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Unread 01-24-2008, 02:53 PM
dtilman dtilman is offline
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Location: Daytona Beach, FL
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Default No performance improvement - fixed pitch

Question: I installed a Power Flow on a 68 Cardinal with a Lycoming 150 HP engine, and a fixed pitch propeller. Everyone I know with the exhaust loves it and sees a dramatic improvement. I performed careful flight tests and I don't think I saw any improvement. What is happening and why?

Answer:
The Power Flow exhaust should provide an immediate increase in RPM when at full throttle on the ground (Static run-up) and during flight, as compared to the original exhaust system's performance. If you aren't seeing any real improvement in climb with the Power Flow, or any increase in RPM either at full Static or in flight, then something else is limiting the Power Flow exhaust from doing its' job.

The typical result is to see 30-130 more RPM with the average gain being about 75 more RPM.

There are two variables that are most likely to be the culprit:

1) A worn camshaft; Insufficient lift or duration shortens the time period that we use to scavenge or to empty the cylinder or

2) the engine might be running exceptionally lean. Assuming the mixture control is rigged correctly and is full rich, and are using true “full throttle” there is nothing a pilot can do to control the amount of fuel going in as that is controlled by the vents and main metering jet on the carburetor.

Generally, too lean of an engine is the more likely issue.

If a plane is only equipped with a single EGT gage, we don’t know what is going on in terms of temperature on each cylinder or the fuel flow and have only got the engine's RPM to indicate power.

The flight test methodology used appeared to be very comprehensive and consistent. However, the leaning of the engine done during the climb was done to a constant EGT and I explained we have found that this method is not as valid as leaning to peak RPM. Peak EGT changes with altitude and atmospherics as well as with the kind of Exhaust system used. It is typical to see a 50-75 degree higher “peak” with a PFS than the stock exhaust. If the stock peak was 1425 at 3500 feet, and they lean 75 rich of that (1350), they are likely at or very near best power/peak RPM. However, the PFS peak may be 1475 or 1500 degrees at 3500 feet, so leaning to 1350 is actually a great deal cooler/richer – resulting in less than best power and therefore lower performance. This is the problem with the constant EGT method – it doesn’t reliably produce best power.

To rule in or out too lean of a carburetor, perform the carb richness test.

Basically, get the engine up to operating temperature, go to full throttle at full rich mixture, wait for stabilization and record the RPM and other parameters. Then, slowly lean the mixture back to look for a rise in RPM. Lycoming and Power Flow’s target is 30 to 50 RPM rise, or a rise of 150-200 degrees in EGT. This extra fuel is necessary for cylinder cooling and detonation margin. However, it is more than enough fuel and chokes the engine when you get above 3500-5000 feet, hence the reason to lean to obtain “best power.”

If you don't see any RPM rise whatsoever when going from full rich to any point leaner, then you likely have a very lean running carburetor. Without extra fuel, the PFS cannot make extra power.

In addition, the engine is likely running much hotter than it should and this can reduce cylinder longevity. It is also possible that the engine isn't generating true full power and this could be a problem. Regardless of the exhaust system used, the problem should be addressed.

At this point, we recommend contacting us to discuss other testing methods or options.
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Darren Tilman
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Power Flow Systems, Inc.
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