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woodyedmiston
Well-Known Member
In this day of fighting the false god of global warming, engineers may do any number of things to mix, mingle or change the burn characteristics of exhaust flow. Those may change from year to year and as rules change. But in general you do not want backpressure in a performance exhaust system.
In THEORY (I'm primarily quoting an old book called The Scientific Design and Building of Exhaust Systems + a lot of dyno and flow bench time working with race engines.) an internal combustion engine should run best with an absolutely undiluted air/fuel mixture. If you want to know why and how go google "flame front" or something like that and learn about the way the burn is dispersed over the piston.
Moving on. To keep the mixture at optimum. Several things have been tried - and these are but a few of them. Raming the air in, increasing the pipe size to lower restriction, mis-patching port/manifold connections, creating a low pressure area at the end of the exhaust stream.
The problem with ramming air in is that it bounces off itself as it enters the inlet . . . until the throttles open wider/less wide - thereby making the "ram effect" not much more than atmospheric – and certainly not a steady state that would keep all things equal. Sure doesn't hurt anything as far as cleaning up the fuel mixture, but it's not a cure. (And you cannot approach anything like a turbo or supercharger.)
Increasing pipe size is usually a winner. But . . . with increase pipe size there is an even greater tendency toward what is called laminar flow this causes dilution or contamination of the fuel mixture in the cylinder. If you imagine a drop of water hitting the top of a container of water, you see that around the center of the drop is a splash going in the opposite direction. Each pulse of combusted gas causes the same reaction in your exhaust pipe. Imagine too, that each pulse is a ping-pong ball going down the pipe. Each time the pulse moves forward there is a reverse flow cause by the pulse of energized gas going down the pipe. Therefore, there is a laminar flow going backwards between the pulse and the sides of the pipe. That "reversion" flow is what contaminates the fuel mixture and where you get the idea of "anti-reversion" flow headers.
Many engine builders mis-match the exhaust ports at the head/header point to combat reversion. That is part of the story. The reason it works is not just that it creates a small "dam" to hold back the reverse flow. It also creates a small low-pressure area that helps return that flow back into the exhaust stream. A properly sized set of headers that opens the pulse to reverse flow clearance up also allows for the low pressure area to work. Assume then that you take this onto the rest of the system - mismatching the header to exhaust, eliminating all mufflers etc. as in a racing engine. At the end of the exhaust, where it can get into the air flow, you can add an even larger pipe or flat washer so that the air stream creates a low pressure area at the mouth of the tail pipe. At that point you have placed several low pressure areas that tend to "suck" the air out of the tailpipe. (This has been used several times by small displacement cars running long races like Sebring and Le Mans) The fish-tail exhaust extension that is put on a lot of "dresser" Harley's is actually a version of this low-pressure area device originally made in the early 1900's for airplane engines. The engineers at Ford and Chev have been adding an engineered tip to their trucks exhaust lately.
Many engines designers have used these ideas to design engine systems. Many of them have discovered that by maximizing these techniques (that include high flow in a tube or pipe) that a single exhaust is superior to not only 4 and 6 Cylinder enginies but also the V-8. Even when the exhaust system could be short stacks, as in racing cars and aircraft engines, the longer tailpipe produced higher dyno horsepower. It's not always practical to use long tailpipes, but in the early days of aircraft it was not unusual to see a tailpipe running down the side of an old biplane.
Backpressure in general is not a good thing. Engines that are required to run mufflers, and those of you who don't like backpressure or noise can easily add a second, smaller pipe - say 1" to the inlet side of your muffler and run it out the back alongside the regular tail pipe that will effectively eliminate the backpressure and most of the problem of noise. I've only done this in practice on a race engine at tracks that required mufflers, so I don't know if your computer will accommodate that change without dyno work.
In THEORY (I'm primarily quoting an old book called The Scientific Design and Building of Exhaust Systems + a lot of dyno and flow bench time working with race engines.) an internal combustion engine should run best with an absolutely undiluted air/fuel mixture. If you want to know why and how go google "flame front" or something like that and learn about the way the burn is dispersed over the piston.
Moving on. To keep the mixture at optimum. Several things have been tried - and these are but a few of them. Raming the air in, increasing the pipe size to lower restriction, mis-patching port/manifold connections, creating a low pressure area at the end of the exhaust stream.
The problem with ramming air in is that it bounces off itself as it enters the inlet . . . until the throttles open wider/less wide - thereby making the "ram effect" not much more than atmospheric – and certainly not a steady state that would keep all things equal. Sure doesn't hurt anything as far as cleaning up the fuel mixture, but it's not a cure. (And you cannot approach anything like a turbo or supercharger.)
Increasing pipe size is usually a winner. But . . . with increase pipe size there is an even greater tendency toward what is called laminar flow this causes dilution or contamination of the fuel mixture in the cylinder. If you imagine a drop of water hitting the top of a container of water, you see that around the center of the drop is a splash going in the opposite direction. Each pulse of combusted gas causes the same reaction in your exhaust pipe. Imagine too, that each pulse is a ping-pong ball going down the pipe. Each time the pulse moves forward there is a reverse flow cause by the pulse of energized gas going down the pipe. Therefore, there is a laminar flow going backwards between the pulse and the sides of the pipe. That "reversion" flow is what contaminates the fuel mixture and where you get the idea of "anti-reversion" flow headers.
Many engine builders mis-match the exhaust ports at the head/header point to combat reversion. That is part of the story. The reason it works is not just that it creates a small "dam" to hold back the reverse flow. It also creates a small low-pressure area that helps return that flow back into the exhaust stream. A properly sized set of headers that opens the pulse to reverse flow clearance up also allows for the low pressure area to work. Assume then that you take this onto the rest of the system - mismatching the header to exhaust, eliminating all mufflers etc. as in a racing engine. At the end of the exhaust, where it can get into the air flow, you can add an even larger pipe or flat washer so that the air stream creates a low pressure area at the mouth of the tail pipe. At that point you have placed several low pressure areas that tend to "suck" the air out of the tailpipe. (This has been used several times by small displacement cars running long races like Sebring and Le Mans) The fish-tail exhaust extension that is put on a lot of "dresser" Harley's is actually a version of this low-pressure area device originally made in the early 1900's for airplane engines. The engineers at Ford and Chev have been adding an engineered tip to their trucks exhaust lately.
Many engines designers have used these ideas to design engine systems. Many of them have discovered that by maximizing these techniques (that include high flow in a tube or pipe) that a single exhaust is superior to not only 4 and 6 Cylinder enginies but also the V-8. Even when the exhaust system could be short stacks, as in racing cars and aircraft engines, the longer tailpipe produced higher dyno horsepower. It's not always practical to use long tailpipes, but in the early days of aircraft it was not unusual to see a tailpipe running down the side of an old biplane.
Backpressure in general is not a good thing. Engines that are required to run mufflers, and those of you who don't like backpressure or noise can easily add a second, smaller pipe - say 1" to the inlet side of your muffler and run it out the back alongside the regular tail pipe that will effectively eliminate the backpressure and most of the problem of noise. I've only done this in practice on a race engine at tracks that required mufflers, so I don't know if your computer will accommodate that change without dyno work.
