The Mystery of "Tuned Pipes"

4 Part Series

How to Determine Length




The "tuned pipe" or properly called Expansion Chamber has long been the subject of considerable mystery. After long and intense study, with verification on the Inertial Dynamometer, I believe I can simplify its application to our RC Hobby! This device is not a "Black Box" which can not be solved with mathematics, chemistry and physics. The definitive study completed by a research team at The Queens University of Belfast and presented in their Design and Simulation of Two-Stroke Engines gives emperical data upon which to solve this mystery. I have applied the mathematics and physics, as well as the chemistry of our fuel concoction, to their formulations and am able to solve these mysteries. I will present this in 4 parts to be discussed in detail. Part 1 addresses the effect of Exhaust Timing on the pipe length. Part 2 addresses the effect of Temperature on the Pipe Length. Part 3 addresses the effect of Compression Ratio on pipe length. Part 4 deals with the RPM at which you make Maximum HP. At the end of EACH of the 4 parts, I will provide BASIC STATEMENTS TO ALLOW YOU TO MAKE CHANGES IN LENGTH BASED ON SPECIFIC CRITERIA. If this seems to be much too technical, just look at the conclusions at the end of each of the 4 parts!


Length Defined: The tuned length of an expansion chamber MUST be accurately measured from the FACE OF THE PISTON to the START OF THE STINGER.



The compression ratio of the engine you are running has a DIRECT bearing on the length of the pipe you run! If you run a very low compression ratio, you will run your needle VERY LEAN in order to make sufficient horsepower to move your model. As you increase your compression ratio, the engine makes MORE HP, thus not having to work as hard. However; you must supply more fuel to the engine to make this horsepower. If you supply more fuel, the exhaust gas temperature goes down and you must SHORTEN your pipe.


An Example: My Nova Rossi 21 was run this past racing season, with a compression ratio 1 point below where I have been currently testing it on the Inertial Dyno. In order to supply this current increased compression ratio, I had to richen the needle considerably. By this richening, I lowered the exhaust gas temperature by 50 degrees. Using the 10 degree temperature pipe increments of 21 - .044", 45 - .050", 67 - .052", 90 - .059", it was necessary to SHORTEN my pipe 5 X .044" or .220" (almost a quarter of an inch)!!!


In conclusion for every 1 point increase in compression ratio, SHORTEN your pipe by an amount of 21 - .225", 45 - .250", 67 - .264", 90 - .299". All the information presented here can be learned by using "The Pipe Design Program" from MWD & Associates.



The effect of Operating RPM Range on Pipe Length………………….

ENGINE Analysis Software for the Serious RC Competitor



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