What are tuned pipes and how do they work?
A Tuned Pipe is something that mystifies many modelers. They all know that the "top" fliers 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.