stg61abc, I ran the engine again with a new ST muffler.
Powermaster 5/22 GMA
Rev Up 10x6
14,111 peak
6.33lbs static thrust
1.24hp
Wow! I gained 622rpm. With an APC 10x6 it would probably rev a bit higher still (but, I don't have one).
Thanks for the muffler suggestion! I wonder what it would do on a pipe?
David