My test show, the tail pipe is everything... It all has to do with engine temp..

A large tail pipe on a small engine will drop the engine temp quit a bit and result in lower static thrust..

I found a big bypass on a small engine works fine. It seems these engines are sensitive to back pressure. To much back pressure and you get tons of thrust
But it may over temp… For more common is, to little back pressure. Somehow big tail pipes will help
Suck the air out the back of a small engine, resulting in lower temp, less fuel burn, and less static thrust.

The way you maximize your tail pipe is this..

First run you engine on the test stand and take note of the temp at Max power.

Install you engine in you plane and run it again. Install a tail pipe that reduces the engine temp the least..

I found the best way to do this is taper the tail pipe just a bit to get the temp back up to something close
To the test stand temp..

Hope this helps..

Eddie Weeks
http://www.variablepitchhobbies.com/