Harry,

Thank you for your detailed report of OS .91 engine modifications. I looked up OS EX502 on the Tower Website and got three hits:

1. LXCC14 Tuned Pipe Set
2. LXCN34 Header Pipe In
3. LXCN35 Header Pipe Out

It appears that the Tuned Pipe Set requires either the LXCN34 Header Pipe In (Inside fuselage installation) or the LXCN35 Header Pipe Out (Outside fuselage installation) to operate using the tuned pipe. From your description, it looks like you used one of the two header pipes without the tuned pipe to achieve a 400 RPM power increase to 9600 RPM.

I have two questions:

1. Which header pipe did you use, LXCN34 or LXCN35?
2. Did you try running the OS .91 without any header pipe at all, and if so, what were the results?

Here is where I am headed with all this. I am starting to think that you might be experiencing a mild tuning effect from the header pipe installation rather than a reduction in exhaust system restriction (back pressure). This might account for the significant RPM drop when using an APC 14-8 prop if the engine dropped below the header pipe tuning band. This is sometimes referred to as "falling off the pipe". Two stroke engines can behave this way with tuned pipes, and it is my understanding that it can affect four strokes as well but to a much lesser extent. Any information that you might have would be appreciated.

Thank you,

Bruce