From the March 1967 issue of Model Airplane News

Tuned Pipes
A New Era In Speed

By Bill Wisniewski

This is an article on tuning the exhaust gases in a miniature two-stroke engine to produce an increase in power speed as well as effectively decrease the noise level. I hope that the information in this article can help further interest in speed as well as other events that put an emphasis on speed and power.

When done properly, a tuned exhaust manifold can not produce at least a 10% increase in power output, but will make an excellent muffler, reducing the noise level approximately 40%. This silencing action could bring back some of the lost flying fields and the power increases could make the silencer a popular item.

I have been working on exhaust tuning with Roger Theobald for the past five years. The initial experiments prompted by conversations with Jack Smith, a motorcycle enthusiast and old time model airplane enthusiast were, therefore, patterned along motorcycle practice.

We have had quite a few problems along the road to success starting from those first experiments. The first bench tests showed an immediate improvement so they were encouraging. However, we were unable to realize this gain in flight.

Progress was slow after these tests, but during late 1965 1966, prior to the Control Line World Championships in England, work in this direction was accelerated. Progress was made in two directions: first in improving the dimensions of the exhaust pipe and, second, in developing a technique which would let us realize the bench test potential in flight.

The dimensions of the first pipes were taken using a typical motorcycle proportions and scaling the length to model engine r.p.m. by using the exhaust gas wave velocity corresponding to gasoline fuel. These figures were found to be considerably higher than the correct one for alcohol. In addition to the difference in fuel, model engine fuel is much higher in oil content than that used in motorcycles, which also adds mass to the exhaust gas.

By trial and error and with the help of thermocouple equipment, we were able to measure the temperature along the length of a fairly successful pipe and estimate the average velocity for our engines. It was determined that the temperature is quite high (as high as 750^oF) and because of this, the exhaust pipe was insulated from the engine.

This insulation is a silicone rubber coupling and is constructed by casting General Electric RTV‑90 compound in a plastic mold which has been machined to the same dimensions as the end of the pipe. The RTV‑90 coupling is bonded to the pipe by priming it with G.E. ss4004. Without this treatment, the coupling will not adhere to the metal.

The temperature measurements and the lack of success in flight tests led us to suspect that the pipe was cooling off a great deal in the air and reducing the temperature of the exhaust gases. The reduced temperature reduces the wave velocity in the exhaust and effectively makes the pipe too long. The pipe was insulated with silicone rubber and this modification was fairly successful. The F.A.I. model jumped in speed from the low 140's to 150 m.p.h. Experiments. with various coatings continued until the presently used black Sperex VHT exhaust paint was tried. This coating resulted in the largest improvement in efficiency and resulted in speeds around 160 m.p.h. with the F.A.I. model on the standard 80‑20 fuel ( i.e. no nitromethane)

A few words on the principle of exhaust tuning are in order. The engine on the intake compression stroke pulls air and fuel into the crankcase and also compresses the fuel and air in the cylinder. The power and exhaust stroke is next. This is where we make use of the hot outgoing gases to scavenge the cylinder and pull the excess fuel and air in the crankcase through the engine, fill the cylinder and pull part of the mixture into the headpipe of the exhaust system. Then the pressure builds up in the pipe sending back a positive pressure just as the transfer port closes and the exhaust port is still open, thus, pushing the mixture in the headpipe back into the engine under positive pressure giving a supercharging effect.

Now that the principle is known, we will have to design a pipe for an engine. First, we must measure the volume in the crankcase with the piston at bottom center. From practical experience, I have found that the internal volume of the pipe should be about ten times the crankcase volume and the headpipe cross sectional area should be 1.6 times the exhaust port area. The next step is to find the length of the exhaust system excluding the tailpipe length. This is done by picking a useful RPM. This must be converted into time. To make it less complicated take the RPM and reduce it to revolutions per second (cps) by dividing RPM by 60 then divide cps by 1 to get the amount of time for one cycle.; 1/(cps)

Then we must figure the percentage of exhaust opening less the overlap or difference between the exhaust and transfer ports on the upstroke. For example, if an engine has 170deg exhaust opening and 130deg transfer opening, we have 40deg difference total then divided by two is 20deg .
170deg - 20deg = 150deg. Then divided by 360deg will give us the required percentage. Let's call this number in the formula (P). Now we must use a constant which is the speed of sound at the average exhaust temperature in inches per second. Practical experience has come up with 22000 in/sec for our constant.

Then to reduce this to a half wave divide by 2 so the formula resolves itself to

(P * 22000)/(2* c.p.s.)

For proportions see Figure 1.. These proportions are derived from experience also.

The tailpipe cross section area is 1/3 the cross section area of the headpipe and the length is the intersection of the convergent cone plus one diameter of the tailpipe.

Now to the engine: It must have no sub-piston induction, that is, at top center there should be no gap showing under the piston. The reason for this is that the pipe creates such a violent negative wave just after the exhaust opens that at top center you are pulling some of the crankcase charge into the pipe which leaves you with a very weak mixture in the crankcase with a decrease in power rather than an increase. Also, the more difference between the port heights on transfer and exhaust, the more range of RPM you have. For example, 5deg overlap = 1000 RPM range.

Construction of the pipe is not too difficult, but it is time consuming. All the pipes used were machined from aluminum and magnesium bar stock. A taper attachment for the lathe is handy, but not essential. Here are the steps we followed.

1. Bore the inside diameter of the headpipe to about 2 inches deep. Turn the outside diameter of the headpipe 1.5 inch in length. Thread the end of 1/8 inch. We used a forty pitch thread on the headpipe so that extensions can be made for varying conditions. Then turn a 1 inch diameter x 1 inch length. Face to 42 % L + 3/32nds inch. (See Figure 2.)

2. Reverse part hold on one inch diameter and bore press fit diameter for mating part 3/32 inch deep. Bore major inside diameter 1/8in deeper than press fit diameter. Set taper with a dial indicator. Bore taper blending at major inside diameter. (See Figure 3.)

3. Make a plug to fit the major inside diameter of pipe as shown in Figure 3. Hold on the headpipe with the plug in the end supported by a live center. Turn the outside taper to a .016 in, wall thickness. Then turn major outside diameter to major inside diameter plus .050. Blend outside taper to the headpipe. (See Figure 4.)

4. Make rear cone using the same procedure as the front cone except as shown in Figure 5.

5. Bond the two cones together using a good high temperature adhesive at the press fit joint.

The engine which was used in the World Champs and which has had the bulk of the development effort is a special engine of our own design which also uses some K&B 15 R components. During the past few months, however, we have been testing these systems on standard K&B engines with very gratifying results. We have gained up to 1500 rpm with no other change to the engine other than adding a tuned exhaust pipe. Experiments with raising the exhaust port are still going on, but could yield a further performance increase.

The photos show Experimental Pipes
the World Champs engine and and 29 R for comparison
TWA and 29R and the experimental KB 29 R

I believe that the tuned exhaust system can work on anysize of engine although there is a lot of development work for each new application. The tuned engine exhaust seems to have no effect whatever on the engine. We had 30 to 40 high-speed flights on each engine with no apparent wear at all. Fuel consumption is approximately 10% than normal even though the power output is increased considerably. We have gotten as many as twenty and never less than five flights per glow plug.

The engines are not hard to start adjusting the needle valve is quite different due to the garbled exhaust and reduction in noise. Once it is set however it does not have to be changed with each flight. There seems to be some controversy about the tuned exhaust system. The tuned exhaust is not a startling new concept. It has been used for a good years in several racing sports. Indianapolis racers, motorcycles, sports cars all use some form of exhaust tuning. Roger and I have merely applied a 30 year-old physics principle to model aeronautics.

True, this is not immediately available on the commercial market, but this has not stopped modelers before. They have a knack of turning the difficult task into an accomplished fact. This is where we progress in our sport.

Of course, you can go faster by using so called super fuels. Most of these fuel ingredients are prohibitive in cost as well as being difficult to obtain. If not handled with extreme care, they can be very dangerous to your health .

In this article, I have tried pass on to you modelers the benefit of our six yers of work and study. All construction details are given so that those who do not have the facilities to do the work may have someone else make it.

Most important, let us keep an open mind and not regress to the past, rather progress to the future.