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 imperial 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!

A special thanks to several people who helped with technical information in the Pipe Program and continuing two-stroke research. Bob Bonahoom (chemistry), Tom Grannis (mathematics), Brian Callahan (engineering), Norris Sparks (machining), John Ackerman (facility), Joe Kramer (electronics & engineering).


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.



In simple terms for each 2 degrees of additional exhaust duration, the pipe should be lengthened as follows: 21 engines - .107", 45 engines - .131", 67 engines - .135", 90 engines - .147".


An example for an application of this: "I run a Nova Rossi .21 engine, on which I always set my exhaust duration at 183 degrees. The engine comes stock with an exhaust duration of 175 degrees. With my modified engine how much must I lengthen my tuned pipe to operate at the same rpm? 183 degrees – 175 degrees = 8 degrees. I told you that I must lengthen .107" for each 2 degrees, so I must lengthen 4 X .107" or .428" (That's over 3/8")!!!! The same example on a .91 engine would be 4 X .147" or .588"!!!"



The effect of Operating Temperature on Pipe Length………………….

ENGINE Analysis Software for the Serious RC Competitor



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