit, weak. So there are a lot of waves, both positive and negative bouncing back and forth, but a properly designed collected system with proper collector length can extend the rev range where there is negative pressure at the exhaust port during the overlap period.
If you want to understand basic exhaust system wave dynamics without all the math get a copy of Phillip H. Smith's "The Scientific Design of Intake and Exhaust Systems" He talks about the concept of an "exhaust box" that I doubt any of you have ever heard of, but when you see a Sprint Cup car flying through the air upside down, it's clear that the exhaust system is designed exactly as recommended by Smith, exhaust box and all.
One ofter basic concept in exhaust (and inlet) design is you want to keep maximum bulk fluid velocity at no more than about Mach 0.3 to avoid excessive viscous friction losses in the fluid. The flow is non-steady, but you can usually get away with making sure that average flow velocity does not exceed Mach 0.3, which would be about 400 ft/sec in the primary exhaust pipes and 300 ft/sec on the inlet side.
Duke
Fluid mechanics is not simple to explain to say the least, but there are some basic qualitative ideas that most should be able to understand. Take a single cylinder engine with a straight exhaust pipe. When the exhaust valve begins to open a primary pressure wave begins traveling down the pipe at sonic velocity, which is a function of fluid temperature. Sonic velocity in the hottest part of the exhaust system is about 1500 ft/sec and about 1100 ft/sec in the inlet system.
When that positive pressure wave reaches the end of the pipe, it is reflected back as a rarefaction wave that creates a slight negative pressure when it arrives back at the cylinder, and we want this to arrive during the valve overlap period within a rev range that we want peak torque or power.
There is a simple formula to compute this, sometimes called the "organ pipe" formula that I'm sure can be found on the Web.
In a multicylinder engine things get considerably more complicated, In addition to the primary wave reflection from the end of the primary pipe, there is a weaker reflection from the end of the collector that is reflected back into all the primary pipes, and those waves are then reflected back as pressure waves, albeit, weak. So there are a lot of waves, both positive and negative bouncing back and forth, but a properly designed collected system with proper collector length can extend the rev range where there is negative pressure at the exhaust port during the overlap period.
If you want to understand basic exhaust system wave dynamics without all the math get a copy of Phillip H. Smith's "The Scientific Design of Intake and Exhaust Systems" He talks about the concept of an "exhaust box" that I doubt any of you have ever heard of, but when you see a Sprint Cup car flying through the air upside down, it's clear that the exhaust system is designed exactly as recommended by Smith, exhaust box and all.
One ofter basic concept in exhaust (and inlet) design is you want to keep maximum bulk fluid velocity at no more than about Mach 0.3 to avoid excessive viscous friction losses in the fluid. The flow is non-steady, but you can usually get away with making sure that average flow velocity does not exceed Mach 0.3, which would be about 400 ft/sec in the primary exhaust pipes and 300 ft/sec on the inlet side.
Duke
I see that you now understand part of it by what you posted above but use time as exhaust valve open to exhaust valve close instead.
Use about 1670 fps in your calculation instead of the 1500 that you posted abive.
By the way, you STILL owe me lunch because of the wager with me that you lost over there.
Last edited by Critter1; 10-24-2016 at 12:54 PM.
The following users liked this post:
wmf62 (10-24-2016)
10-24-2016, 12:53 PM
#36
Team Owner
Member Since: Mar 2003
Location: Greenville, Indiana
Posts: 26,118
Received 1,843 Likes
on
1,398 Posts
....
10-24-2016, 12:57 PM
#37
Race Director
Thread Starter
Member Since: May 2000
Location: Redondo Beach USA
Posts: 12,487
Received 1,974 Likes
on
1,188 Posts
I did my graduate work circa 1970 and have had Smith's book since the mid-seventies, so my simplified explanation of exhaust system wave dynamics is the same now as I would have given ten or forty years ago.
Duke
Duke
10-24-2016, 02:16 PM
#38
Tech Contributor
Member Since: Jun 2004
Location: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Posts: 18,360
Received 768 Likes
on
550 Posts
I spent six years in engineering schools including a Master's thesis modeling what goes on in a carburetor that was basically a fluid mechanics problem. Even after making considerable simplifying assumptions regarding friction and heat transfer I ended up with a set of six simultaneous partial differential equations that were a bitch to solve.
Fluid mechanics is not simple to explain to say the least, but there are some basic qualitative ideas that most should be able to understand. Take a single cylinder engine with a straight exhaust pipe. When the exhaust valve begins to open a primary pressure wave begins traveling down the pipe at sonic velocity, which is a function of fluid temperature. Sonic velocity in the hottest part of the exhaust system is about 1500 ft/sec and about 1100 ft/sec in the inlet system.
When that positive pressure wave reaches the end of the pipe, it is reflected back as a rarefaction wave that creates a slight negative pressure when it arrives back at the cylinder, and we want this to arrive during the valve overlap period within a rev range that we want peak torque or power.
There is a simple formula to compute this, sometimes called the "organ pipe" formula that I'm sure can be found on the Web.
In a multicylinder engine things get considerably more complicated, In addition to the primary wave reflection from the end of the primary pipe, there is a weaker reflection from the end of the collector that is reflected back into all the primary pipes, and those waves are then reflected back as pressure waves, albeit, weak. So there are a lot of waves, both positive and negative bouncing back and forth, but a properly designed collected system with proper collector length can extend the rev range where there is negative pressure at the exhaust port during the overlap period on a naturally aspirated engine.
On a boosted engine at high load inlet pressure is nearly always greater than exhaust pressure (even with a well designed road exhaust system), so the most efficient exhaust system has large individual pipes just long enough to safely clear the hot exhaust gas from the chassis and driver, which is what you see on Top Fuel and Funny Cars, and for the same reason, high specific output boosted engines don't need as much overlap as a high output naturally aspirated engine. Supercharged hot rods with headers are typical, but large diameter streamlined manifolds would be better.
If you want to understand basic exhaust system wave dynamics without all the math get a copy of Phillip H. Smith's "The Scientific Design of Intake and Exhaust Systems" He talks about the concept of an "exhaust box" that I doubt any of you have ever heard of, but when you see a Sprint Cup car flying through the air upside down, it's clear that the exhaust system is designed exactly as recommended by Smith, exhaust box and all.
One ofter basic concept in exhaust (and inlet) design is you want to keep maximum bulk fluid velocity at no more than about Mach 0.3 to avoid excessive viscous friction losses in the fluid. The flow is non-steady, but you can usually get away with making sure that average flow velocity does not exceed Mach 0.3, which would be about 400 ft/sec in the primary exhaust pipes and 300 ft/sec on the inlet side.
Duke
Fluid mechanics is not simple to explain to say the least, but there are some basic qualitative ideas that most should be able to understand. Take a single cylinder engine with a straight exhaust pipe. When the exhaust valve begins to open a primary pressure wave begins traveling down the pipe at sonic velocity, which is a function of fluid temperature. Sonic velocity in the hottest part of the exhaust system is about 1500 ft/sec and about 1100 ft/sec in the inlet system.
When that positive pressure wave reaches the end of the pipe, it is reflected back as a rarefaction wave that creates a slight negative pressure when it arrives back at the cylinder, and we want this to arrive during the valve overlap period within a rev range that we want peak torque or power.
There is a simple formula to compute this, sometimes called the "organ pipe" formula that I'm sure can be found on the Web.
In a multicylinder engine things get considerably more complicated, In addition to the primary wave reflection from the end of the primary pipe, there is a weaker reflection from the end of the collector that is reflected back into all the primary pipes, and those waves are then reflected back as pressure waves, albeit, weak. So there are a lot of waves, both positive and negative bouncing back and forth, but a properly designed collected system with proper collector length can extend the rev range where there is negative pressure at the exhaust port during the overlap period on a naturally aspirated engine.
On a boosted engine at high load inlet pressure is nearly always greater than exhaust pressure (even with a well designed road exhaust system), so the most efficient exhaust system has large individual pipes just long enough to safely clear the hot exhaust gas from the chassis and driver, which is what you see on Top Fuel and Funny Cars, and for the same reason, high specific output boosted engines don't need as much overlap as a high output naturally aspirated engine. Supercharged hot rods with headers are typical, but large diameter streamlined manifolds would be better.
If you want to understand basic exhaust system wave dynamics without all the math get a copy of Phillip H. Smith's "The Scientific Design of Intake and Exhaust Systems" He talks about the concept of an "exhaust box" that I doubt any of you have ever heard of, but when you see a Sprint Cup car flying through the air upside down, it's clear that the exhaust system is designed exactly as recommended by Smith, exhaust box and all.
One ofter basic concept in exhaust (and inlet) design is you want to keep maximum bulk fluid velocity at no more than about Mach 0.3 to avoid excessive viscous friction losses in the fluid. The flow is non-steady, but you can usually get away with making sure that average flow velocity does not exceed Mach 0.3, which would be about 400 ft/sec in the primary exhaust pipes and 300 ft/sec on the inlet side.
Duke
Respectfully, that's a broad brush you're using with your post, particularly the bolded above. Vizard talks about EPT boxes frequently. I did the math, accepted some of the many packaging restrictions under a C3, and built one for my '69 (looking for some increased power for corner exit speeds). Ran it for about a year. No dyno work to measure the actual results, but finally had to remove it from under the car as it was like sitting on top of a bass drum under WOT conditions.
10-24-2016, 05:54 PM
#39
Le Mans Master
Here's a story I always found quite interesting - right from the horse's mouth - a good friend of mine that worked for Chrysler for probably 35 years. His final 10 or so years with Chrysler he spent with the Viper group constantly developing them, so it was all high-performance stuff all the time. In this particular story, it involves exhaust & how Chrysler was constantly trying to make the funky-sounding V-10 sound better through many exhaust experiments. Trust me, they tried almost everything within their decibel threshold limit, & there was no magic bullet exhaust system. The Viper still sounded like a Viper - to me very reminiscent of a straight 6. I wanted them to run a pair of my chambered Powerstick mufflers in 2.5" on a test mule & I built a pair of mufflers for them, but they never got around to installing them, so a year later they came back to me & eventually sold to a Viper owner in Saginaw, MI.
Anyway, in a nutshell, they took a car with recent dyno data & removed the OE Walker straight-through mufflers & straight-piped the exhaust with 3" pipes. Results showed a loss of around 32 HP, plus the engine ran about 30 degrees hotter - the engineers were baffled & wondered if they harmed the motor during the dyno run somehow, although it didn't seem like anything happened. They let it all cool down again & installed a pair of the OE mufflers in the sidepipe locations, fired it back up, & re-ran the dyno testing. Everything came back into normal parameters. In conclusion, you need an expansion chamber somewhere in the system, & I believe the best location is around the middle of the system - call it resonators or mufflers. If you place mufflers/resonators at the very rear of the system, you'll do additional decibel reduction & possibly some tuning (depending on what you install). The few posts above this get very technical & sound very confusing - those go beyond how I design or recommend a setup. Basically, you need good-flowing headers or manifolds, the RIGHT pipe diameter, & mufflers that are not so restrictive that you lose a bunch of flow. So you can put a system together & test it out & see what power the engine makes. Then you can get real scientific & do it all by the book, then test it out & see where you are. I believe you will have a draw in the end, & keep in mind different dyno pulls with the same setup will yield minor differences. The dyno runs shown above show some differences, but they are so minor, I think you could get the same variations with the SAME exhaust by doing 3 pulls.
Anyway, in a nutshell, they took a car with recent dyno data & removed the OE Walker straight-through mufflers & straight-piped the exhaust with 3" pipes. Results showed a loss of around 32 HP, plus the engine ran about 30 degrees hotter - the engineers were baffled & wondered if they harmed the motor during the dyno run somehow, although it didn't seem like anything happened. They let it all cool down again & installed a pair of the OE mufflers in the sidepipe locations, fired it back up, & re-ran the dyno testing. Everything came back into normal parameters. In conclusion, you need an expansion chamber somewhere in the system, & I believe the best location is around the middle of the system - call it resonators or mufflers. If you place mufflers/resonators at the very rear of the system, you'll do additional decibel reduction & possibly some tuning (depending on what you install). The few posts above this get very technical & sound very confusing - those go beyond how I design or recommend a setup. Basically, you need good-flowing headers or manifolds, the RIGHT pipe diameter, & mufflers that are not so restrictive that you lose a bunch of flow. So you can put a system together & test it out & see what power the engine makes. Then you can get real scientific & do it all by the book, then test it out & see where you are. I believe you will have a draw in the end, & keep in mind different dyno pulls with the same setup will yield minor differences. The dyno runs shown above show some differences, but they are so minor, I think you could get the same variations with the SAME exhaust by doing 3 pulls.
10-24-2016, 06:41 PM
#40
Team Owner
Member Since: Mar 2003
Location: Greenville, Indiana
Posts: 26,118
Received 1,843 Likes
on
1,398 Posts
I have heard of this happening years ago. Wonder how many will pick up on this?