Background on Beech Sundowner testing - why the mixture is so critical
Power Flow for the Beech Sundowner
You might recall that in 2007, Power Flow Systems, Inc. postponed the release of a tuned exhaust system for the Beech Sundowner series of aircraft equipped with the Lycoming O-360 engines.
We took this action based upon our testing results on our own Sundowner aircraft. Although we saw an improvement with the PFS exhaust, it was not as dramatic of an improvement as we and our customers have come to expect. Although we had already completed all the FAA certification work to develop the exhaust system for the Sundowner, we choose to not bring the exhaust to the market until we had answered two additional questions.
1) Why doesn’t the PFS exhaust appear to work nearly as well on the Sundowner as other aircraft with the same engine installation?
2) Is the lower boost in performance we saw related to the Sundowner design, or is it unique to our actual aircraft?
After further testing and research, we have made the decision to release our FAA certified exhaust system for the Sundowner. It appears that the Sundowner is more prone to having a “lean” carburetor, so we caution any prospective installer to verify that their engine is “rich” enough to gain the most benefit from the Power Flow Exhaust.
Click on the attachment at the bottom of this page to download the PDF testing document.
Please read on for further details about this testing
The basic operational benefit of a Power Flow exhaust is the improved capability to empty each cylinder more completely than a traditional exhaust, resulting in a higher effective power output of the engine. However, as part of this process, the engine has to have the ability to increase its fuel flow to allow the engine (and exhaust) to make additional power. On nearly every Lycoming engine we have tested, the carburetor sees a higher amount of air flow when a Power Flow exhaust is installed and adds a proportional amount of extra fuel. The engine will burn more fuel at full throttle with a PFS exhaust as compared to a stock exhaust. In other words; the engine uses more fuel to make more power at full throttle.
We suspected that our particular Sundowner might have a leaner than normal carburetor. We suspected this because you should be able to run the engine at full power, full rich mixture on the ground and achieve the maximum (static) RPM as listed in the aircraft type certificate datasheet. Further, when leaning the engine you should see a 30-50 RPM rise, (or at least a 200 degree rise in EGT) when leaning for best power (peak RPM). When we purchased our Sundowner, we saw no more than a 10 RPM gain when leaned during the full power ground run.
We did our series of tests and concluded that the engine was not sufficiently “rich” to see a significant power improvement from the PFS tuned exhaust or adequate cooling for the cylinders. We later changed the carburetor for a “richer” model. That testing showed a slight improvement in ground testing, but only a gain on the ground of 25-30 RPM from full rich to peak RPM.
We have talked with a number of engine and airframe experts, including Mike Rellihan, an experienced Beechcraft technician in the Beech Aero Club. According to Mike, the Beech carburetor system design is not a common configuration, as it allows fuel to enter the carburetor from two different inlets. The more typical configuration is for a single inlet for fuel. Based on his considerable experience working on the Beech, it is his opinion that the carburetors on Beech Sundowners often can be “too lean.”
Mike’s information confirmed what our research had indicated. We started looking for another test aircraft and the first volunteer conducted a series of tests at our request. The result? His engine had absolutely no rise in RPM on the ground or in flight. Not only does this mean he currently isn’t a good candidate for a Power Flow Exhaust, but his engine has insufficient fuel for adequate cooling and detonation margin.
We then conducted flight tests on another Sundowner that had been identified as having a “richer” carburetor. This aircraft was well equipped with an engine monitor and fuel flow instrumentation; it gained approximately 30 RPM when leaned on the ground. 30 RPM is at the bottom of our “rich enough” scale, but it provided a good starting point. Because this aircraft had a richer carburetor, we thought that this aircraft would respond even better to the Power Flow exhaust than our test aircraft. When flight tested with a Power Flow exhaust, the sundowner did gain an average of 30 RPM during the climb, and the ability to develop up to 50 more RPM in cruise flight, as compared to the original/stock exhaust.
Although these results aren’t quite as strong as we would like to see, there is a measurable and noticeable improvement in the performance on the Sundowner from a Power Flow exhaust, and it is within the range of what we promise.
We have stated that the range of expected benefits from installing a Power Flow exhaust include:
• 30-130 more RPM
• 50-250 feet per minute higher rate of climb
• Fuel savings at current speeds
• Smoother running engine
Both Sundowners that we have tested have seen improvements in line with our promised benefits. Although not in the top end of our results, the installation of the Power Flow exhaust provided a genuine improvement.
As a result of this testing, we have made the decision to release our FAA Certified exhaust system for the Sundowner. As it appears that the Sundowner is more prone to having a “lean” carburetor, so we caution any prospective installer to verify that their engine is “rich” enough to gain the most benefit from the Power Flow Exhaust. We want any customer that buys a Power Flow System to be happy, so we strongly suggest conducting a simple test on your engine before buying a Power Flow to avoid disappointment.
The test is available at the bottom of this message.
We are now accepting orders and can ship the exhaust system within 6 weeks. We are offering the complete exhaust system for the previously promised price of $4,290 for a polished tailpipe and $4,490 for a ceramic coated tailpipe. There were 8 previous deposit holders for the Sundowner exhaust, and we will honor our “pre-production” discounts to those individuals.
Power Flow Systems, Inc. offers an unconditional 60 day money-back guarantee on its exhaust systems. Install it, try it and if you aren’t totally happy, return it for a full refund of the purchase price of the exhaust. Labor and shipping charges are excluded from the money-back guarantee.
For more details, please go to our “product” page for the Beech Sundowner series, or click the link below.
Power Flow Systems, Inc.
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