Hi!

I bought and used my .91FX with this original short muffler and also tried it with a tuned pipe and never had any running problems.
Unfortunately I don't have an APC 14x6 and have not tried yet.
With a 15x6 MARO wooden prop it turns about the same RPM as with the 14x8 APC.
I also have an APC 15x4w but cannot remember what it turned...