Better Breathing

Tuned exhaust for the Cessna 172 adds a couple of knots and shaves fuel flow. Wed like to see it applied to larger engines.

To one degree or another, many of the systems found on the typical light airplane are afterthoughts tacked on after the airframers got done bending the aluminum into a flyable shape.

This is especially true of the engine exhaust system. When Lycoming and Continental design an engine, they don’t pay all that much attention to what flows into the thing and you can bet even less time is lavished on what comes out, unless it has to drive a turbocharger.

Traditionally, the engine makers have left exhaust system design to the manufacturers, who have tended to view it merely as a plumbing problem. The pipes are bent and routed so as to fit into the cowling, rarely with any regard to volumetric efficiency or maintenance issues. Yet as every high school hot rodder soon learns, throw-away exhaust system design costs performance by constricting the engines ability to suck in a fresh charge of fuel/air and burn it efficiently. In extreme cases, poor exhaust design aggravates marginal cooling, causing high cylinder head temps.

In the quest for performance, homebuilders have tinkered extensively with tuned exhaust systems. In fact, much of the serious recent research on the topic of tuned exhausts for small aircraft engines has been done by the EAAs CAFE Foundation. Unfettered by the frustration of FAA meddling, some homebuilders have designed tuned exhaust systems, occasionally with dramatic results.

Spam Cans, Too
Naturally, given all the aftermarket engine R&D going on these days, it was simply a matter of time until someone focused on tuned exhausts. Now one company has and recently began shipping new tuned systems for the popular Lycoming O-320 found in thousands of Cessna 172s.

Power Flow Systems, Inc., of Daytona Beach, Florida, has been researching the tuned exhaust system for a couple of years. Power Flows owner and chief engineer-Robin Thomas-is no newcomer to the aircraft mod world, having pioneered aerodynamic speed mods during the 1980s, marketed under the tradename Laminar Flow.

Of late, that market has flattened somewhat so Thomas recently turned his attention to engine mods, building on some of the work done by the CAFE Foundation. The upshot: A bolt-on tuned exhaust system that appears to boost the Lycoming O-320s output by at least 20 HP, while simultaneously reducing fuel flow and lowering CHTs.

Geez, what a deal. Why didnt someone think of this before? (Were not sure we have an answer.) Weve flown with one of the systems and reviewed dyno data and Power Flows rather modest claims seem to we’ll substantiated, in our view. That said, it may be a toss-up for some owners on the value of converting to this system, although we certainly think its worth considering at overhaul when the exhaust system would normally be replaced.

Price: $2795 for the complete kit, including the STC. Thats about $300 more than a new stock Cessna system but considerably more than a reworked or repaired system. In most cases, installation will be a simple bolt-up operation with no modifications required. Power Flow plans to rapidly expand the model coverage to other Lycoming and Continental-powered aircraft.

The Basic Theory
Tuned anything but especially tuned exhaust sounds like getting something for nothing. Actually, a more apt description is getting back what you already had, in exchange for some attention paid to pipe design.

Pestered by the government to improve fuel economy, the automotive manufacturers have tinkered with tuned induction, along with other such goodies as electronic ignition and variable timing, concepts that are only now finding their way into light aircraft and with markedly limited success at that. In the auto racing business, tuned exhaust is considered standard equipment.

When aircraft engines are certified by the factory to their claimed horsepower, the tests are normally done in a dyno chamber with whats called a neutral exhaust. These are essentially straight pipes that extend the exhaust manifold only far enough to direct heat away from the engine during testing. Neutral exhaust systems arent perfect, from a volumetric efficiency standpoint, but for certification purposes, no one cares.

Unfortunately, when the OEMs get hold of the engine, its usually downhill, power wise. Stock exhaust systems pay scant attention to efficient gas flow and what started out as a 150 HP sea-level engine in the factorys dyno chamber may give up 20 HP or more by the time the airframe builder is done cobbling up an exhaust system.

To understand why thats so, think of the engine as what it is: A multi-cylindered air pump. Considering just one cylinder for a moment, when the piston is on the downstroke of the intake cycle, it sucks in a fresh charge of fuel and cool air. At the bottom of its stroke (BDC), the piston reverses direction for the compression stroke and, in most aircraft engines, the sparkplug fires the mixture at a point when the piston is still on the upstroke at 25 degrees before top dead center (BTDC.) Heres where things get interesting. Cam and valve train design dramatically effect how efficiently the cylinder sucks in fresh fuel/air and how it purges the spent gasses. Flow patterns related to how the fuel/air charge gets into the cylinder and how the hot gasses get out after combustion is another critical factor that impacts power and efficiency. There are significant power gains to be had by manipulating these variables. Of course, there are some penalties, too.

One critical factor here is called overlap TDC; the brief period when both intake and exhaust valves are open, exposing both the intake and the exhaust tracts to the pressure pulses created by combustion. In a carefully tuned exhaust system, there’s a slight negative pressure-sort of like the draft of a properly designed chimney-which helps to both empty the cylinder of spent gasses and suck in a fresh charge for the next power cycle.

This process is called scavenging and is we’ll understood by builders of high-performance engines. If overlap TDC is long enough, the cylinders negative pressure can actually flow induction air across the exhaust valve and seat, cooling it in the process. Unfortunately, that cool air contains unburned fuel which then spews out the exhaust pipe in the form of reduced economy.

Most stock aircraft exhaust systems arent tuned we’ll or at all and these tend to cause unpredictable chaos during overlap TDC. Aircraft systems use individual header pipes which empty into a common collector at or near the heat muff, after which theyre directed out the exhaust pipe.

If the headers are too short or of less than ideal diameter-as they usually are on stock systems-the exiting gasses create pressure waves that reduce and may all but eliminate any scavenging effect for one or more cylinders. A pilot will see this in the form of reduced power, higher fuel consumption and probably higher cylinder head temperatures, since the hot gasses tend to hang around the exhaust valve rather than being expelled as the next cool charge of fuel/air arrives. Characteristic engine roughness may be a side effect, since a bad breathing cylinder makes less power and too much variability in power pulses between cylinders is noticeable.

Its The Length, Stupid
If youre going to tackle improved performance solely from the exhaust pipe forward, one critical element to consider is exhaust pipe length between the header-where the gases exit the cylinder-and the collector, where all the pipes come together. (Oddly, shape isn’t much of a consideration, since at the speed exhaust gasses flow, twists and turns don’t hinder the flow appreciably. But pipe diameter is important, as it controls gas velocity and how exhaust pressure pulses interact.)

Power Flows Thomas told us that both his research and that of CAF Foundation suggested that lengths between 35 and 40 inches are the ideal. Obviously, in any tuned system-intake or exhaust-tuning is married to a fixed set of variables. In the case of tuned exhausts on engines with fixed-pitch propellers, its RPM. Power Flows pipes are about 40 inches long, which Thomas tells us represents a middle compromise between climb and cruise RPM. The mechanical trick here is to fit four sections of three-foot long stainless steel pipe inside a stock Cessna cowling. Power Flow managed this by bending the pipes into a series of switchback bends which fit into a combined muff assembly thats the same size as the stock exhaust system. The pipes terminate in a common collector that flows into a glasspack exhaust pipe.

Even though it has more piping, the Power Flow system is only 1.7 pounds heavier than the stock system. Since the pipes are continuous with only bends for routing, there are fewer welds to break. Theoretically, that ought to reduce exhaust cracking incidents but as with any new system, a year or two of field service will tell more about durability than will speculating about apparent good design.

The pipes are made of the same 321 stainless used for stock systems but the wall thickness is 0.049 inch rather than the 0.035 inch found in the stock version. The heat muff/collector housing is made of aluminum.

Thomas told us that installation in most Cessna models is no different than installing or replacing a stock system, which is to say it takes a couple of hours of shop labor. To support the exhaust pipe, a steel bracket bolts to the back of the engine and runs through the bottom of the cowl. On some models, this will require boring a small hole in the bottom of the cowl. On the Cherokee 140-a soon-to-come STC-a modified airbox is required, which Power Flow provides at no extra charge.

Thus far, Thomas has examined tuned exhausts only for Lycomings 320/360 series. At the moment, STCs exist only for the Cessna 172 O-320 installation but Thomas says he sees no showstoppers in extending approvals to lots of other models, including the Tampico, Grumman Cheetah, Beechcraft Musketeer, Cessna Cardinal and various Maule models. Power Flow expects to offer systems for those aircraft eventually and will expand the product line to cover Lycoming 540 series engines.

If you own a turbocharged engine, you can pretty much forget about tuned exhaust. Although its theoretically possible to tune a turbocharged exhaust system, the difficulty of doing it in an aircraft engine is probably too costly to consider. Its mainly a piping problem and whatever gains might be realized arent likely to pay for the research to get it to work.

Measuring incremental performance gains that aftermarket mods supposedly deliver is always tricky business. The improvements are rarely night and day and if they exist at all, they may fall within the noise level of the instrumentation.

That said, we examined Power Flows exhaust system in a Cessna 172 and flew an airplane equipped with one of the pre-production systems. When we flew the Cessna-last February-Thomas told us he had been measuring power and economy improvements largely through empirical flight testing.

Due to rudimentary instrumentation in the Cessna and an airspeed indicator of questionable accuracy, we have to say our test flight was inconclusive. To be fair, Thomas warned us not to expect dramatic results. Using a handheld GPS and three perpendicular groundspeed runs, however, we calculated a true airspeed of 118 knots at full-throttle RPM (about 2450) at 7500 feet. Thats about 10 knots faster than book speed at this altitude and if the speed increase is real, its probably due to the higher RPM the Power Flow equipped engine can muster.

Thomas reports that recent flight tests indicate improved climb rate on the order of an additional 260 FPM for the tuned exhaust, compared to an identically loaded Cessna with the factory exhaust. Again, because of turbulence on our flight trial, we werent able to confirm this.

In our view, the more conclusive (and convincing) data comes from dyno tests carried out by Ly-Con, Inc., a respected California overall shop. According to Ly-Cons test run-the results of which are posted in part on Power Flows Web site-the Power Flow exhaust system on an O-320 delivered 157.1 HP at full throttle against 133.35 HP for the stock system. Naturally, the power gains come at higher RPM. The stock system was capable of 2563 RPM while the Power Flow version delivered 2665 RPM, using the same prop.

At the RPM settings youd be more likely to use, 2400 RPM for example, power output for both systems is essentially identical. However, the fuel flow savings with the Power Flow system were noticeable: 13.5 gallons per hour for the stock system, versus 11 GPH for the Power Flow system. If these seem high, its because the dyno tests are done at full rich mixture. Aggressive leaning will yield even lower but proportional results.

Ly-Cons dyno run also showed that CHTs and EGTs were significantly lowered by the Power Flow System. On the stock system, the average CHT was 283 degrees, versus 218 degrees for the tuned system.

From the dyno data and limited flight test, we conclude that the Power Flow systems lives up to the modest claims the company makes. True, a 23 HP gain for a 150 HP engine isn’t all that modest, but keep in mind this gain is limited to max RPM, so youre likely to see it in improved climb, not faster cruise at low altitudes.

At higher altitudes, you would expect to see slight speed gains, since the Power Flow equipped engine delivers more RPM and thus more power. If Ly-Cons data pans out in the real world, fuel savings of 15 to 18 percent at equivalent airspeeds seem achievable. (Again, our flight test aircraft had no fuel flow instrumentation.)

Does it make sense to plow this much money into a Cessna 172 exhaust system? In our view, its attractive only if considered at overhaul, when you might otherwise replace the exhaust system anyway.

Considering that repairing an old exhaust system is always an alternative to buying a new one, the real cost of a Power Flow System is probably $1500 to $2000 over the competing option, unless your existing exhaust system is serviceable, in which case the cost premium is the full price of the new tuned system.

For the investment, you’ll get a more sprightly climb and noticeable fuel savings out the deal. Ly-Cons Steve Mehalek told us the Power Flow-equipped engine was both smoother and quieter, too. We didnt notice much difference on our flight trial.

Converting a serviceable exhaust to the Power Flow system doesnt strike us as a cost effective choice for an individual owner, unless fuel savings are important to you for range considerations. (Not likely in a 172.) The performance gains, while impressive, arent commensurate with the investment, at least for a low-performance, fuel-economical aircraft such as the Cessna 172.

However, a flightschool or club might want to consider converting a servicible exhaust system. Power Flow told us that a Daytona Beach flightschool operated a 172 equipped with a tuned exhaust for three months and noted a composite monthly fuel consumption of 2 GPH less, over a range of typical training operations. For 100 hours of flying per month, thats $400 to $500 in lower fuel bills.

If Power Flow applies this technology to higher performance aircraft, such as the Cessna 182, the Mooney and the Arrow, among others, the equation may very we’ll shift in favor of owners converting a serviceable exhaust system, solely for the fuel savings. We think the system and the company have real promise in the high-performance market, where owners will be more willing to make the investment.

Power Flow System, Inc.
1585 Aviation Center Parkway
Hangar 805
Daytona Beach, FL 32114

Also With This Article
Click here to view the Tuned Exhaust Checklist.
Click here to view the Power Flow Test.