Tuned Exhaust Explained

Tuning Theory

Tuning an exhaust system is critical for an aircraft engine. It is easier to tune an aircraft engine than a car or motorbike, because the range of RPM used is much narrower. The major obstacle is that space inside the cowl tends to be very scarce!

Although motorcycle and automobile racers have been tuning their exhausts for over forty years, until the late 90's, there were no tuned exhausts on certified aircraft. In order to work, a tuned exhaust must reduce the pressure at the exhaust port just before it closes, to below the pressure in the intake manifold at that time... called the overlap phase. At that moment, fresh mixture can be drawn into the cylinder and simultaneously spent gases removed from it. In an untuned exhaust, the pressure in the exhaust at that moment will be greater than that of the intake, and spent gases will remain in the cylinder or even flow backwards into the intake manifold.

The power developed by a stroke in which fresh cool gas is being compressed will be much greater than the same stroke in which the cylinder is partly filled with spent hot gases.

But before we get into scavenging, let's review what is happening inside the cylinders of your engine. Your engine goes through four "strokes" during the combustion process. The Intake, Compression, Combustion, and Exhaust strokes.

An "intake" valve is located at the top of the cylinder. During the intake stroke, the piston moves down the cylinder, sucking a new fuel/air mixture through the open intake valve. The intake valve closes, and the piston moves back up the cylinder, compressing the fuel air mixture, at which point a spark from the spark plug ignites and explodes the compressed mixture. This explosion forces the piston back down the cylinder, producing the necessary power to turn your prop. Next, the exhaust valve opens as the piston pushes out most of the spent fuel air mixture, and the cycle starts all over.

Having said that, here is what is happening when you have your original exhaust system.

As the spent fuel air mixture leaves the cylinder, it is routed through a set of headers, into a common collector area, and pushed out the tail pipe by the remaining pressure. Like a garden hose with a kink in it, pressure builds up through out the exhaust system, making it more difficult for the spent gas mixture from the next cycle to leave. So now, the exhaust isn't flowing as freely as it should, leaving some exhaust in the cylinder, taking up space better used for a clean fuel/air charge.

Our Tuned Exhaust System is totally different. When you break down our exhaust system, you have two "sets" of pipes; the primary (header/collector combination) and the secondary (pipe after the collector). The primary contains four independent tubes that all join at the "4-1 Collector" (see below). As each "exhaust puff" of spent fuel travels through the primaries, it leaves a vacuum behind. In our tuned exhaust system, we have adjusted the length of each tube so that the vacuum reaches the collector just in time to "suck out" the exhaust from the next cylinder. The exhaust gases are not only being pushed out by the cylinder, but "extracted" by the vacuum, so a bigger, fresher charge will enter the cylinder during the next cycle. A tuned exhaust system really promotes a smoother, cooler, and finally more powerful running engine.

How an Exhaust Is Made

Creating a "Power Flow Systems Tuned Exhaust" is not as easy as you may think. Manufacturing a tuned exhaust system all starts with the tube bending. Tube bending machines are powerful hydraulic machines that run a flexible "mandrel" inside of the tube at the same time as the tube is being bent around a specific radius. We mandrel bend the tubes so that the cross section area of the tube is not reduced - helping to ensure smooth and non turbulent air flow wherever possible. We mandrel bend the collector tubes, headers, and muffler assemblies. In some cases we are able to manufacture tubes with multiple bends in them as a one piece assembly. This process is very costly to develop - and isn't cost effective for some parts.

Once back in Daytona Beach, our home city, the exhaust tubes are welded. This is the heart of our fine craftsmanship, as all of our exhaust systems are hand made. Each system is built on a calibrated jig. We monitor the production every step of the way, and ensure that everything is built using only the tightest tolerances.

To greatly reduce the risk of Carbon Monoxide Poisoning, we "Pressure Test" each collector assembly. We accomplish this by plugging the slip joints, submerging it in water, and pressurizing the system to at least 2 PSI. We then check around the welds for air bubbles signifying a leak. If there is just the smallest leak, we reject the part. However, if the collector passes the test, it then leaves Daytona for annealing.

Testing for Leaks

Each of our welded collectors are annealed. This process includes heating the collector to over 1800 degrees F in a huge furnace then slowly cooling the assembly in a controlled environment. This relieves the stress of the welds, down to the molecular level, reducing the chance of premature cracks. Our collectors are once again, pressure tested, and then shrouded using aircraft quality Stainless Steel.

Our support rods are bent (if necessary), welded, and powder coated.

After welding, our exhaust tubes are electro-polished or ceramic coated. The electro-polishing process chemically alters the surface of the stainless metal by dipping the metal into a a chemical bath and sending an electric charge through both the chemical and the stainless part. This has the effect of removing the dirt and grime and bringing the shiny part of the metal, the chromium, to the surface of the metal - leaving nothing but a beautiful shine. Over time the high temperatures in the exhaust will affect the shiny muffler - resulting in the gradual discoloration of the muffler tube. The tube can be returned to its original "shine" by polishing it with a commercially available stainless polish and keeping it free from oil. The alternative to letting the part discolor was to find a way to not let heat affect it.

As of May 2003, Power Flow Systems, Inc. received FAA approval on an exclusive ceramic coating process for its tailpipes. A ceramic coating will prevent the tailpipe from discoloring or "blue"ing. This process is only available on new muffler assemblies - it cannot be applied to used mufflers. Existing customers can purchase a new muffler and clamp assembly, or you can choose to upgrade to the ceramic upgrade during order placement.

After polishing or ceramic coating, the mufflers are "stuffed" using a stainless steel wire mesh with a basalt fiber outer wrapping.

Finally, after all the parts come together, the system is ready for shipping. We test fit each system by assembling it, and setting it on our quality control jig. We clearly mark the position of each header as it fits into the collector making installation that much easier for your mechanic.

In order to ensure that your exhaust will arrive to you intact, we use a special machine that deploys an expandable foam. This foam, wrapped in a sheet of plastic, totally surrounds your exhaust system ensuring it arrives in complete protection from careless shipping companies.

Quality Control

All products manufactured by Power Flow Systems, Inc. undergo strict and rigorous quality assurance procedures. We are a holder of FAA Parts Manufacturer Approval (PMA) for all of our certified products. This means that our entire manufacturing system is under the strict supervision of the Federal Aviation Administration (FAA.)

Each component goes through no less than three in-house quality assurance inspections. To ensure extra safety and reliability, we also take additional steps beyond industry typical procedures.

For example:

  • All tubing on our systems is made from 321 stainless steel and the tubes are mandrel bent, not cast or formed. Our headers/risers and internal collector tubes are .049 inches in thickness. Typical original exhaust systems use .035 inches thickness. This means our systems are up to 40% thicker than the original.
  • After initial manufacture, our central collector system is vacuum annealed to relieve any stresses introduced during the manufacturing process and to ensure greater longevity of the system.
  • Our exhaust flanges are laser cut, not stamped, and are welded two times to each header/riser (internally and externally) whereas it is typical to find only an external weld on the original systems. The cabin heat design used on Power Flow’s tuned exhaust systems are not only as much as 50% more effective, but they have far fewer welds cabin heat section, which greatly reduces the chance of carbon monoxide entering the cabin.
  • The carburetor heat design used on Power Flow’s tuned exhaust systems has up to 50% more surface area than the original. Our exhaust system uses no clamps where the headers/risers join the central collector system. Instead we use close-tolerance slip fits that not only reduce maintenance, but also greatly diminish the possibility of cracking related to vibration and other stresses. Each of our complete systems are assembled on an engine jig prior to shipment to ensure trouble-free installation on the customer’s aircraft.
  • Each exhaust system goes through approximately 160 quality inspections before shipment. There are no recurring Service Bulletins (SBs) or Airworthiness Directives (ADs) on any of our products. Our cabin heat section only requires a visual inspection every 100 hour or annual inspection in accordance with the original aircraft manufacturer’s instructions.

Testing Procedure

Once we have committed to our next aircraft, we do a number of before and after tests. Prior to any fabrication, we will install our data collecting instruments. These include a full Electronics International calibrated instrument package, cylinder head temperature (CHTs) and exhaust gas temperature (EGTs) probes for each cylinder, a fuel flow meter, OAT gauge, carburetor heat temperature gauge, a sound level meter, an optical tachometer, a calibrated altimeter and airspeed indicator.

We then do time to climbs, and full throttle runs at predetermined altitudes. Each flight is recorded to video tape for review afterwards. Our data is then fed into our computer and analyzed.

Taking that analyzed data into consideration, we then fabricate a prototype tuned exhaust. This process alone may take up to three weeks.

After installing our prototype we pursue further flight testing. After reviewing the data, we make any necessary changes to tube length or routing. When we have a product we are happy with, we submit our final requests for certification to the FAA.

Dyno Testing

It is common practice to use a dynamometer (Dyno) to measure an engines performance. People frequently use this machine to collect data on exactly what the engine is doing. The Dyno can tell us Cylinder Head Temps, Exhaust Gas Temps, fuel flow readings, and most important, horsepower!

We had the privilege of visiting Ly-Con. Located in California, Ly-Con is leading the way in high performance engine overhauls. They own state of the art equipment, including an aircraft engine dyno. In order to prove what our system can do, and hopefully combat some skepticism, we installed a Power Flow Tuned Exhaust on one of their test stands, and compared the dyno numbers to the same engine with the OEM Cessna exhaust. The engine used was a Lycoming O-320 A1A, rated at 160hp.dyno rig

Here is what we found:

Peak Horsepower Developed:

PFS Tuned Exhaust:157.1hp

OEM Cessna exhaust:133.3hp

That's a 23.8hp difference! Those extra horses really give you a dramatic pitch angle on Vx and Vy climbs. It is by far, the most impressive difference you will notice on your first flight after installation.

Certification Process

The main goal of the certification process is to be issued a Supplemental Type Certificate (STC) and a Parts Manufacturing Approval (PMA) by the FAA. These allow us to sell and install our Tuned Exhaust Systems.

While there aren't that many steps involved, the whole process may take up to a year before an STC is issued!

The first step involves submitting an application for an STC/PMA to the local FAA office. This application includes which product we seek to certify, and on which aircraft the product will be installed. The application also includes a schedule of when the different phases of the project are to be completed, as well as some administrative data.

Next, after all of the assembly drawings (headers, collectors, etc.) are made, they are submitted for review. We are then required to wait for a Statement of Conformity which, when signed by the FAA states that we are permitted to move on to the approved test proposal. Getting past this hurdle is 75% of the battle. After everything has been approved, we move on to the inspection and testing of the completed assembly. Here, an FAA representative witnesses an install, and flight test. If everything is with in compliance, the FAA representative submits a recommendation for STC. This recommendation is then reviewed, and if there are no problems, we are submitted an STC!

Sound pretty easy? Well, it would be, but keep in mind that if something is found that doesn't meet the strict tolerances of the FAA, it may take weeks to correct. This could range from a simple re-weld of a header, to putting an aircraft into an experimental category for three months.

We make an effort to work with the FAA as efficiently as possible, but sometimes, the only answer is patience.