2 Cycle Resonant Exhaust Diverter Systems

After building and flying electric planes for many years, I'd become used to flying CLEAN planes. When I started building Wet Power planes, alI of the oil everywhere immediately caught my attention! I decided to try to set up to use a long exhaust & "crud diverter" pipe. I found in the process that if the internal volume of the add-on tube wasn't matched to the engine, it would detune the engine dramaticly!

However, if the exhaust diverter is sized properly, you can end up with all of the oil well back & directed down away from the plane, and not lose any RPM on most engines. (A GAIN in top end RPM is reported on some 2 Cycle engines!) The key variable factor seems to be INTERNAL VOLUME.

For a 2 cycle .46, such as the ASP .46 and Thunder Tiger Pro .46, I am useing a 12" piece of J'TEC 7/16" ID black neoptrene tube with a 8-1/2" long piece of 7/16" OD brass tube ; the brass is inserted into the neoprene 1/2", for an overall length of 20". The neoprene is then slid onto the end of the engine's stock muffler as far as it will go.

Splicing two pieces of the neoprene, which comes from J'TEC only in 12" lengths, results in a slightly lighter total installation, as seen below on the WILD HARE. For mounting, I use nylon cable bundle clamps, available from electronics supply sources.

Above is the WILD HARE with an ASP .46, exhaust diverter mounted on fuselage side.

This results in something I refer to as a RESONANT EXHAUST EXTRACTOR; it's not a tuned pipe, but you can "Tune" your own resonant system; it's done best by trial and error, starting with the following as aproximate lengths:

The 20" length works for me for the .46 size 2 cycle engines mentioned, with my prop selection, and the particular RPM that the particular prop turns; I fly at 9,000 to 12,000 feet elevation most of the time, however. so for your own running setup & environment, you may want to start on the test bench with a shorter piece of the 7/16" ID brass tube (or slide it farther into the neoprene temporarily); then slide another piece of mateing brass tube over the outside. With the engine running at full RPM, slide the outer brass tube back and forth to determine the optimum length. (A tachometer is most helpfull for fine tuneing.) A change of length of 1/4" longer or shorter than optimum will show a drop in RPM which you probably can't detect with your ears. Recheck your mixture adjustment at the top end while doing this tuning process.

For the .61 & .75 size 2 cycle engines, use 1/2" ID neoprene, and !/2" OD Brass, for about 22" total length; the same fine tuning process as above can be used.

Other materials have been tried by others in the area; vinyl tubing doesn't hold up well with the hot exhaust; the same is found with Polybutylene plumbing tubing. Automotive neoprene hoses are much heavier walled than that supplied by J'TEC. (If you come accross a bulk supplier of the 7/16" ID and 1/2" ID thin wall neoprene, please pass the word! )The sound dampening effect will be greater with softer tubing.

Starting with a long piece of hose and cutting 1/4" at a time until a Peak RPM is found would be another approach. The ASP 1.08's don't like anything I've tried so far; without anything added to the stock muffler, they run well, but definitely are oiling the DUSTER thoroughly. I'll start next with about 30" of automotive 1/2" ID hose, cutting back a bit at a time until I find a resonant length for this system. (Two fliers have related poor running when a "PITTS Style Muffler" was bolted to the ASP 1.08; the result was a badly detuned exhaust system, resulting in Destructive Resonance.

I'll report results of the tests on the ASP 1.08 when I complete them.

Above is the "FLOATMASTER" first prototype fuselage, exhaust diverter attached to direct exhaust and oil residue down away from the plane.

Planes using both sizes of exhaust diverters were flown at the London Bridge Seaplane Classic at Lake Havasu City, Arizona, with no changes; Props are used which turn in the 11,000 to 13,000 RPM range here at 10,000 feet.

An added advantage is the substantially reduced noise level; the length of neoprene tube damps out the exhaust noise of these 2 cycle engines very noticably. (Our club doesn't have a sound meter now, but if we get one for use in the area, I'll try to report in the future on the actual drops in measured noise levels.)

NITEOWL with side mounted TT Pro .61, diverter under belly.

Above is the NITEOWL, with a Thunder Tiger Pro .61 side mounted on the nose. The exhaust diverter system is mounted under the fuselage on this plane. (Another SKYFOXX fuselage is partly visable behind, again with the side mounted engine, and exhaust diverter running under the fuselage.)

Some engines can show an increase in top end RPM over the stock exhaust system; the K&B .65 Sportster was reported to gain significant RPM with around a 12" tube added, as reported in Clarence Lee's review article in RC Modeler Magazine many years ago.

Now it's time to stop talking about it, and go ahead and try it! A cleaner, quieter running 2 cycle engine on your plane can be your result; and the possible GAIN in RPM is certainly worth pursuing in the process.

Bruce Stenulson

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