pipes
06-19-2016 | 03:17 AM
#3
Tech Regular
Joined: Feb 2015
Posts: 336
From: Menasha, Wisconsin
Exactly! Different sized and shaped pipes and headers will cause the engine to perform differently. This can be used as a method of tuning for conditions. Bashing, run a torque or mid range pipe. Racing in a large open track, RPM. A good pipe and header is one of the most important mods one can install on their nitro. I've seen low performance RTR engines wake up with a simple pipe and header swap.
06-21-2016 | 02:02 PM
#4
Tech Master
Joined: Nov 2003
Posts: 1,811
From: lima-peru
Tuned Pipes
By Steve Pond
I've been touting the virtues of tuned pipes for many years. They help increase power, improve fuel efficiency and reduce engine temperatures. It's a no-brainer! The nitro public at large understands much of this, but as nitro enthusiasts have become more aware of the benefits of the tuned pipe, the focus has shifted: the question now is which tuned pipe is the best.
I'm no expert on tuned pipes—or expansion chambers, as they're often called; I simply have years of experience with different exhaust configurations. This gives me the street smarts to suggest which pipe would be best for a given application, but I needed to brush up on the science to tell you why one pipe performs differently from another. I spent months consulting some of the best minds in the RC industry and others who have made careers in science of exposing the "how" and "why" of "the pipe." I won't bore you to tears with technical mumbo jumbo, but I hope that the combination of my experience sprinkled with a little technical information will help point you toward the right pipe for your application.
WHAT IS A TUNED PIPE?
A tuned pipe is actually part of a tuned exhaust system that begins with a header that's attached to the engine. The header is usually a constant-diameter pipe that can bend in many directions to direct the exhaust flow to the tuned pipe. It's the outlet for the exhaust gases, and we hope it deadens engine noise slightly. The header and the tuned pipe are usually connected with a short section of flexible tubing and a couple of zip-ties.
THE BASICS
For the benefit of those who want to know why a tuned pipe helps every aspect of 2-stroke-engine performance, I'll cover some of the basics.
A 2-stroke engine's intake transfer ports and the exhaust port are open at roughly the same time. This means that a portion of the fuel/air mixture that enters the engine simply passes straight out through the exhaust. It doesn't take a rocket scientist to figure out that the escaped fuel is wasted and can't contribute to making power. Many years ago, engineers figured out that by cleverly crafting the exhaust pipe into a specific shape, the exhaust itself could be used to keep more unburned fuel in the engine to increase power. Now, when the exhaust port opens, the pressure created during the combustion process creates a pressure "wave" that travels through the header and into the tuned pipe. The tuned pipe's small exhaust outlet releases some of the exhaust pressure, but a significant amount remains in the pipe. The engineers carefully "tuned" the length and shape of the exhaust pipe (hence the name "tuned pipe") to send some of the pressure wave back up the pipe to force the escaping fuel mixture back into the engine. In practice, tuned-pipe design is more involved than I make it sound, but this is generally why tuned pipes can be so beneficial.
THE DETAILS
Racing enthusiasts, or those who simply want to get maximum possible performance, may want to take a longer look at what will work best for their application. Listed below are some facts about the best tuned-pipe configuration. Much of this information has been borrowed from the development of pipes for larger 2-stroke engines, but it still serves as a guide to many pipe manufacturers and engine tuners in the RC industry. Nitro-burning engines are a slightly different animal from the engines for which these guidelines were formed, so it isn't an exact crossover, but most of our tuned pipes are formulated according to this data.
• Header. The ideal header would start with an area that's roughly 10 to 15 percent larger than the cross-section of the exhaust port. The ideal header would also expand outward at an angle of roughly 2 to 4 degrees. This subtle expansion, when combined with those of the tuned pipe, helps scavenge, or suck, the exhaust from the engine. Many headers designed for RC car engines don't follow these guidelines, however; they are usually too large in diameter and are straight. Straight pipes are less expensive to manufacture, and the gains offered by a tapered pipe are debatable, at best, for RC applications.
• Coupling. The header and tuned pipe are usually joined with a flexible coupling of mostly heat-resistant flexible tubing. The coupling has little to do with the effectiveness of the pipe, unless it fails prematurely or it has not been properly installed. The best installation has only a slight gap between the header and the pipe. This ensures smoother exhaust flow and avoids exposing the coupling to excessively high temperatures. Be careful not to allow the pipe and header to touch each other, however, as the vibration will cause minor damage to both. The real puzzler is that I have also seen this metal-to-metal high-frequency vibration cause radio interference. Yup; for some reason, when the two pieces of metal rub together, it can cause the radio to glitch.
• Divergent cone. The first part of a tuned pipe is the divergent cone. It's the part just beyond the coupling, and it's flared outward. As the exhaust-pressure wave travels through this expanding section of the pipe, it creates a vacuum of sorts behind it. This helps pull out (scavenge) any residual exhaust gases in the cylinder, and it aids in the delivery of fresh fuel. The ideal angle of the divergent cone is in the ballpark of 10 degrees. The angle of this section of the pipe determines the duration and intensity of both, negative and positive pressure waves. A sharper angle of the divergent cone makes the negative pressure wave more intense but shorter in duration. This makes scavenging more efficient but limits it to a more narrow rpm range. A more gradual divergent cone reduces the intensity of the negative wave, but it lasts much longer. This increases the rpm range at which the negative wave reaches the cylinder while the exhaust port is still open. It's a balancing act of sorts: more performance over a limited rpm range or less performance over a broader rpm range. The divergent cone also has the same effect on the returning pressure wave. The positive exhaust pressure that doesn't escape from the pipe bounces back toward the header. The angle of the divergent cone has the same effect on the return trip. A more pronounced angle makes the positive pressure wave more intense but shorter in duration, while a more gradual angle makes it less intense over a longer duration. This pressure wave helps stuff fresh fuel back into the cylinder when the exhaust port closes.
• Belly. The belly is the straight section of a tuned pipe; the important dimension here is length. Remember I said that as the exhaust-pressure wave enters the divergent cone, a negative wave is sent back toward the engine. The positive pressure wave then continues through the belly of the pipe, bounces off the convergent cone (I'll get to that in a minute) and heads back toward the engine; the length of this belly section determines how long it takes for the pressure wave to reach the engine. A shorter belly means the pressure wave has less distance to travel to the end of the pipe and is therefore on its way back more quickly. Logic dictates that a longer belly increases the time it takes for the pressure wave to return to the engine. The relative timing between the positive wave and the negative wave is what's at stake here. A short section reduces the delay between the two waves and is best for developing maximum power at high rpm ranges; a longer section increases the delay between the two waves and is more suited to low-rpm power.
• Convergent cone. This is at the end of the belly section and reflects the pressure wave back toward the engine. The cone's taper angle influences how long the pressure wave takes to return along the tuned pipe. Again, more pronounced taper causes a more intense pressure wave of short duration; a gentle taper results in a longer, less intense wave. A taper angle in the range of 15 to 20 degrees, or roughly double the angle of the divergent cone, is thought to be ideal.
• Stinger. This is the last part of the tuned pipe. The stinger's diameter and length are important to performance but are largely irrelevant in RC car applications. The diameter and length of the stinger are governed by most sanctioning bodies, therefore, most pipes are designed to meet these regulations, and there is less variety of stinger sizes. The ideal stinger diameter is about 60 percent of that of the header pipe, and its length should be 10 to 12 times its own diameter.
PUT IT ALL TOGETHER AND ...
The exhaust port opens and a positive pressure wave of exhaust in the cylinder is released into the header and travels through the header to the divergent cone. As it travels through the divergent cone, a negative pressure wave (vacuum) is sent back up the header. The negative pressure helps to pull any remaining exhaust out of the cylinder and helps to draw in a fresh charge of fuel/air mixture. Meanwhile, the original positive pressure wave runs through the pipe's belly, bounces off the convergent cone (some of it sneaks out through the stinger) and returns along the pipe and through the header. This positive wave should reach the engine while the exhaust port is still open so it can force any fresh fuel/air that has tried to sneak out of the cylinder back into the engine before the port closes. That, in a nutshell, is how a tuned pipe is supposed to work.
TUNING THE TUNED PIPE
You thought it was over, didn't you? Now that you know how a tuned pipe works, you're better educated for the next time you select one. "But," you may be asking, "how does that help me now?" Well, unless you have a machine shop and are prepared to modify a pipe or make an entirely new one, it won't help you until you buy a new pipe. But you can do one thing with your existing tuned pipe that can improve performance: tune it! That's right; you can tune a tuned pipe!
The header's length has a significant impact on performance. It's much like a trombone: you slide it out farther, and it resonates at a lower pitch. Slide it out less, and it resonates at a higher pitch—same thing with tuned pipes. Experiment by cutting 1/8 inch at a time off the header, and run the engine between cuts. The idea is to keep cutting until, as the boys from across the pond say, it's really "on song." That means the engine and exhaust system are in sync and singing in harmony. This is when the engine makes the most power. The rule of thumb is that a longer header is better for bottom-end power, while a shorter header makes your engine scream on the top end. You can go too far in either direction, so don't be reckless; you'll do more harm than good.
WHICH TUNED PIPE IS BEST?
So, you've decided that you want a tuned pipe; inevitably, your next question is "Which one should I get?" Man, is that a loaded question!
Its intended use will determine how long you'll want to research your pipe purchase. If you want to improve your vehicle's performance just to have fun at a faster pace, then any header/tuned-pipe combination will do. Choose one that is properly sized for your engine, of course, but you almost can't go wrong if you switch from a muffler to a tuned pipe. You may not get all the power that could be expected if you had spec'd the ideal setup, but the point I'm trying to get across is that anything is better than a muffler; mufflers are for leaf blowers!
By Steve Pond
I've been touting the virtues of tuned pipes for many years. They help increase power, improve fuel efficiency and reduce engine temperatures. It's a no-brainer! The nitro public at large understands much of this, but as nitro enthusiasts have become more aware of the benefits of the tuned pipe, the focus has shifted: the question now is which tuned pipe is the best.
I'm no expert on tuned pipes—or expansion chambers, as they're often called; I simply have years of experience with different exhaust configurations. This gives me the street smarts to suggest which pipe would be best for a given application, but I needed to brush up on the science to tell you why one pipe performs differently from another. I spent months consulting some of the best minds in the RC industry and others who have made careers in science of exposing the "how" and "why" of "the pipe." I won't bore you to tears with technical mumbo jumbo, but I hope that the combination of my experience sprinkled with a little technical information will help point you toward the right pipe for your application.
WHAT IS A TUNED PIPE?
A tuned pipe is actually part of a tuned exhaust system that begins with a header that's attached to the engine. The header is usually a constant-diameter pipe that can bend in many directions to direct the exhaust flow to the tuned pipe. It's the outlet for the exhaust gases, and we hope it deadens engine noise slightly. The header and the tuned pipe are usually connected with a short section of flexible tubing and a couple of zip-ties.
THE BASICS
For the benefit of those who want to know why a tuned pipe helps every aspect of 2-stroke-engine performance, I'll cover some of the basics.
A 2-stroke engine's intake transfer ports and the exhaust port are open at roughly the same time. This means that a portion of the fuel/air mixture that enters the engine simply passes straight out through the exhaust. It doesn't take a rocket scientist to figure out that the escaped fuel is wasted and can't contribute to making power. Many years ago, engineers figured out that by cleverly crafting the exhaust pipe into a specific shape, the exhaust itself could be used to keep more unburned fuel in the engine to increase power. Now, when the exhaust port opens, the pressure created during the combustion process creates a pressure "wave" that travels through the header and into the tuned pipe. The tuned pipe's small exhaust outlet releases some of the exhaust pressure, but a significant amount remains in the pipe. The engineers carefully "tuned" the length and shape of the exhaust pipe (hence the name "tuned pipe") to send some of the pressure wave back up the pipe to force the escaping fuel mixture back into the engine. In practice, tuned-pipe design is more involved than I make it sound, but this is generally why tuned pipes can be so beneficial.
THE DETAILS
Racing enthusiasts, or those who simply want to get maximum possible performance, may want to take a longer look at what will work best for their application. Listed below are some facts about the best tuned-pipe configuration. Much of this information has been borrowed from the development of pipes for larger 2-stroke engines, but it still serves as a guide to many pipe manufacturers and engine tuners in the RC industry. Nitro-burning engines are a slightly different animal from the engines for which these guidelines were formed, so it isn't an exact crossover, but most of our tuned pipes are formulated according to this data.
• Header. The ideal header would start with an area that's roughly 10 to 15 percent larger than the cross-section of the exhaust port. The ideal header would also expand outward at an angle of roughly 2 to 4 degrees. This subtle expansion, when combined with those of the tuned pipe, helps scavenge, or suck, the exhaust from the engine. Many headers designed for RC car engines don't follow these guidelines, however; they are usually too large in diameter and are straight. Straight pipes are less expensive to manufacture, and the gains offered by a tapered pipe are debatable, at best, for RC applications.
• Coupling. The header and tuned pipe are usually joined with a flexible coupling of mostly heat-resistant flexible tubing. The coupling has little to do with the effectiveness of the pipe, unless it fails prematurely or it has not been properly installed. The best installation has only a slight gap between the header and the pipe. This ensures smoother exhaust flow and avoids exposing the coupling to excessively high temperatures. Be careful not to allow the pipe and header to touch each other, however, as the vibration will cause minor damage to both. The real puzzler is that I have also seen this metal-to-metal high-frequency vibration cause radio interference. Yup; for some reason, when the two pieces of metal rub together, it can cause the radio to glitch.
• Divergent cone. The first part of a tuned pipe is the divergent cone. It's the part just beyond the coupling, and it's flared outward. As the exhaust-pressure wave travels through this expanding section of the pipe, it creates a vacuum of sorts behind it. This helps pull out (scavenge) any residual exhaust gases in the cylinder, and it aids in the delivery of fresh fuel. The ideal angle of the divergent cone is in the ballpark of 10 degrees. The angle of this section of the pipe determines the duration and intensity of both, negative and positive pressure waves. A sharper angle of the divergent cone makes the negative pressure wave more intense but shorter in duration. This makes scavenging more efficient but limits it to a more narrow rpm range. A more gradual divergent cone reduces the intensity of the negative wave, but it lasts much longer. This increases the rpm range at which the negative wave reaches the cylinder while the exhaust port is still open. It's a balancing act of sorts: more performance over a limited rpm range or less performance over a broader rpm range. The divergent cone also has the same effect on the returning pressure wave. The positive exhaust pressure that doesn't escape from the pipe bounces back toward the header. The angle of the divergent cone has the same effect on the return trip. A more pronounced angle makes the positive pressure wave more intense but shorter in duration, while a more gradual angle makes it less intense over a longer duration. This pressure wave helps stuff fresh fuel back into the cylinder when the exhaust port closes.
• Belly. The belly is the straight section of a tuned pipe; the important dimension here is length. Remember I said that as the exhaust-pressure wave enters the divergent cone, a negative wave is sent back toward the engine. The positive pressure wave then continues through the belly of the pipe, bounces off the convergent cone (I'll get to that in a minute) and heads back toward the engine; the length of this belly section determines how long it takes for the pressure wave to reach the engine. A shorter belly means the pressure wave has less distance to travel to the end of the pipe and is therefore on its way back more quickly. Logic dictates that a longer belly increases the time it takes for the pressure wave to return to the engine. The relative timing between the positive wave and the negative wave is what's at stake here. A short section reduces the delay between the two waves and is best for developing maximum power at high rpm ranges; a longer section increases the delay between the two waves and is more suited to low-rpm power.
• Convergent cone. This is at the end of the belly section and reflects the pressure wave back toward the engine. The cone's taper angle influences how long the pressure wave takes to return along the tuned pipe. Again, more pronounced taper causes a more intense pressure wave of short duration; a gentle taper results in a longer, less intense wave. A taper angle in the range of 15 to 20 degrees, or roughly double the angle of the divergent cone, is thought to be ideal.
• Stinger. This is the last part of the tuned pipe. The stinger's diameter and length are important to performance but are largely irrelevant in RC car applications. The diameter and length of the stinger are governed by most sanctioning bodies, therefore, most pipes are designed to meet these regulations, and there is less variety of stinger sizes. The ideal stinger diameter is about 60 percent of that of the header pipe, and its length should be 10 to 12 times its own diameter.
PUT IT ALL TOGETHER AND ...
The exhaust port opens and a positive pressure wave of exhaust in the cylinder is released into the header and travels through the header to the divergent cone. As it travels through the divergent cone, a negative pressure wave (vacuum) is sent back up the header. The negative pressure helps to pull any remaining exhaust out of the cylinder and helps to draw in a fresh charge of fuel/air mixture. Meanwhile, the original positive pressure wave runs through the pipe's belly, bounces off the convergent cone (some of it sneaks out through the stinger) and returns along the pipe and through the header. This positive wave should reach the engine while the exhaust port is still open so it can force any fresh fuel/air that has tried to sneak out of the cylinder back into the engine before the port closes. That, in a nutshell, is how a tuned pipe is supposed to work.
TUNING THE TUNED PIPE
You thought it was over, didn't you? Now that you know how a tuned pipe works, you're better educated for the next time you select one. "But," you may be asking, "how does that help me now?" Well, unless you have a machine shop and are prepared to modify a pipe or make an entirely new one, it won't help you until you buy a new pipe. But you can do one thing with your existing tuned pipe that can improve performance: tune it! That's right; you can tune a tuned pipe!
The header's length has a significant impact on performance. It's much like a trombone: you slide it out farther, and it resonates at a lower pitch. Slide it out less, and it resonates at a higher pitch—same thing with tuned pipes. Experiment by cutting 1/8 inch at a time off the header, and run the engine between cuts. The idea is to keep cutting until, as the boys from across the pond say, it's really "on song." That means the engine and exhaust system are in sync and singing in harmony. This is when the engine makes the most power. The rule of thumb is that a longer header is better for bottom-end power, while a shorter header makes your engine scream on the top end. You can go too far in either direction, so don't be reckless; you'll do more harm than good.
WHICH TUNED PIPE IS BEST?
So, you've decided that you want a tuned pipe; inevitably, your next question is "Which one should I get?" Man, is that a loaded question!
Its intended use will determine how long you'll want to research your pipe purchase. If you want to improve your vehicle's performance just to have fun at a faster pace, then any header/tuned-pipe combination will do. Choose one that is properly sized for your engine, of course, but you almost can't go wrong if you switch from a muffler to a tuned pipe. You may not get all the power that could be expected if you had spec'd the ideal setup, but the point I'm trying to get across is that anything is better than a muffler; mufflers are for leaf blowers!
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