Running a Fox Eagle IV .60 engine, current production;
Using a Zinger 11x7 Prop I ran the Stock muffler, the Fox Quiet Muffler and the Performance Specialties (AKA Nelson) Ultrathrust muffler on the New Eagle IV .60 engine. With the stock OEM muffler it runs around 11,500 rpms or so. With the Fox Quiet Muffler it runs better still with a little over 13,000 RPMs rpms so it has maybe a little bit of a tuned pipe effect.
But with the Performance Specialties UltraThrust muffler you can get it to peak out a little over 14,500 rpms but I backed it off as I didn't want to hold it there for too long with a new engine. But it is still new and may do even better with more time on it.

I ran my old Eagle III with the Ultrathrust muffler and got it to peak out at a little over 14,000 rpms too.
Going over 14,000 RPMs exceeds the design specs for the .61 size Ultrathrust muffler. The instructions say the .61 muffler is tuned for the 11,000 to 14,000 rpms range.

Running the Eagle IV 60 engine with the Ultrathrust muffler video clip:
[youtube]http://www.youtube.com/watch?v=qOeTpYEouiU[/youtube]


Fox Eagle IV with Ultathrust muffler: I fabricated a bridge adapter to adapt a OS 61SF sized muffler to fit the Fox engine.



Fox Eagle IV .60 engine tachometer reading using the Ultrathrust Muffler:


Fox Stock muffler on the Fox Eagle IV .60 engine


Fox Eagle IV .60 engine the the Fox Quiet Muffler.


Running the older Fox Eagle III .60 engine with a Ultrathrust muffler:


Fox Eagle III tach reading: