In the ongoing search for more speed and performance, aircraft designers have only two variables to manipulate: aerodynamics and power. But either option can cost real money. Hot-rodders and motorcyclists, among others, long have known the benefits of less-restrictive exhaust systems as a means of tweaking the engine equation.
Power Flow Systems figured that out a decade ago and applied it to its series of tuned exhausts for various airframes with small-displacement Lycoming (O-320 and O-360) engines. The company says one of its exhausts often can increase an airplanes performance for not too many dollars. And if you fly a lot, it might pay for itself through fuel savings. Are the results worth the expense? Are Power Flows customers seeing real-world improvements? To find out, we surveyed Power Flow customers for their real world results compared to our own data, collected when Power Flow first came on the market.
What It Is
Airplane manufacturers rarely spend time optimizing the engine compartment. Instead, they just make sure everything fits, which can result in some Byzantine exhaust pipe routing. That means exhaust gases from each cylinder travel unequal distances before being dumped overboard. Power Flow likens the resulting increased back pressure in the system to a kinked garden hose. Improve the engines ability to breathe, according to theory, and youll improve performance without altering anything else. This something-for-nothing phenomenon only works if the engine and its exhaust are not working at top efficiency: Some combinations of carburetors/engines/airframes may not see any benefits, according to the company.
Power Flow says its header pipes from each cylinder are of equal length, so when the negative pressure following one cylinders power stroke occurs, the lower pressure helps extract the exhaust from the next cylinder. Since the length of time each exhaust pulse needs to travel from the cylinder to the collector varies with engine speed, the company has optimized its systems for 2450 RPM. At that speed, Power Flow says more of the exhaust is scavenged from the cylinder and the incoming fuel/air charges volume is greater. Early Power Flow designs were fitted with a rather ungainly pipe assembly, extending below the cowling and supported by a rod. This rod usually extends through a hole cut in the cowling during installation. Later designs use a so-called short stack, which is about the length of a stock system, eliminating the support rod. Both the original design and the optional short stack are available in either stainless steel or ceramic-coated stainless. For some applications, only the short stack is available.For a Grumman Traveler, Cheetah or Tiger, Power Flow says youll immediately notice a 30 to 130 RPM increase at full throttle, plus significant climb rate improvement. Similar claims exist for other engine/airframe combinations for which Power Flow offers an exhaust. (See the table at left.)
Since theres no free lunch, especially with airplanes and 100LL, more power means higher fuel burn than in an unmodified airplane. Power Flows fuel-savings claim comes from being able to cruise at the same speed as before the conversion, using less throttle and therefore less power. Many owners report their fixed-pitch propellers tended to turn too fast on takeoff or in cruise after the conversion and have re-pitched the prop, returning RPM to POH values while still seeing increased climb and cruise performance.
Using the Grumman Traveler, Cheetah or Tiger as an example and choosing to cruise at the same airspeed as before, the company says youll save up to 2.2 gallons per hour. If so, and at $4 a gallon for 100LL, the math is easy. Thats about a $10/hour savings in fuel. Or you can go faster and get to your destination sooner.
System prices range from $2695 in polished stainless for an O-320- or O-360-powered Glastar/Sportsman 2+2 to $5600 for a ceramic-coated system to fit a Cessna Cardinal RG. Most installations are straightforward, although some Mooneys may require additional parts and Diamonds DA40 may require a prop change and/or a different engine baffling system. The companys products include an unconditional 60-day (from date of installation) money-back guarantee on the purchase price-you eat the shipping.
Installation should take about four or five hours at whatever rate your shop charges. Plan on that extra expense when factoring how long, if ever, youll recoup the acquisition and installation costs through reduced fuel consumption.
What Customers Say
If readers who responded to our feedback request are any indication, the majority of Power Flow customers are happy with their purchase. Several sent in detailed before and after results, including static RPM readings, fuel flow, time-to-climb and full-throttle cruise speeds. A few said they experienced no performance gains; others only noticed improved climb rates. Those who chose to return their system to Power Flow for a refund seemed happy with the companys response.
A typical report came from Greg Burnett of Dodge Center, Minnesota. “I bought one of the first Power Flow exhausts for my 1974 Cessna 172M with a Lycoming O-320-E2D. The installation was a simple remove/replace and the added strut was easy to install. I found that my static RPM had gone from the low end of the allowable range to the high end, an increase of 150 RPM. In flight, I gained about 150 FPM in climb and in level flight below 3000 MSL, I could exceed redline RPM. I also found I could now run lean of peak, which resulted in lower fuel burns and CHTs and very efficient flight.”
Stan Fetter is a fleet operator based in the Washington, D.C., area. His three O-320-H2AD-equipped 172N Skyhawks saw heavy use flying traffic reporters before being converted to Thielert diesels. He reports 150 FPM or so improvement in climb rate, with fuel savings a little over one GPH, depending on the airplane and pilot, “which added up pretty quickly when we were flying 325 hours or so per month across all three airplanes.”
Fetters Power Flow systems held up well: “There was a bracket that cracked periodically; over the life of one system we probably had to weld it a couple of times and eventually replaced it. I only actually wore one of them out, and that was a first-generation system that went well over 7000 hours. In my case, they paid for themselves between longevity and reduced fuel burn,” Fetter said.
Morton Boyd installed a Power Flow on his 1970 Mooney M20F, with an IO-360. “Generally, I would summarize as follows: 15 percent faster climb, 4 to 6 knots faster cruise, .5 GPH lower fuel burn.” See the table above for Boyds detailed before-and-after numbers.
But not all Power Flow customers saw Boyds results. “After a few hours of us, one of the exhaust pipes separated from the cylinder head flange, in flight, causing a dangerous condition and resulting in an immediate precautionary landing,” wrote another Mooney M20F owner, Alex Melton. “In the months following, several different pipes were sent from Power Flow and installed to try and alleviate the cracking at this flange. The results were always the same: After about three to five hours, the pipe would crack near the flange. Melton got a refund and installed another aftermarket exhaust, which proved trouble-free.
If we had an airplane for which one was available-and we either needed a new exhaust, flew the airplane a lot or wanted some extra performance-wed definitely consider a Power Flow exhaust. For around $4000, or maybe 10 percent of the airplanes value, most customers will see at least a small boost in performance, whether measured in improved climb rate, higher top speeds and/or reduced fuel consumption at the same cruise speed as before. Theres very little available on the aftermarket these days providing the same return rate on investment.
But ensuring the investment pays off requires us to, you know, actually fly the airplane. A lot. If most of your flying is in search of the $100 hamburger, whether you get there two minutes early or two minutes late isnt worth the acquisition and installation expense, in our view. Wed want such a purchase to break even for us after two or three years. Using Power Flows fuel-savings numbers (which might be a bit high for the real world), if we figure $4000 for the parts, plus another $500 for installation, wed need to fly about 450 hours at $4 a gallon before we broke even. At 150 hours a year and 2.2 GPH savings, it would take us about three years to get there.
Of course, if you just want the fastest Skyhawk or Cherokee out there or want a better designed exhaust, Power Flow is worth a look. Although owners report good results, even Power Flow says not all airplanes will benefit from one of its exhausts. But the companys money-back guarantee almost makes it a no-brainer to give it a try.