February 2009 Issue
Power Flow Exhaust: Owners Say Thumbs UP
Available for four-cylinder Lycomings, Power Flow is a bolt-on solution that tweaks power and efficiency. If it doesn’t work out, there’s a money-back guarantee.
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 engine’s ability to breathe, according to theory, and you’ll 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 cylinder’s 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 charge’s 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 you’ll 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 there’s no free lunch, especially with airplanes and 100LL, more power means higher fuel burn than in an unmodified airplane. Power Flow’s 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.
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