Tuned Exhaust Explained

Tuning Theory For Aircraft Exhaust Systems

The basic workings of the four cycle piston engines which power our aircraft are familiar to most pilots.  The maximum horsepower that any piston engine can produce is a function of the internal volume of the engine’s cylinders which determine the size of the fuel/air charge that can be burned during the power stroke.  In the “ideal scenario”, when the exhaust valve opens, the piston pushes spent exhaust gases out of the cylinder so that a fresh fuel/air charge can enter during the intake stroke.

Although we’re only looking at one cylinder here, in a small aircraft engine at least three other cylinders are going through the same process about 1,000 times per minute, generating over 200 cubic feet of hot, expanding exhaust gas in the process.  Where does all that exhaust go?  In a stock system, a steel “header” tube is bolted on to each exhaust port and routes each blast of hot gas to the muffler – a relatively small metal can with four inlets (one for each header tube), BUT usually only one outlet – the tail pipe.

So, we have four cylinders blasting over 200 cubic feet of hot gas each minute into a muffler roughly the size of a large coffee can with only one “good” exit.  But the laws of physics being what they are, the gas can’t flow out through the single tailpipe as fast as it is coming in from the four headers.  So, the pressure inside the muffler increases and then flows back up the headers to the exhaust ports on each cylinder.  

Now, when the exhaust valve opens to expel the next pulse, the piston has to push against the compressed exhaust gas from previous combustion cycles.  This has two consequences, both bad: #1.) The engine uses more of its energy to push the spent gas out, generating excessive heat and leaving less energy to turn the propeller; #2.) The back pressure in the header prevents the spent gas from completely evacuating the cylinder before the exhaust valve closes.  So when the intake valve opens there is less space in the cylinder for the fresh fuel/air charge.  In a typical aircraft engine as much as 20% of each cylinder’s volume can be lost in this manner, significantly decreasing the available horsepower.

In a Tuned Exhaust System, by contrast, each header has a clear, unobstructed path from the exhaust port to the end of the tailpipe.  There is no common muffler to accumulate power robbing back pressure.  This feature alone does a lot to reduce internal friction and increase available power.  To improve things even further, the headers in a Tuned Exhaust are all of a length that is calculated for each engine (“Tuned”) to increase combustion efficiency and maximize horsepower.

The process is called “scavenging” and it works like this:  As each high-pressure exhaust pulse blasts out of the cylinder and shoots down the header, it leaves a “wake” of relatively low pressure behind it, like the slipstream generated by a fast-moving vehicle.  This pulse and the resulting low-pressure wake are shown by the green trace on the pressure graph below.  In a Tuned Exhaust, the first exhaust pulse is still racing down the header pipe, trailing its low-pressure wake, when the exhaust valve opens to emit the next blast.  Now instead of encountering high pressure interference from the muffler, the second exhaust pulse is actually pulled (“scavenged”) from the cylinder by the low-pressure wake generated by the previous pulse.  Note the difference in pressure between the Tuned Exhaust and the stock exhaust during the critical overlap phase when both the Exhaust Valve and the Intake Valve are open at the same time. 

Instead of losing volume to residual exhaust gas, each cylinder is able to accommodate a full fuel/air charge for every combustion stroke with a corresponding increase in power.  “Increased Volumetric Efficiency” is the technical term for this benefit and the result is a smoother, cooler, more powerful engine.  You get all of the benefits that you would expect from increased horsepower at a fraction of the cost of obtaining that horsepower by any other means.

Power Flow did not invent Tuned Exhaust Systems.  They have been used successfully in automobile and motorcycle racing circles for over 50 years.  Our unique contribution has been to adapt these systems to the much smaller confines of light aircraft cowlings and to get the resulting products approved by the FAA via STC for use on GA aircraft.