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Tuned vs Equal Length Exhaust Systems
Let me start out by saying that Tuned is the most desired result of a properly designed exhaust system that includes the primary tubes, (pipes that lead from the exhaust flange to the muffler), muffler and tailpipe. The word “Tuned” has become a Buzz Word, a word that a lot of people use thinking it is the way all exhaust systems are produced. The other most common words I refer to as Buzz words are horsepower and torque. The auto industry over the past few years has finally identified torque correctly as pound feet not foot pounds. Foot pounds is the proper term for work or horsepower. Horsepower equates to a unit of power equaling 745.7 watts or raising 33,000 foot pounds one foot in one minute. Torque may or may not result in motion, it is a twisting force. An example is when you torque a bolt to the prescribed torque and the bolt or nut’s movement might be imperceptible if it weren’t for the pointer/indicator or click in the handle. Horsepower results in motion of a given object.
When the term “Tuned Pipes’ is used there is a lot more that goes into the definition and some of the constituents are: Exhaust Pressure Graph (EPG), EVO-Exhaust Valve Opening, IVO- Intake Valve Opening, Valve Overlap, “P”- pressure wave, T-time, blowdown, volumetric efficiency, back pressure, compression ratio, TDC- top dead center, BDC- bottom dead center, WOT- wide open throttle, wave speed, collector size, “Rings”, Doppler phenomenon and so on.
The typical EPG will show steeply rising “P” wave exhaust pressure, which starts upward at the point of EVO. The P wave typically falls to below zero (ambient) pressure later in the exhaust cycle. At overlap TDC, top dead center, both valves are open for a brief interval. The EPG often shows additional waves which come from reflections, turbulence and, in collector-equipped systems, the firings of other cylinders (cross-talk) referred to as R waves. The C waves are those measured in the collector are merged into individual headers. Each cylinder provides a separate C wave. The time, T, between the rise of the P wave and the rise of the attendant C wave is very short and can be used to calculate the velocity of the wave.
The “blowdown” cycle is defined as the period from EVO to firing BDC. It is during this interval that the steep rise in the P wave takes place as the cylinder discharges or blows down through the exhaust valve and the in-cylinder pressure rapidly falls. Positive in-cylinder pressure during blowdown is still doing useful work by pushing downward on the top of the piston.
Overlap is a very important interval. When both the intake and exhaust valves are open, the pressure in the exhaust pipe, combustion chamber and intake tract can all influence one another.
During overlap TDC, the suction in both tuned exhaust’s header can help empty the combustion chamber of its burnt gas residues. The effect, collapsing of the exhaust gases, is called scavenging. The exhaust suction may even enhance the combustion chamber’s filling thus improving volumetric efficiency and horsepower.
Higher compression pistons should scavenge better due to their smaller combustion chamber volume.
Typically the intake is greater than the exhaust pressure during valve at overlap @ TDC.
Such a pressure will encourage scavenging. At part throttle the intake valve pressure would be much lower, and unfavorable reverse flow could occur at overlap. This is one argument for using wide open throttle (WOT) whenever possible in high altitude cruise flight.
Normally, engine producers try to design EVO about 40-75 degrees prior to firing BDC so that the peak of the very high in-cylinder pressure can be dissipated during blowdown before BDC. A tuned exhaust system with a very low opening pressure at EVO, can assist in evacuating the cylinder quickly, and can thus allow EVO to be delayed later in the cycle. The later EVO allows the positive in-cylinder pressure to do more work pushing the piston downward prior to EVO. Thus, a tuned exhaust system works best if the timing of the EVO is delayed to take advantage of the tuning.
After the blowdown in the exhaust stroke the piston begins to rise from BDC. A rising piston pushing against a high in-cylinder pressure causes a loss of power known as “pumping loss”. Instead, the piston should be pulled upward by a negative pressure in the cylinder, thus producing a “pumping gain”. Suction in a tuned exhaust system can produce such a pumping gain in mid to late exhaust stroke. The earlier in the exhaust cycle the P wave subsides and goes negative or below ambient (zero) pressure, the more pumping gain occur, making greater horsepower.
Thus an ideal exhaust system should produce a highly negative pressure at the exhaust valve at both EVO and again as soon as possible after dissipating the P wave. This negative pressure should be made to persist throughout the overlap stroke so that favorable scavenging can occur.
There are many other things that influence an exhaust system performance such as intake pulsations that can show how much scavenging effect might be expected, and the character of the cylinder filling. The latter can serve as a guide to the relative volumetric efficiency of the engine.
The W.O.T. intake pulses can reach as high as 6-7” Hg. above atmospheric pressure. This effect thus gives an instantaneous pressure of about 37” Hg. and if timed correctly can act somewhat like supercharging.
EPG can show the average speed of wave traveling through the pipe if plotted on an X-Y graph. The wave speed can equal the sum of the average sonic wave speed and the average mass flow velocity.
The exhaust gas expands and cools as it goes down the pipe, and the wave velocity varies directly with the square root of the ratio of the absolute exhaust gas temperatures.
Other influencing factors are bends in the tubing, tail pipe down turns at cowling egress have shown an insignificant back pressure increase. Some tests have shown that the correct collector length appeared to optimize at 20-30”. Those particular tests showed it must be long enough to develop continuum of flow and fully contain each pulse. Some suitable collector tests have shown a suitable collector was one with about 50-60% greater cross sectional area than each individual header and with a length of at least 18” or so.
The small waves which occur after the P wave in independent pipes are called “rings’, as in a doorbell ringing. The negative portions of these rings are of such short duration that they might require the designer to choose between positioning them early in the cycle to obtain pumping gains or late in the cycle to scavenge during overlap. The collector system has a long duration negative wave after the P wave that it serves both purposes, i.e., gives pumping gain as well as scavenging at overlap.
A crossover exhaust joins the headers of the cylinders whose firings occur 180 crankshaft degrees apart. The P wave of one cylinder will then travel upstream to the other cylinder where it will bounce off the closed exhaust valve and return. Pipe lengths in the crossover can be chosen so that the returning wave will produce a negative pressure for scavenging.
Header size in one particular test showed the optimum header size for an engine operating in the 2500 to 2700 rpm range, at sea level, was 1.750” outside diameter. The length of the headers with a 4 into 1 system optimized at 28-36” long. Longer length probably raises back pressure and delays onset of scavenging while shorter lengths reduce the ability to contain a fully developed, powerful wave.
Lean vs. Rich EPG’s- a richer produces a lower EGT and thus a slower wave speed than does a lean mixture.
Ball joints in some tests have shown a very slight increase in backpressure but no change in fuel flow, RPM or thrust.
Valve jobs have also shown that smoothing the sharp edge transition of the exhaust valve’s seat bevel cut where it blends into the valve’s tulip portion. This apparently greatly improves the fuel flow past the valve both at initial valve opening and during the small valve openings at overlap.
Most simple mathematical formulae for calculating the ideal length for exhaust pipes fail to account or to recognize that there is a Doppler phenomenon occurring in an exhaust pipe because the sonic exhaust wave is riding on the “wind” of the steaming mass of fuel and air. The sonic wave moves at 1500-1800 fps while the mass flow moves at 200-400 fps. The sonic wave travels faster to the tailpipe than does the returning reflected sonic wave which must “swim upstream” to reach the exhaust valve. Computer programs have been developed and can address these complexities called “method of characteristics”.
It must be remembered that a 200Hp engine becomes a 130Hp engine at cruise altitudes of 8000-12,000 ft. Optimization of exhaust tuning at these altitudes, with the attendant reduced air density, will call for use of smaller diameter headers and collectors. A compromise must be found to not rob the engine of its sea level climb power.
To fully realize the potential benefits of tuned exhaust systems for the aircraft engine, the camshaft timing must be suitably altered by making exhaust valve closure occur later and the valve overlap period of longer duration and higher lift.
In closing I want to emphasize the preceding text is given simply to explain the very complicated procedures required to make the claim that an exhaust system is “Tuned”.
Leading Edge Exhaust Systems, LLC, (LEES) has never used this term in any advertising and probably never will. The next time you hear the word “Tuned”, a “buzzword” as I call it, ask the company or person, (s) making the claims to show you how they came to that conclusion; formulae and calculations. The formula might dictate the primary tubes and collectors are required to be so long that they would not fit within the cowling and you might not be able to access the, all important, exhaust flange nuts. Some of the exhaust components we manufacture are direct replacements and must be able to attach to existing mufflers and tailpipes.
At LEES we use the term “Equal Length” pipes for the following reasons; We build FAA/PMA approved exhaust systems that must fit within the confines of the aircraft’s cowling; our systems must comply with the FAR’s regarding climb/cooling and carburetor heat tests, if applicable, as well as EPA noise level regulations.
Portions of the preceding can be attributed to the CAFE Foundation Aircraft Performance Report, “Sport Aviation of EPG IV, January 1997” sponsored by the Experimental Aircraft Association and the Federal Aviation Administration. Research work that named, developed and defined the features of the EPG is attributed to Brien Seeley and Ed Vetter of the CAFE Foundation.
Copyright, Dane Wagner Sr., December, 2005
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