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 "Mystery" 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 imperical 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 Selection of RPM range for 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 RPM at which you make Maximum HP and its EFFECT ON PIPE LENGTH:

The RPM at which you make Maximum HP also has a direct bearing on pipe length. Attached is a screen capture of a dyno run showing the upper limit of the pipe and what happens when the pipe runs out of rpm range. It looks like the power "drops off a cliff". You must decide the RPM at which you make Maximum HP. This is to a large degree dependent upon the horsepower output of your engine. If you have a stock engine with low compression ratio and low exhaust open duration, the RPM at which you make Maximum HP will be somewhat lower than the high output, high compression engine. Why is this so? In order to use an engine successfully in a rc application, it must have good low/mid/high end power. If you over tax your engine, or run a pipe which is excessively short, the engine will not be able to accomplish consistent running. If you run a .21 or smaller sized engine, you know that it is necessary to have good low/mid/high end power to go very fast. If you try to run a pipe that is to short, it will not have enough low end power. So, if you are running an engine that is stock you probably will run at the RPM at which you make Maximum HP of about 28000. If you are running a high compression engine with lots of nitro (60-70%) you will be able to run at 30000 - 35000. This difference of 2000-7000 rpm will necessitate a pipe shortening of .351" per 1000 rpm or in this case a shortening of .702 - 1.053". Unbelievable! The necessary shortening for each 1000 rpm movement in the RPM at which you make Maximum HP is as follows: 21 - .351", 45 - .413", 67 - .438", 90 - .536". This is the largest contributor to pipe length change.


In conclusion:

Exhaust Timing:

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".

Exhaust Temperature:

For every 10 degree rise in pipe gas temperature, you will have to lengthen your pipe as follows: 21 - .044", 45 - .050", 67 - .052", 90 - .059".

Compression Ratio:

For every 1 point increase in compression ratio, SHORTEN your pipe by an amount of 21 - .225", 45 - .250", 67 - .264", 90 - .299".

The RPM at which you make Maximum HP:

For every 1000 rpm increase in the RPM at which you make Maximum HP, the pipe must be shortened as follows: 21 - .351", 45 - .413", 67 - .438", 90 - .536".

All the information presented here can be learned by using the pipe design program built into "The Engine Analysis Software" from MWD & Associates.




ENGINE Analysis Software for the Serious RC Competitor



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