Tune your engine exhaust like a pro
Eric Henderson [email protected]
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 different reactions to different sounds, and even to the same sound in different circumstances. 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: the high-pitched sound of an insect that we instinctively know could hurt us. Flying insects produce sounds ranging from the unmistakable whine of an "attack" mosquito to the lumbering tone of an incoming bumble bee "bomber," and they 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 both draw attention and can provoke reactions 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 correctly designed mufflers improve the situation for everyone.
Enjoying man-made noise is subjective. When I compare the noise of a Cox .049 at maximum rpm to the roar of six 12-cylinder Rolls-Royce Merlin engines at the Battle of Britain Memorial Flight making low passes (Lancasters, Hurricanes, and Spitfires), I enjoy the Merlins. The massive roar and throbbing of a World War II 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 control-line 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 decibels measured at 9 feet from the 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 too much it would lose heading and points in the round.
Tuning an engine with an exhaust system was not a new concept. Most work focused on getting more top-end power. What happened in the Pattern world was a shift to improving midrange and adding torque at the lower top-end rpm. A slower-turning propeller produces less noise, so larger propellers with higher pitch were used to get the same speeds with significantly less noise. Engines were heavily cowled, and even 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 accompanies 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 compress the previously ingested fuel/air mix. There is no ignition timing system in a glow engine, so the gases explode when the right compression is reached and the glowing glow-plug element ignites them. (Gas engines do the same, except they use a spark plug, which requires external current to fire.) The piston travels down again and the exploded—and expanded—gases begin to escape from the exhaust port. The piston then begins to compress the fuel/air mix waiting below, forcing that mix into and up the transfer porting. These channels run up the outside of the cylinder liner in the engine casing and guide the fuel/air 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 as on an automobile; instead, control of gas arrival and departure is achieved by the positioning of the ports in the cylinder barrel. 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, is exploded, and is expelled. There is a period of time when the exhaust port(s) and the inlet port(s) are both open.
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 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 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 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 more mid-range torque for acceleration.
Before I address tuning, it 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 prior to the next ignition. 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 stud.
Then something magical happens. At certain rpms and at a certain frequency of explosions, the sound/energy waves tune and have a marked effect on engine performance. 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 returns to the exhaust port and prevents the fresh fuel/air charge from escaping with the last of the exhaust gases. This puts more combustible mixture 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 it 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 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 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.
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.
Correction
In my June 2012 "The Engine Shop" column on servicing a two-stroke engine I wrote, "If you have a 1/4 x 32 tap it will do no harm to clean up the glow plug threads in the head."
At the end of my August 2012 column I added a late update on glow-plug thread size that was incorrect. Please be clear that it is still 1/4 x 32 for most glow engines.
Transcribed from original scans by AI. Minor OCR errors may remain.
