Tuned Exhaust: How it impacts your aircraft engine

A brief primer on tuned exhaust systems.

March 2005

A tuned exhaust system is available for some aircraft piston engines. Installation of a tuned exhaust engine can result in increased engine efficiency, increased power, and better performance. We will take a look at a tuned exhaust system, how it affects the engine, and some maintenance items to keep in mind when working on engines with one of these systems installed.

Where are they developed?

The company that specializes in tuned exhaust systems is Daytona Beach, FL-based Power Flow Systems Inc. I talked with general manager Darren Tilman, an A&P and test pilot for Power Flow Systems to learn how it works. In order to understand how a tuned exhaust system works, we need to first learn what is happening on a typical OEM exhaust system.

Traditional exhaust system

"Traditionally, the exhaust system on an engine was purely functional -- to remove gases from the cylinder and get them out of the nacelle," Tilman shares. "And then the FAA came along and said 'we need to muffle it' and so they got rudimentary exhaust systems. And then you got pilots that didn't like to freeze, so the manufacturers had to come up with a heating system. So they wrapped a piece of metal around the other metal and came up with a heating system. But everything was sort of a band-aid approach. It was an afterthought."

This traditional type of exhaust system has worked out fine through the years but could be improved. The main way it is able to be optimized is the method that the exhaust gases are able to escape. Tilman explains how this works on an OEM installation. "Picture your typical Lycoming engine running, starting with the number one cylinder. The cylinder fires. The exhaust valve opens and the gases enter a relatively random short length of tubing. Then they will enter a central chamber, and then they have a choice. They can go out, or go up three other chambers, or three other manifolds if you will. So some of it leaves, but some of it goes up the other three. Well, then the number three cylinder fires. When the valve opens, instead of being able to evacuate itself, it's met with the pressure from the number one. The bottom line is that only 80 percent of a cylinder typically 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."

Tuned exhaust

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 tubing. "There are multiple waves of both pressure and heat that are going to come out when the exhaust valve opens," Tilman shares. "Length dictates whether the pulses are going to benefit you or not. The key thing about tuning is if you send those pulses through a long enough tube, so that they are not interfered or influenced from another one at the time the valve is open, then you can utilize that to create essentially a mirror image -- a negative wave or a suction at the time the valve opens."

So the engineers working on a tuned exhaust system determine what the optimum length of tubing is. A tuned system is technically 100 percent optimized for one given set of conditions. Typically, an exhaust is tuned for a peak boost down at sea level at 2,450 rpm.

The four tubes then join at the four to one collector. When they meet at the collector, they are the same length. They then go on to the muffler and out the system.

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