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Written by Eric Henderson
The Engine Shop
As seen in the October 2012 issue of
Model Aviation.

This month, I will explore engine exhaust tuning. One of the most interesting aspects of being involved most of my life with internal-combustion engines is sound. There is something hardwired in us that causes us to react differently to different sounds, and differently to the same sound. A jet passing overhead is so commonplace today that most never look up unless it is low or unusually loud.

There is one sound that seems to have a built-in degree of sensitivity to our brains. Namely, the high-pitched sound of an insect that we instinctively know could hurt us. Flying insects that produce sounds such as the unmistakable whine of an “attack” mosquito, all the way to the lumbering tone of an incoming bumble bee “bomber,” will get our attention.

It may be that these primitive reflexes saved early humans. One thing for sure is that those reflexes are still there!

My theory is that the autonomic response to that kind of noise is why high-revving model engines have the ability to both draw attention and invoke reactions in and from people who live near our flying sites.

Before I go any further, it is worth stating that this is not an anti-engine-noise piece, but a deeper look at why our engines can annoy and why mufflers, designed correctly, improve the situation for all of us.

A soda can is used as a muffler extension, circa 1974, in an attempt to reduce exhaust noise without losing power.

Enjoying man-made noise is subjective. When I compare the noise of a Cox .049 at maximum rpm to the noise of six 12-cylinder Merlin engines at the Battle of Britain memorial flight (Lancasters, Hurricanes, and Spitfires) making low passes, I enjoy the Merlins.

The massive roar and throbbing of a World War II Rolls-Royce Merlin engine is music to my ears, but the Cox .049 is like fingernails on a blackboard! And yet, as a small boy, nothing was sweeter than getting that little CL engine to scream and fly in circles until I was too dizzy to stand.

As a Pattern pilot in the 1990s, I was required to get my 1.60 two- and four-stroke engines down to 90-decibel sound readings at 9 feet from the measuring meter. At the same time, I fought to get more power to haul my competition airplanes straight up without losing airspeed. If the airplane slowed down too much it would lose heading and points in the round.

Tuning an engine with an exhaust system was not a new concept. Most of the work was focused on getting more top-end power. What happened in the Pattern world was that the focus shifted to the mid-range and more torque at the lower top-end rpm. This was because a slower-turning propeller produces less noise.

Extreme silencing can be achieved using an exhaust header and a carbon-fiber tuned pipe to gain power while decreasing noise.

Larger propellers were used with higher pitch to get the same speeds as before with significantly less noise. Engines were heavily cowled, and even the carburetor intake noise was reduced with air filters similar to those in automobiles.

Let’s take a simplified look at what a glow-powered two-stroke engine does to produce exhaust, and the noise that goes with it. The engine draws a fuel/air mix from the carburetor into the main crankcase under the piston. It does this as the piston moves up the bore to explode the previously ingested fuel/air mix.

There is no timing system in a glow engine, so the gases explode when the right compression is reached to allow the glowing glow-plug element to set it off. (Gas engines do the same, except there is a spark plug instead of a glow plug. Spark plugs require external current to activate them.)

The piston travels down again and the exploded—and expanded—gases begin to escape from the exhaust port. The piston begins to compress the fuel/air mix waiting below. This forces the mix into and up the transfer porting. These channels run up the outside of the cylinder liner in the engine casing. They guide the fuel/mix into the combustion chamber to provide a charge for the next explosion.

This is where you will hear the word “timing” again. There are ports in the cylinder liner that are positioned to let the exhaust gases out before the fuel/air mix comes in through its own inlet port or ports. There are no cylinder-head valves similar to an automobile in this type of engine, so the control of gas arrival and departure is achieved by the positioning of the ports in the cylinder liner where the piston moves up and down.

The movement of the piston past the cylinder liner ports creates a timed control that dictates when fresh fuel/air mix arrives, is closed off, exploded, and expunged. There is a period of time when the exhaust port(s) and the inlet port(s) are open.

The growl of big gas-ignition engines can be tamed using tuned “cans” designed to also increase the power.

The reason for explaining this operation is to point out that this timing is primarily preordained by the designer and manufacturer. It is not something that most mere aeromodeling mortals would mess with.

This brings me to exhaust port management. We have the opportunity to influence what goes on as the gases leave the engine. You can run a model engine with no muffler at all, especially if you live in the middle of nowhere, but you will lose two key opportunities and possibly some hearing!

One of the most common practices is to use a pressure tap in a two-stroke glow engine muffler to supply a pressure pulse to the air intake of the fuel tank. The exhaust gases cool quickly and do not harm the silicone fuel lines, the fuel, or the fuel tank. This pulse of pressure helps the fuel reach the carburetor and, in a way, matches the increased need for a good fuel supply as the rpm increases.

Less common—or perhaps less understood—is exhaust tuning. You can tune a two-stroke engine’s exhaust to get more high-end rpm or mid-range torque for acceleration.
Before I address tuning, you need to know what I am writing about. It probably helps to think about the exhaust gas flow as a series of pressure waves, similar to those in a wind instrument.

No matter how well you design the porting in a two-stroke engine, there is a period when fresh fuel/air mix escapes from the exhaust port before the piston closes the gap before igniting the gasses. With a muffler, the exhaust does not immediately escape into the atmosphere. It bounces around as random waves inside the muffler and leaves via the exhaust stub.

A standard muffler can do the job, but too often it doesn’t fit inside the cowl.

Then, something magical happens. At certain rpms and at a certain frequency of explosions, the sound/energy wave tunes and has a marked effect on the performance of the engine. It’s much like when you blow into a flute or the top of a soda bottle. That’s, in essence, why it is called tuning!

At some point in the rpm range, all mufflers tune. You might be asking, “What does this tuning do?” The back pressure of the tuned exhaust wave comes to the exhaust port and prevents the fresh fuel/air charge from escaping with the last of the exhaust gases.

This puts more combustible product in the cylinder for the next explosion. Repeat this cycle and you get more power at the same throttle setting.

This tuning boosts your power output. However, you often must slightly richen the fuel mix to feed the combustion needs of the engine and support the new power output. It might appear that the engine is running leaner, but is actually running faster—often with an increase of many hundreds of rpm.

Depending on the exhaust design, the tuning only happens at a certain rpm and is not available throughout the rpm range. Two primary factors influence the width of the rpm range in which such tuning can occur. The first one is the preset timing of the cylinder ports.

You may hear terms such as “broad” or “wide timing,” which mean that the design is less sensitive to tuning and will perform fine with a standard bolt-on muffler.

The second factor is the length of the tuning pipe. (In this case you should regard a stock muffler as a short, fat pipe). Generally, the longer the muffler, the broader the rpm range.

The same airplane, a Great Planes Super Chipmunk, with an aftermarket muffler looks better, but does it do the job?

Longer pipes typically give better mid-range power and shorter pipes give higher rpm at the top end. From a sound reduction point of view, the longer pipes give better results.

Because there is no way for us to change the hardwired genetic programming that dictates how we react to sound, it falls to us to change the sounds our engines make!

-Eric Henderson

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