Tuned Pipe Info

This page will have info from various authors (used with their permission, of course) regarding the setup and feeding of tuned pipes, with an emphasis on the larger 2 stroke powerplants for pattern (F3A, Masters, etc).

If you'd like to contribute, please email me using the link at the bottom of the page.

First article comes from Ron Lockhart, who knows a thing or two about this stuff, and probably has forgotten more than many of us ever learn (yes, he's been around THAT long)

WHY DO I HAVE TO  "TUNE" A “TUNED” PIPE?    Isn't it already tuned?  

Fair question.  Tuned Pipe is an incomplete name.  A so-called "tuned pipe" is really one part of a “tuned exhaust system” for a two-stroke engine. The rest of the parts are the engine porting, prop, fuel, header, connecting couplers and the pipe.  It is the system that needs to be tuned.  

There are tuned pipes, and there are tuned/muffled pipes. Both are tuned the same way.  The engine inlet end of a tuned pipe tapers upward to a maximum diameter, then tapers more rapidly down to an exhaust outlet.  They are loud with a crackling, ringing sort of sound.  They are not suitable for our field.  A tuned-muffled pipe has the same taper from the inlet end to a maximum diameter, then stays at that diameter for several inches.  Then it tapers very rapidly, or even bluntly, to the exhaust outlet.  A tuned muffled pipe may be quieter than a stock muffler set up.  

The single most critical factor is the length of the pipe.  The length that matters is not what you see on the outside.  It is the distance from piston face around the curve of the header, to the first baffle in the pipe.  Or if the pipe does not have a baffle, to the point where the pipe first starts tapering down from it's maximum diameter.  That distance is what "tunes" the pipe to an rpm. The shorter that length, the higher the rpm it is set for.  

Several factors effect the length.  The diameter of the pipe, the taper ratio of the pipes that use a reverse cone type constriction, vs. the ones that use a baffle, the timing of the engine, compression ratio, etc.   There is a "black art" aspect of tuning a pipe, but also some rules of thumb that are useful while you experiment with different length and prop and needle valve setting combinations.  

Very approximate starting points - for 10,000 rpm use about 19 to 21 inches. For 12,500 rpm use about 14-15”.  

Here is a way to go about tuning the pipe:  

Start with pipe at guestimated length of way too long.  Test run the engine, then shorten the pipe and test run again.  Here is the engine behavior to expect with a pipe that's set too long and how it changes as you gradually shorten it.  (Assuming prop & fuel remain the same)  

Way long-         Little power gain.  No problems.  The Pipe is performing like a better-than-average muffler.

Some shorter-   Start to see a small power gain. 

Still shorter-      See even more power gain.  High-speed needle may get a little more sensitive. This is the length you may want for general sport flying, or aerobatics.

Still shorter-      Yet again more power gain, high-speed needle is probably getting sensitive.  Engine idles fine, but may run a bit ragged through mid range.  This is close to maximum power gain.  If you don't mind the ragged mid-range, and do want lots of power, this may be the right length for you. You may need a cooler glow plug to tolerate full throttle, high rpm operation.

Too Short-        The high-speed needle is very sensitive and maybe erratic.  The mid-range is inconsistent or poor all the time.  Engine tends to go overlean in the air.  May blow plug often.  May be very fast and powerful till it goes overlean.  

You can use the extra power of the tuned pipe set on the short side to turn the same prop faster, or set a bit long to turn a larger prop at same rpm.   Either way, performance of the model is improved.  

For instance, a .40 with standard muffler might turn a 10x6 at 12,000 rpm.  With a pipe set kind of short for maximum power, it might turn the 10x6 at 13,000.  You get lots more power, more fuel consumption, and more noise. Or, you could set the pipe a bit longer, and get the engine to turn an 11x6 at 12,000.  You still get more power, and only slightly more fuel burn, not much more noise.  Yet another choice, you could set it even longer, and turn an 11x7 at perhaps 11,000.  That would result in a large noise reduction, and little extra fuel burn.     

To find the baffle point, or where the reverse cone constriction starts, stick a wire in the pipe.  Tuned-muffled pipes have a muffling section behind the baffle or cone, so there may not be an exterior clue of where that point is.  

Exhaust pressure taps may be in the engine header, but will be most effective in the pipe at the point where it just gets to maximum diameter.  

The mid-range ragged running and richness problem - As the engine rpm comes up to the point where it gets "on the pipe" more power is produced, which takes more fuel.  When the high-speed needle setting is rich enough for maximum power on the pipe, it is likely too rich for the mid-range.  (Most of the mid 1980s vintage pattern ships with tuned pipe .60s set for max power had this classic problem.  Many got around the problem by using an in-flight mixture control.  They set the engine slightly lean for take off to fix the midrange richness, then richened the mixture as the engine got on pipe on the first high-speed pass.)  

There are other types of tuned exhaust systems.  There are so called "mini-pipes" or tuned mufflers.  There are also systems originally homemade from hair mousse cans that mount on a header.   These systems operate in a similar manner to tuned pipes, but often are not adjustable in length.  

Most "pre tuned" systems tend to be set up for nearly maximum power, and are therefore on the short side.  

Prop size changes are an alternative to changing pipe length.  If you have an engine that behaves as though the pipe is too short, changing to a smaller prop may improve engine behavior.  The smaller prop lets the engine turn at higher rpm, which should match better to a short pipe.

                                                                                            Ron Lockhart,  May 2002

Mail Bob here:  mailto:rcaerobob@cox.net