dwbebens
Posts: 207
Joined: 4/25/2005
From: Dickson,
TN, USA
Status: offline
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Over the years, I’ve designed a number of tuned pipes and applied them to engines from .15 glow engines all the way up to 340 cc two cylinder two stroke engines. I’ve also just bought tuned pipes and put them on various engines. In most cases, I’ve gotten around 30% power increases.
According to my copy of the “Two-stroke TUNER’S HANDBOOK”, the formula is:
Tuned Length = (E x V)/RPM where
Tuned Length = the distance (in inches) from the exhaust port window, at the piston face, measured along the exhaust system’s centerline to the mean point of reflection. This “mean point of reflection” is ½ way down the converging cone (also called the baffle cone). The baffle cone starts where the diameter just starts to decrease and ends at an imaginary point as if the cone ended at a sharp tip. Draw a 2D side view of the cone going to this imaginary sharp tip, and then find the ½ way point.
E = Exhaust open timing (in degrees). Measure it.
V = speed of sound (in feet\second) in the exhaust gasses (approximately = 1700 ft / second)
RPM = Revolutions/Minute
x is just the mathematical operator “times”
This formula seems surprisingly simple, and can be derived from theory fairly easily.
There are a couple other important features of full wave tuned pipes that have an effect on performance. The diverging cone (also called the diffuser cone) controls how broad the power band is before the peak power point. The sharper, more abruptly the cone tapers, the narrower the power band will be before peak power. The baffle cone on the other hand controls how broad the power band is after the peak power point. Again, the more sharply the cone tapers, the narrower the power band will be after the peak power point. The basic trade-off concerning these tapers is that you get narrow power band along with higher peak power with sharply tapered cones, and conversely, you get a broader power band along with lower peak power with shallowly tapered cones. By “before” the peak power point, I mean at lower RPMs. By “after” the peak power point, I mean at higher RPMs. There are a number of other factor which effect various aspects of the performance, but these are the main ones.
The final effect of all this is that the pipe’s power band seems to “pull” the RPM way up in a “jump” like manner, and thereafter keeps it locked in.
My procedure for determining a proper tuned pipe length is as follows:
1. In the first place, get in the ballpark with the correct prop size, using your experience or by copying what’s typically being used on the engine already. As DHG said, make sure that the prop can take the RPM and power that you will be applying by using a tuned pipe. Also, take great care to keep yourself clear and safe, as you normally would, right. Use a test stand. I too have had an unfortunate experience with a high velocity piece of prop! Some kind of safety glasses are a MUST!
2. Run the engine to peak “static RPM” with no muffler using this prop and record peak RPM.
3. Estimate the “flying RPM”; this depends on how clean and fast the plane is (for a clean fast plane this could be easily 20% or more greater than static RPM)
4. Add maybe 2,000 to 3,000 RPM to this estimated “flying RPM”. This added amount also depends how clean and fast the plane is. You now have the "in-air-on-pipe RPM".
5. Calculate the Tuned Length from the formula using the "in-air-on-pipe" RPM calculated from #4 above.
6. Trim or extend the header pipes so you get this Tuned Length.
7. Now comes the part where you have to be careful concerning needling. You have to be very careful how you needle a tuned pipe engine. The engine needs to be needled richer than you think. It will NOT be fully on the pipe when running on the ground. It should sound dirty-rich on the ground. DO NOT lean it to peak RPM. If you do lean it to peak on the ground, any subsequent richening may be false. You would need to throttle back down and then up again before re-setting the needle. At this “dirty-rich” setting, it will be running partially on the pipe - - it will be on the part of the power band before the peak. In other words, the diffuser cone will be dominant. After it is launched, as the speed increases, the RPM will come up as it unloads. At some point during this speed-up process, the RPM will get into the full effect range of the pipe and the RPM will abruptly come way up. It now should be running 2,000 to 3,000 RPM or more higher than it would have if it hadn’t had the pipe in the first place.
You DON’T want to adjust the Tuned Length for maximum RPM on the ground. It will just lock-in the RPM like a governor at a too low level, and stay that way in the air.
When it gets fully on the pipe in the air, it will proceed to a much more lean condition, hence the necessity of needling it rich on the ground.
Most of this procedure is geared toward a “speed” type plane installation. A full wave tuned pipe would NOT be good for a 3D type airplane because the RPM would “lock-in” and be difficult to control in a linear manner.
Example:
Tuned Length = (E x V)/RPM
I believe that E = 145 degrees for the Rossi 15, anyway, let’s say that it is
V = 1700
Let's say that static RPM = 22,000 and we estimate it would unload in the air to 25,000 RPM
Therefore "in-air-on-pipe" RPM = 25,000 + 2,000 = 27,000
And then; Tuned Length = (145 x 1700)/27000 = 9.13 inches.
Get ready for a really surprising jump in performance.
Doug Bebensee
< Message edited by dwbebens -- 11/1/2006 12:56:18 AM >
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Rossi .15 tuned pipe question - 
.... but I want more still... 
) 
And I think that pipe was originally intended for a CL speed engine. It may be old technology, but still... 
. Our rules limit 15% max nitro. The prop of choice was APC 7x6 wide. The stock 7x6 spins good to the ear but the 7x6W pulls better. 7x7 is too much, it falls outside the happy powerband.