Maintainers Corner - Tuned Exhaust Systems
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TUNED EXHAUST SYSTEMS
I’ve been asked by a few people about my thoughts on tuned exhausts, so I thought this would make a good subject for the Newsletter.
Let’s start by briefly describing a traditional exhaust system. The purpose of the engine exhaust system was purely functional – to remove gases from the cylinders and get them out of the nacelle. There was also a need to muffle the sound and so the rudimentary exhaust system was born.
This traditional type of exhaust system has worked fine throughout the years but could be improved and the primary method for doing this is to optimize the way the exhaust gases are able to escape. Picture your typical aircraft engine (running). When the number one cylinder fires, the exhaust valve opens and the gases travel through a relatively random short length of tubing into a central chamber. The gases then have a choice. They can go out through the system, or they can go up any of the tubes leading back to the other cylinders. Some of the gases will leave through the chamber, but some will go up the other tubes. Then the number three cylinder fires and when the valve opens, instead of the gases being evacuated, they meet with pressure from the number one cylinder. The bottom line is, that typically, only 80 percent of the cylinder is being emptied. Twenty percent of it is being wasted because it couldn’t empty itself fully. So you’ve lost 20 percent of your efficiency.
Now consider a tuned exhaust. The main goal is to more efficiently evacuate the exhaust gases from the cylinders. This is mostly controlled by the length of the tubing. There are multiple waves of both pressure and heat that are going to come out when he exhaust valve opens. Length dictates whether the pulses are going to benefit you or not. The key thing about tuning is, that if you send those pulses through a long enough tube, so that they are not interfered with or influenced by other pulses when another valve opens, then you can utilize that to create essentially a mirror image – a negative wave or suction at the time the valve opens.
The muffler on a tuned exhaust system is different from an OEM (original equipment manufacture) muffler. On an OEM muffler the exhaust gases entering the muffler are diverted by baffles. On a tuned exhaust system, the muffler is typically an external muffler - a round can. It is about 3 ½ inches in diameter and has an insert inside of it. It is a perforated tube. You can shine a flashlight in and see from one end to the other. As the gases enter this perforated tube, they diffuse into an absorption material behind it and as the gases pass down, they get quieter.
The bottom line is that with a tuned exhaust system, you are always suctioning out and emptying out the cylinder more effectively. So less gas is wasted to overcome the inertia. The engine gets a more complete fuel burn. The effect in the cockpit is that it will take less throttle to get the same rpm that you are used to. This means less fuel flow. The EGT ends up going up a little bit because more of the exhaust gases are being emptied from the cylinder.
Of course I've heard from both sides of the coin – some say yes it works and some say no difference. I guess it is up to the individual and their AME and whatever information they can gather together before making the decision.
The foregoing information provided courtesy of Kori Ibey of Maximum North Aviation.