Tuning O.S. 46 w/tuned pipe
09-03-2002, 09:11 PM
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Tuning O.S. 46 w/tuned pipe
I have a H9 Aresti 40 that is powered by an O.S. .46FX with a Mac's pretuned muffled pipe. Since I have never had a piped engine before I need some advice on how to adjust the engine for peak RPM and proper mixture.
Currently I am running an APC 11 x 5 prop, O.S. A3 plug and Wildcat 15% fuel in the engine. With this combo it will tach 14,400 RPM once the needle is at the max lean setting and backed off about 2 clicks rich.
The engine had previously had about a gallon of fuel run through it since new prior to installing the pipe. My major concern is the exhaust residue is really grey and dark, in the past this usually indicated high wear with metal particles in the residue responsible for the dark color.
I have another .46FX on a great planes Big Stick 40 that has had about 5 gallons through it and the residue is pink like the color of the fuel so this has me a bit concerned. With the exception of the pipe installation on the Aresti the two engines run the same prop, fuel and plug.
Any help would be greatly appreciated.
Scott
Currently I am running an APC 11 x 5 prop, O.S. A3 plug and Wildcat 15% fuel in the engine. With this combo it will tach 14,400 RPM once the needle is at the max lean setting and backed off about 2 clicks rich.
The engine had previously had about a gallon of fuel run through it since new prior to installing the pipe. My major concern is the exhaust residue is really grey and dark, in the past this usually indicated high wear with metal particles in the residue responsible for the dark color.
I have another .46FX on a great planes Big Stick 40 that has had about 5 gallons through it and the residue is pink like the color of the fuel so this has me a bit concerned. With the exception of the pipe installation on the Aresti the two engines run the same prop, fuel and plug.
Any help would be greatly appreciated.
Scott
09-03-2002, 09:19 PM
#2
Tuning O.S. 46 w/tuned pipe
Tuned Pipes
A Tuned Pipe is something that mystifies many modelers. They all know that the “top” flyers use tuned pipes on their airplanes, and that racing cars and boats use them to get top performance. What modelers don’t know is what the pipe actually is and how it works. They just see that great big, odd-looking thing hanging on the engine, and are told that it gives a very noticeable increase in power.
When the average modeler hangs a tuned pipe on his engine, he’ll most likely get very little increase in the engine’s power output, if at all. Why?
The secret to a tuned pipe is in the word “tuned”. It is like an organ pipe in the sense that it must be a specific length to be any good at all.
A few pointers...
There is no such thing as a pre-tuned, tuned-pipe assembly. Some manufacturers advertise that their pipes are just “bolt-on”. All that means is that the manufacturer is offering a broadly tuned pipe system that should work in most cases. It’s not a sure thing. For proper operation, every tuned-pipe installation must be tuned for the engine/propeller/fuel combination. Change prop or fuel, and the pipe will have to be re-tuned.
If a pipe is not adjusted properly, it could result in the engine running too rich or too lean. If the pipe system is too short for the RPM the engine runs, it will make the engine run too lean, which will cause damage to the engine. If the pipe system is too long for the RPM the engine runs, the engine will trend rich, which will result in carbon buildup and wasted fuel.
How it works:
When a two-stroke engine runs, it lets out a burst of exhaust once every revolution of the engine. This burst of exhaust creates a high-pressure sound wave at the frequency of the engine’s RPM. So if an engine is turning 13,000 RPM, the sound has a frequency of 13,000 cycles per minute, or 216 Hz, which is just below middle C on a piano keyboard. The high pressure of the sound wave is what our ears interpret as the loudness of the sound. Because it is a very high-pressure sound wave, we hear it as very loud. The result is that we can use the wave behavior of the exhaust to help the engine generate more power.
A tuned pipe is typically has a constant-size header pipe, which connects to the pipe body. The pipe body is made of metal and has a diverging cone connected to a converging cone, which ends in a small tailpipe.
The pipe works in the following manner: Each time the exhaust port opens, it expels a puff of high-pressure exhaust gas. This high-pressure pulse goes into the diverging cone of the pipe and slows down as it expands in the pipe. It also loses pressure and cools down. When it hits the converging pipe section, the sound wave is reflected back towards engine. The exhaust gases, themselves, continue out of the pipe, slower and cooler when they went in. The slowing and cooling of the exhaust gasses help muffle the noise.
The sound wave, though, is what does the work. When the engine emits the exhaust gases in a high-pressure pulse, there is a low-pressure area immediately behind that high-pressure pulse. If the exhaust port of the engine stays open long enough, not only does that low-pressure area help suck out any remaining exhaust gases from inside the cylinder, but it also pulls some of the new, fresh fuel/air mixture into the cylinder, and even out into the manifold a bit.
A sound wave consists of high and low-pressure pulses. The sound wave that was reflected back from the tuned pipe will eventually come back to the engine. That wave has a high-pressure area. If it hits the fresh fuel/air mixture at the right time, it will push it back into the engine just before the exhaust port closes. Because the pressure waves pulled some of the fuel/air mixture out of the engine, the fresh mixture that is pushed back in is above what the engine could do for itself, so the result is a kind of “supercharging” of the engine. The extra fuel/air mixture crammed into the engine is what gives it the extra power a tuned pipe is known for.
For all of this to happen, the timing has to be exactly right. That means that the engine has to be turning at just the right RPM for the pipe to work. Or, conversely, the pipe must be the right size for all of this to happen. The engine must also be designed so that the intake and exhaust ports are cut correctly so that the engine will benefit from a tuned pipe.
To repeat, a tuned pipe sets up an alternating series of high and low pressure just outside of the exhaust port of the engine. When the exhaust port opens, the expelled gases come out into a low-pressure area, which helps pull the gases out. Some of the fresh intake charge is also pulled out. As the exhaust port closes, a high-pressure wave comes down the header and pushes the fresh intake charge back into the engine. The engine gets a bit of supercharging.
Tuned pipes only work within a relatively narrow RPM range. If the engine is running faster or slower than that range, the engine gets no benefit from having a pipe. Also, if the engine wasn’t designed for pipe use, then there’s also no benefit.
Tuning a Pipe
When a pipe is installed, it must be tuned to the engine. To begin, it must be left a bit long. This ensures that the pipe will be tuned correctly.
The engine must be started, allowed to warm up, and then run at full-throttle with the mixture peaked. The RPM must be tached. Stop the engine. Shorten the pipe assembly by 1/4". Restart the engine, peak it, tach it, and stop it. There should have been a increase in the engine’s RPM after the tuned pipe assembly was shortened.
This procedure is done over and over. At some point, the RPM will not increase when the pipe is shortened. Back off 1/4" and clamp it all down. Fly the airplane (or drive the car or boat). Listen to it carefully. The pipe may need to be adjusted a bit to make sure the engine’s running “on the pipe”. There is a very distinctive change in the sound of the engine when it “comes onto the pipe”.
A tuned pipe cannot be tuned by running the engine and sliding the pipe in its silicon tube to shorten it. This “trombone” type of tuning will work for that run, but won’t work once the engine has been stopped. The engine must be stopped between adjustments.
Some modelers use the pipe to help the midrange, if it’s too rich. You’ll sacrifice top-end performance, though. Peak the needle for full throttle, then set the throttle to the midrange setting where you wish to run and tune the pipe as above. With some pipes, this may not work because a pipe that works for the full-throttle RPM and exhaust volume of your engine may have a too-large volume for the midrange of the engine, and the tuning effects may not work well.
Types of Tuned Pipes
There are a few variants of tuned pipes that are seen around. The most well-known is the double-cone tuned pipe. Properly-made, these pipes allow the engine to develop the highest power it can. They can be quite loud, though. They may not be as effective as a good “can” type muffler, as far as quieting is concerned.
Next, is the “silenced” tuned pipe. These pipes have a diverging cone section that blends into a constant-diameter cylindrical section. The end of the cylinder is capped, with a small tailpipe coming out of its center. This type of tuned pipe will not give as much of a “supercharging” effect as the double-coned type, but it will be much more effective in quieting the exhaust noise. Sometimes silenced pipes are really a double-coned pipe with a large shield around them. The pipe’s outlet is into the shielded area, and then let out the tailpipe. This type isn’t used much because it’s just too heavy.
A third type of pipe is the “mini” tuned pipe. This type looks like a rather large version of a muffler, but it’s attached to a long header pipe. Sometimes, these pipes look like regular tuned pipes that have been shortened a bit. Because they use the wave effects of the exhaust note, they are tuned exhaust systems, but not really true tuned pipes. They make exhaust flow from the engine more effective, which helps the power output. They don’t really do much as far as the supercharging effect is concerned. It takes a full-sized double-coned pipe to be best there. The mini-pipes are used where space or weight is a serious concern.
---
Finally, the main thing to remember about tuned pipes is that they aren’t really suitable for most modelers’ uses. They require quite a bit of work to get them properly installed and tuned. Once
tuned, the modeler must keep using the same combination of fuel, engine, and propeller on that one model. Any change means that the tuning process must be done all over again. This not something for a novice modeler to attempt.
__________________
Sincerely,
Mrs. AnnMarie Cross
Senior Manager, Proprietary Services and Support
Great Planes Model Distributors
[email protected]
www.greatplanes.com
www.bestrc.com
A Tuned Pipe is something that mystifies many modelers. They all know that the “top” flyers use tuned pipes on their airplanes, and that racing cars and boats use them to get top performance. What modelers don’t know is what the pipe actually is and how it works. They just see that great big, odd-looking thing hanging on the engine, and are told that it gives a very noticeable increase in power.
When the average modeler hangs a tuned pipe on his engine, he’ll most likely get very little increase in the engine’s power output, if at all. Why?
The secret to a tuned pipe is in the word “tuned”. It is like an organ pipe in the sense that it must be a specific length to be any good at all.
A few pointers...
There is no such thing as a pre-tuned, tuned-pipe assembly. Some manufacturers advertise that their pipes are just “bolt-on”. All that means is that the manufacturer is offering a broadly tuned pipe system that should work in most cases. It’s not a sure thing. For proper operation, every tuned-pipe installation must be tuned for the engine/propeller/fuel combination. Change prop or fuel, and the pipe will have to be re-tuned.
If a pipe is not adjusted properly, it could result in the engine running too rich or too lean. If the pipe system is too short for the RPM the engine runs, it will make the engine run too lean, which will cause damage to the engine. If the pipe system is too long for the RPM the engine runs, the engine will trend rich, which will result in carbon buildup and wasted fuel.
How it works:
When a two-stroke engine runs, it lets out a burst of exhaust once every revolution of the engine. This burst of exhaust creates a high-pressure sound wave at the frequency of the engine’s RPM. So if an engine is turning 13,000 RPM, the sound has a frequency of 13,000 cycles per minute, or 216 Hz, which is just below middle C on a piano keyboard. The high pressure of the sound wave is what our ears interpret as the loudness of the sound. Because it is a very high-pressure sound wave, we hear it as very loud. The result is that we can use the wave behavior of the exhaust to help the engine generate more power.
A tuned pipe is typically has a constant-size header pipe, which connects to the pipe body. The pipe body is made of metal and has a diverging cone connected to a converging cone, which ends in a small tailpipe.
The pipe works in the following manner: Each time the exhaust port opens, it expels a puff of high-pressure exhaust gas. This high-pressure pulse goes into the diverging cone of the pipe and slows down as it expands in the pipe. It also loses pressure and cools down. When it hits the converging pipe section, the sound wave is reflected back towards engine. The exhaust gases, themselves, continue out of the pipe, slower and cooler when they went in. The slowing and cooling of the exhaust gasses help muffle the noise.
The sound wave, though, is what does the work. When the engine emits the exhaust gases in a high-pressure pulse, there is a low-pressure area immediately behind that high-pressure pulse. If the exhaust port of the engine stays open long enough, not only does that low-pressure area help suck out any remaining exhaust gases from inside the cylinder, but it also pulls some of the new, fresh fuel/air mixture into the cylinder, and even out into the manifold a bit.
A sound wave consists of high and low-pressure pulses. The sound wave that was reflected back from the tuned pipe will eventually come back to the engine. That wave has a high-pressure area. If it hits the fresh fuel/air mixture at the right time, it will push it back into the engine just before the exhaust port closes. Because the pressure waves pulled some of the fuel/air mixture out of the engine, the fresh mixture that is pushed back in is above what the engine could do for itself, so the result is a kind of “supercharging” of the engine. The extra fuel/air mixture crammed into the engine is what gives it the extra power a tuned pipe is known for.
For all of this to happen, the timing has to be exactly right. That means that the engine has to be turning at just the right RPM for the pipe to work. Or, conversely, the pipe must be the right size for all of this to happen. The engine must also be designed so that the intake and exhaust ports are cut correctly so that the engine will benefit from a tuned pipe.
To repeat, a tuned pipe sets up an alternating series of high and low pressure just outside of the exhaust port of the engine. When the exhaust port opens, the expelled gases come out into a low-pressure area, which helps pull the gases out. Some of the fresh intake charge is also pulled out. As the exhaust port closes, a high-pressure wave comes down the header and pushes the fresh intake charge back into the engine. The engine gets a bit of supercharging.
Tuned pipes only work within a relatively narrow RPM range. If the engine is running faster or slower than that range, the engine gets no benefit from having a pipe. Also, if the engine wasn’t designed for pipe use, then there’s also no benefit.
Tuning a Pipe
When a pipe is installed, it must be tuned to the engine. To begin, it must be left a bit long. This ensures that the pipe will be tuned correctly.
The engine must be started, allowed to warm up, and then run at full-throttle with the mixture peaked. The RPM must be tached. Stop the engine. Shorten the pipe assembly by 1/4". Restart the engine, peak it, tach it, and stop it. There should have been a increase in the engine’s RPM after the tuned pipe assembly was shortened.
This procedure is done over and over. At some point, the RPM will not increase when the pipe is shortened. Back off 1/4" and clamp it all down. Fly the airplane (or drive the car or boat). Listen to it carefully. The pipe may need to be adjusted a bit to make sure the engine’s running “on the pipe”. There is a very distinctive change in the sound of the engine when it “comes onto the pipe”.
A tuned pipe cannot be tuned by running the engine and sliding the pipe in its silicon tube to shorten it. This “trombone” type of tuning will work for that run, but won’t work once the engine has been stopped. The engine must be stopped between adjustments.
Some modelers use the pipe to help the midrange, if it’s too rich. You’ll sacrifice top-end performance, though. Peak the needle for full throttle, then set the throttle to the midrange setting where you wish to run and tune the pipe as above. With some pipes, this may not work because a pipe that works for the full-throttle RPM and exhaust volume of your engine may have a too-large volume for the midrange of the engine, and the tuning effects may not work well.
Types of Tuned Pipes
There are a few variants of tuned pipes that are seen around. The most well-known is the double-cone tuned pipe. Properly-made, these pipes allow the engine to develop the highest power it can. They can be quite loud, though. They may not be as effective as a good “can” type muffler, as far as quieting is concerned.
Next, is the “silenced” tuned pipe. These pipes have a diverging cone section that blends into a constant-diameter cylindrical section. The end of the cylinder is capped, with a small tailpipe coming out of its center. This type of tuned pipe will not give as much of a “supercharging” effect as the double-coned type, but it will be much more effective in quieting the exhaust noise. Sometimes silenced pipes are really a double-coned pipe with a large shield around them. The pipe’s outlet is into the shielded area, and then let out the tailpipe. This type isn’t used much because it’s just too heavy.
A third type of pipe is the “mini” tuned pipe. This type looks like a rather large version of a muffler, but it’s attached to a long header pipe. Sometimes, these pipes look like regular tuned pipes that have been shortened a bit. Because they use the wave effects of the exhaust note, they are tuned exhaust systems, but not really true tuned pipes. They make exhaust flow from the engine more effective, which helps the power output. They don’t really do much as far as the supercharging effect is concerned. It takes a full-sized double-coned pipe to be best there. The mini-pipes are used where space or weight is a serious concern.
---
Finally, the main thing to remember about tuned pipes is that they aren’t really suitable for most modelers’ uses. They require quite a bit of work to get them properly installed and tuned. Once
tuned, the modeler must keep using the same combination of fuel, engine, and propeller on that one model. Any change means that the tuning process must be done all over again. This not something for a novice modeler to attempt.
__________________
Sincerely,
Mrs. AnnMarie Cross
Senior Manager, Proprietary Services and Support
Great Planes Model Distributors
[email protected]
www.greatplanes.com
www.bestrc.com
09-03-2002, 09:22 PM
#3
Tuning O.S. 46 w/tuned pipe
TUNED PIPES
Chuck Auger, Pampa, Texas
Pipes work off of reflected waves, so props that turn the same RPM can run on the same length pipe.
If you are moving your pipe 1.5" and do not notice any difference, you may not be on the pipe to begin with. Without running them, I would think the RPM difference between a 9-7 and 10-6 wouldn't be much (if they are of the same brand). The same pipe length would probably be OK.
Also, if you are running a LA 40, this engine has very conservative timing and may not show much of a gain with a pipe.
Here is a little article passed on via the internet by ChuckN et all…. it will answer a lot of questions about pipe length, but be warned…. you will need a good tach and a degree wheel to be able to use it.
*************************************************
”This is how I get it done. Do you have a test stand? I hope so, as this makes the job tremendously easier. First off, pick a prop that is right for your plane. Be sure to watch your tip speeds for sake of prop integrity (See below). You really shouldn’t spin sport props faster than the rpms listed below for fear of losing blades. (Remember, props unload a good bit in the air!) Special competition racing props, like APC D-props, would be far safer above these rpms.”
Divide 172,200 by the diameter of the prop and that will give you the maximum rpm to keep the propeller tip speed below approximately 512 mph. This is necessary to keep the tip of your propeller far enough below the speed of sound in order to keep efficiency up and noise down. This also will help to maintain the integrity of the prop.
8 inch dia. - 21,500rpm
9 inch dia. - 19,100rpm
10 inch dia. - 17,200rpm
11 inch dia. - 15,600rpm
12 inch dia. - 14,300rpm
13 inch dia. - 13,200rpm
14 inch dia. - 12,300rpm
15 inch dia. - 11,500rpm
16 inch dia. - 10,800rpm
17 inch dia. - 10,100rpm
18 inch dia. - 9,600rpm
19 inch dia. - 9,000rpm
20 inch dia. - 8,600rpm
21 inch dia. - 8,200rpm
22 inch dia. - 7,800rpm
23 inch dia. - 7,500rpm
24 inch dia. - 7,200rpm
”That being said, mount the prop of your choice and run the engine with an open exhaust. (Be sure to wear ear protection!) Measure the RPM you get at max power. This RPM reading is what you will use for your starting point in pipe length.
True pipe length is measured from the skirt of the piston to the mean reflection point of the converging cone of the pipe. On an unmuffled pipe, like the Anitro pipes@ the boat racers use, the mean reflection point is easy to find as it is halfway down the length of the converging cone (which is the back half of the pipe). On a Macs muffled pipe the converging cone is covered up so you will have to use some ingenuity here. The formula for pipe length is Exhaust Duration times Wave Speed divided by RPM. I will use an O.S. 46VX-DF engine as an example. If I remember right, an O.S. 46VX-DF has an exhaust duration of 170 degrees. (You are going to need a degree wheel to find the exhaust duration of your engine.) 1450 ft/sec is the usual value for the wave speed in a glow motor. So if this O.S. turned 16,000rpm with an 8 inch prop and open exhaust then calculating pipe length would be as follows:
Pipe Length = 170 degrees X 1450 ft per second / 16,000rpm = 15.4 or about 15 3/8 inches.
Now we need to cut the header the appropriate amount to obtain a true pipe length of 15 3/8 inches and hook the whole exhaust system up to our engine and run it. More than likely it will turn a few hundred RPM higher than it did without the pipe. If not, we would need to adjust the 1450 ft/sec in the above formula to a value more accurate for your conditions (ambient air temp, % nitro, etc). This formula will calculate Wave Speed:
Wave Speed = Pipe Length X RPM / Exhaust Duration
Now we need to decide what RPM we want the engine to turn on the ground. (Be realistic here!) An 8 inch sport prop is safe up to about 21,000 RPM so we probably want to shoot for 18,500 on the ground. Using the Pipe Length formula you get:
Pipe Length = 170 degrees X 1450 ft per second / 18,500 RPM = 13.3 or about 13 3/8 inches.
In this particular situation, we would need to cut 2 inches from the header. The reason I went through all of this is that this system of finding optimum pipe length can be used on any engine with any size prop. And it sure beats cutting off 1/4 inch at a time and running it!
JUST REMEMBER THIS: If your engine is difficult to needle and won’t settle down at one RPM when leaned out, then the pipe is TOO SHORT! Go to a smaller pitch prop or lengthen the pipe before you burn your engine up!
**************************************************
I have checked some pipe lengths against this formula, and was coming up with my pipes being too short. Then I re-calculated using the estimated RPM's of the engines unloaded in the air...and my pipes were almost exactly the lengths calculated in this formula. So whip out that tach and degree wheel and have fun!”
The trick is to keep shortening the pipe 1/4" at a time and re-tune after every adjustment. When the RPM's quit going up, you can either quit shortening or back it out a 1/4". Trying to slide the pipe in and out with the engine running has not worked very well for me.
And remember...a pipe that is too long is just a muffler.. it won't hurt anything, but a pipe that is too short can make life miserable and is not too good for your engine. So you might set your pipe for the slower prop and then you can use the faster prop without worries. It won't be getting quite the boost, but it won't hurt anything.
But the bottom line is just try to run props that turn about the same RPM, or if you happen upon a length that works for a slower prop make a spacer out of header material and stick it in between the header and pipe when you run that prop.
This is why I try to run the same prop after I get a pipe set…. or one pretty dang close.
The multi-chambered pipes, like the quite pipes, I have dissected were just a regular double-cone pipe with a muffler chamber tacked on the rear. They should tune just like a non-muffled pipe. If in doubt, just leave it a little long...you won't get every last RPM, but should get a nice boost (at least not lose RPM like a muffler).
A folded tuned pipe is exactly what an Ultra Thrust or Jett muffler (or even the mousse can muffler) is. They are basically a folded tuned pipe. And they have a pretty broad range compared to a pipe. I look around and I see 4 tuned pipes and 5 UT mufflers...LOL. According to A. Graham Bell who has many of his designs used on 2 cycle race bikes since the early 70,s and the writer of the book two stroke performance tuning you can bend a tuned pipe around a lot as long as the measurements stay the same. The waves will follow a curve very well. A critical area is the header pipe which is very sensitive to dents because of the high speeds of the exhaust gasses. The main thing is all the curves are smooth and not abrupt or they will reflect an errant wave.
I was under the impression that 4strokes don't benefit much from exhaust tuning, if at all. The theory behind a tuned pipe is to create a scavenging effect at the exhaust port by tuning the pipe length to position the low pressure region of the acoustic standing wave right at the exhaust port, thereby allowing more intake charge to enter the combustion chamber to yield more power.
Since the overlap of the typical 4stroke engine intake and exhaust valve opening is rather short, the effect of this scavenging is rather small. I know full-size auto racers still do exhaust tuning, but that is on race engines having high valve lift durations and overlaps.
I believe just having a low restriction (i.e. low back pressure) is plenty good enough for boosting 4stroke power.
My older MVVS 40 has a rather peaky mini tuned pipe (tuned silencer). It was easy noticed that very little happened below --- say -- 12 000 rpm. at 13 000 rpm and little higher it was really powerful.
Basically the pipe needs have a longer manifold to give good performance at low rpm. As an example the Weston torque minipipe available from Justengines UK seem to work already at 10 000 rpm. Very good for hovering and 3D. Also the measurements of the silencer can matter.
The design of a minipipe is somewhat complicated but a full length pipe is very easy to tune to any rpm. If the engine runs hot, has critical needle, or burns plugs, the pipe needs to be longer. If power is down, shorten it. A tach is a big help in this process. Aim for max rpm and then lengthen the pipe slightly to suit your flying style.
To me it seems that 11x4 is a pretty small propeller for a 45. 12x4, or 12x3,75 is much more load and you may need to lengthen the minipipe manifold slightly. Also try APC 11,5x4.
How do you know the pipe is working? It should be more or less easy to notice a step in rpm. Some pipes have a big step, but the good ones are quite linear. For 3D you want to have the pipe working fairly low at the power curve, say at 2/3 rpm. You do not max speed or power, you want good power with good throttle response at midrange!
Do not worry. It sounds complicated, but in practice the minipipe will work just fine.
Chuck Auger, Pampa, Texas
Pipes work off of reflected waves, so props that turn the same RPM can run on the same length pipe.
If you are moving your pipe 1.5" and do not notice any difference, you may not be on the pipe to begin with. Without running them, I would think the RPM difference between a 9-7 and 10-6 wouldn't be much (if they are of the same brand). The same pipe length would probably be OK.
Also, if you are running a LA 40, this engine has very conservative timing and may not show much of a gain with a pipe.
Here is a little article passed on via the internet by ChuckN et all…. it will answer a lot of questions about pipe length, but be warned…. you will need a good tach and a degree wheel to be able to use it.
*************************************************
”This is how I get it done. Do you have a test stand? I hope so, as this makes the job tremendously easier. First off, pick a prop that is right for your plane. Be sure to watch your tip speeds for sake of prop integrity (See below). You really shouldn’t spin sport props faster than the rpms listed below for fear of losing blades. (Remember, props unload a good bit in the air!) Special competition racing props, like APC D-props, would be far safer above these rpms.”
Divide 172,200 by the diameter of the prop and that will give you the maximum rpm to keep the propeller tip speed below approximately 512 mph. This is necessary to keep the tip of your propeller far enough below the speed of sound in order to keep efficiency up and noise down. This also will help to maintain the integrity of the prop.
8 inch dia. - 21,500rpm
9 inch dia. - 19,100rpm
10 inch dia. - 17,200rpm
11 inch dia. - 15,600rpm
12 inch dia. - 14,300rpm
13 inch dia. - 13,200rpm
14 inch dia. - 12,300rpm
15 inch dia. - 11,500rpm
16 inch dia. - 10,800rpm
17 inch dia. - 10,100rpm
18 inch dia. - 9,600rpm
19 inch dia. - 9,000rpm
20 inch dia. - 8,600rpm
21 inch dia. - 8,200rpm
22 inch dia. - 7,800rpm
23 inch dia. - 7,500rpm
24 inch dia. - 7,200rpm
”That being said, mount the prop of your choice and run the engine with an open exhaust. (Be sure to wear ear protection!) Measure the RPM you get at max power. This RPM reading is what you will use for your starting point in pipe length.
True pipe length is measured from the skirt of the piston to the mean reflection point of the converging cone of the pipe. On an unmuffled pipe, like the Anitro pipes@ the boat racers use, the mean reflection point is easy to find as it is halfway down the length of the converging cone (which is the back half of the pipe). On a Macs muffled pipe the converging cone is covered up so you will have to use some ingenuity here. The formula for pipe length is Exhaust Duration times Wave Speed divided by RPM. I will use an O.S. 46VX-DF engine as an example. If I remember right, an O.S. 46VX-DF has an exhaust duration of 170 degrees. (You are going to need a degree wheel to find the exhaust duration of your engine.) 1450 ft/sec is the usual value for the wave speed in a glow motor. So if this O.S. turned 16,000rpm with an 8 inch prop and open exhaust then calculating pipe length would be as follows:
Pipe Length = 170 degrees X 1450 ft per second / 16,000rpm = 15.4 or about 15 3/8 inches.
Now we need to cut the header the appropriate amount to obtain a true pipe length of 15 3/8 inches and hook the whole exhaust system up to our engine and run it. More than likely it will turn a few hundred RPM higher than it did without the pipe. If not, we would need to adjust the 1450 ft/sec in the above formula to a value more accurate for your conditions (ambient air temp, % nitro, etc). This formula will calculate Wave Speed:
Wave Speed = Pipe Length X RPM / Exhaust Duration
Now we need to decide what RPM we want the engine to turn on the ground. (Be realistic here!) An 8 inch sport prop is safe up to about 21,000 RPM so we probably want to shoot for 18,500 on the ground. Using the Pipe Length formula you get:
Pipe Length = 170 degrees X 1450 ft per second / 18,500 RPM = 13.3 or about 13 3/8 inches.
In this particular situation, we would need to cut 2 inches from the header. The reason I went through all of this is that this system of finding optimum pipe length can be used on any engine with any size prop. And it sure beats cutting off 1/4 inch at a time and running it!
JUST REMEMBER THIS: If your engine is difficult to needle and won’t settle down at one RPM when leaned out, then the pipe is TOO SHORT! Go to a smaller pitch prop or lengthen the pipe before you burn your engine up!
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I have checked some pipe lengths against this formula, and was coming up with my pipes being too short. Then I re-calculated using the estimated RPM's of the engines unloaded in the air...and my pipes were almost exactly the lengths calculated in this formula. So whip out that tach and degree wheel and have fun!”
The trick is to keep shortening the pipe 1/4" at a time and re-tune after every adjustment. When the RPM's quit going up, you can either quit shortening or back it out a 1/4". Trying to slide the pipe in and out with the engine running has not worked very well for me.
And remember...a pipe that is too long is just a muffler.. it won't hurt anything, but a pipe that is too short can make life miserable and is not too good for your engine. So you might set your pipe for the slower prop and then you can use the faster prop without worries. It won't be getting quite the boost, but it won't hurt anything.
But the bottom line is just try to run props that turn about the same RPM, or if you happen upon a length that works for a slower prop make a spacer out of header material and stick it in between the header and pipe when you run that prop.
This is why I try to run the same prop after I get a pipe set…. or one pretty dang close.
The multi-chambered pipes, like the quite pipes, I have dissected were just a regular double-cone pipe with a muffler chamber tacked on the rear. They should tune just like a non-muffled pipe. If in doubt, just leave it a little long...you won't get every last RPM, but should get a nice boost (at least not lose RPM like a muffler).
A folded tuned pipe is exactly what an Ultra Thrust or Jett muffler (or even the mousse can muffler) is. They are basically a folded tuned pipe. And they have a pretty broad range compared to a pipe. I look around and I see 4 tuned pipes and 5 UT mufflers...LOL. According to A. Graham Bell who has many of his designs used on 2 cycle race bikes since the early 70,s and the writer of the book two stroke performance tuning you can bend a tuned pipe around a lot as long as the measurements stay the same. The waves will follow a curve very well. A critical area is the header pipe which is very sensitive to dents because of the high speeds of the exhaust gasses. The main thing is all the curves are smooth and not abrupt or they will reflect an errant wave.
I was under the impression that 4strokes don't benefit much from exhaust tuning, if at all. The theory behind a tuned pipe is to create a scavenging effect at the exhaust port by tuning the pipe length to position the low pressure region of the acoustic standing wave right at the exhaust port, thereby allowing more intake charge to enter the combustion chamber to yield more power.
Since the overlap of the typical 4stroke engine intake and exhaust valve opening is rather short, the effect of this scavenging is rather small. I know full-size auto racers still do exhaust tuning, but that is on race engines having high valve lift durations and overlaps.
I believe just having a low restriction (i.e. low back pressure) is plenty good enough for boosting 4stroke power.
My older MVVS 40 has a rather peaky mini tuned pipe (tuned silencer). It was easy noticed that very little happened below --- say -- 12 000 rpm. at 13 000 rpm and little higher it was really powerful.
Basically the pipe needs have a longer manifold to give good performance at low rpm. As an example the Weston torque minipipe available from Justengines UK seem to work already at 10 000 rpm. Very good for hovering and 3D. Also the measurements of the silencer can matter.
The design of a minipipe is somewhat complicated but a full length pipe is very easy to tune to any rpm. If the engine runs hot, has critical needle, or burns plugs, the pipe needs to be longer. If power is down, shorten it. A tach is a big help in this process. Aim for max rpm and then lengthen the pipe slightly to suit your flying style.
To me it seems that 11x4 is a pretty small propeller for a 45. 12x4, or 12x3,75 is much more load and you may need to lengthen the minipipe manifold slightly. Also try APC 11,5x4.
How do you know the pipe is working? It should be more or less easy to notice a step in rpm. Some pipes have a big step, but the good ones are quite linear. For 3D you want to have the pipe working fairly low at the power curve, say at 2/3 rpm. You do not max speed or power, you want good power with good throttle response at midrange!
Do not worry. It sounds complicated, but in practice the minipipe will work just fine.
09-03-2002, 09:37 PM
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Tuning O.S. 46 w/tuned pipe
As to the dark residue in the exhaust, this is usually an indicator of aluminum pieces vibrating against each other. Check that the header is tightened properly and that the header outlet and tuned pipe inlet are not touching or vibrating against each other. The tuned pipe should also be securely attached to your plane to avoid vibration.
09-04-2002, 01:49 AM
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Tuning O.S. 46 w/tuned pipe
I have a Mac's one piece pipe on my 46FX. Can't say I really noticed a big difference in performance. It was great already!
Mike
Mike
09-05-2002, 04:04 AM
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pipes
The black one peace Macs mufflers are not tuned pipes, they are simply a more effiecient muffler, sometimes they help RPM sometimes they don't , sometimes they help more with consistancy than anything.
