Monthly Archives: December 2014

Does the PFS have a flame tube?

This was asked on another user group after a pilot experienced a large loss of power using a traditional exhaust.

In a traditional exhaust, gases are shot at each other and have to diffuse through a flame tube to exit. When that flame tube breaks down, it is hoped that the chunks are small enough to not block the exit from the muffler and thereby causing a potentially dangerous power loss or new hole in the muffler for your air pump (the engine) to relieve the pressure it created.

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Example of a stock exhaust system

One of the engineered “improvements” of the Power Flow over traditional Elano style (and other standard) exhaust systems is to allow an uninterrupted pathway for the exhaust gases in the muffler area. For all Exhaust systems except Cessna “short Stack” and our earliest Mooney 200 HP exhaust (PFS-16201 without the quiet pipe), the muffler consists of a 14 inch stainless steel perforated tube that has a 2.0 inch diameter. The gases exit the 4 to 1 collector, go down a 2.0 inch pipe and enter the muffler area and continue down the 2.0 inch inner diameter of the perforated tube. If you look at the PFS muffler, the outer area of the pipe has an expansion from a 2.0 inch area to a 3.5 diameter. On the inside of the muffler, the perforated tube (called a muffler insert) is inside the outer section creating a constant 2.0 inch pathway for the exhaust gas. This is muffler insert is based upon automotive type sound suppression, but adapted for aviation. The inner wrapping that is directly exposed to the exhaust gases is made up of INOX stainless steel fibers, with the outer layer a high density BASALT wrap. It looks a little like fiberglass, but it is brown in color.

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View looking down installed muffler insert inside tailpipe

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Uninstalled muffler insert

This high density wrap around the perforated tube is designed to be sacrificial. It will, over time, break down into very tiny hairs and blow out the tail pipe. The structure of the inner stainless steel tube is designed to stay intact and isn’t standing in the way of the gas pathway. On condition (we have seen between 400 and 800 hours) you remove the entire muffler insert, and replace it with another for about 15-30 minutes labor and $180.00 for the part (pricing correct as of December 2014, subject to change). This can be done in the field, while the exhaust is on the plane, if you want. No overhaul or sending the muffler out for rebuild.

To inspect the PFS muffler, you shine a light up the tailpipe. You are looking to make sure that the inner tube is intact and that you can see the light reflecting back from stainless steel hairs. I recommend that every PFS owner take 10 seconds and do this during pre-flight.

If you have a Cessna Short stack or the earliest 200 HP Mooney Power Flow exhaust, you don’t have a 14 inch muffler insert. The outer diameter of your tailpipe is 3.0 inches instead of 3.5 inches and what you have is a more rudimentary, sound suppressant cone inside the last 5 inches of the tailpipe.

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PFS Short Stack Tailpipe

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Installed Muffler Insert

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Cessna Short Stack with a Silencer Cone

Because this cone doesn’t have any material on it, there isn’t anything to “wear out.” It is possible that over time the cone will deteriorate and when that happens, it can be easily replaced.

INSPECTION & PREFLIGHT

To inspect the PFS muffler, you shine a light up the tailpipe. If you have muffler insert, you are looking to make sure that the inner is intact and not deformed and that you can see the light reflecting back from stainless steel hairs. I recommend that every PFS owner take 10 seconds and do this during pre-flight. If you have a cone type (Cessna short stack and early 200 HP Mooneys), you want to make sure that the muffler cone is still intact.

Darren Tilman
General Manager

Edited 01/08/2015: Previous posting reflected outdated pricing and misstated some technical aspects. We apologize for any confusion.

Cessna Short Stack vs. Classic – difference?

The only measurable performance difference between the short stack vs our classic (long pipe) are found only on the Cessna 172 and 177. This does not apply to Grummans and Piper installations.

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Cessna 172 (Short Stack)

The performance differences are subtle and only seen in three areas, which the average customer/non test pilot may not even notice.

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Short Stack Ceramic tail pipe 

1) The RPM gain on the C172 with the classic we measured at 65 RPM on our test aircraft. The short stack showed 58 RPM. Slight loss in performance, but hard to see.
2) The classic exhaust system allowed aggressive leaning – lean of peak operations for fantastic fuel savings. The reality is that the vast majority of our customers don’t use lean of peak operations so this is an unrealized benefit.
3) We have a report from a customer who routinely flew their C172 at 15500 MSL Feet with the PFS classic. They converted to the Short stack and reported the loss of Lean of peak leaning and they were unable to get above 14000 MSL – so we have a reduction in the gain in service ceiling.

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Classic Kit

From an installation and maintenance aspect, the short stack for the Cessna models wins in most areas:

The short stack is: 1.5 pounds lighter, takes one hour less to install (average) and does not require you to remove the lower pipe to remove the cowling.

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172 with Classic Tailpipe

From a noise perspective:
The noise on the ground in the cabin with the short stack is a bit more “sharp”, but in flight our customers have told us the noise appears to be lower than with the classic system. This is likely because the short stack points down and is 2 feet forward of the cabin. The classic pipe is 2 feet closer to the cabin and pointed straight back (aft.)

Grumman and Piper short stacks are approximately the same length as the “classic” systems they replace, so there is no change in tuned length or performance. The Cessna Short stack is approximately 40% of the tuned length of the classic pipe, hence the slight drop in performance.
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Darren Tilman
General Manager
Power Flow Systems, Inc.