Bill will runing tune pipe on FX 91 give more rmp.
Some thing like a Jet pipe and hurt it ,Cheer's
Thanx now.

A tuned pipe will give a bit more performance, if only because it will allow a more-free passage of exhaust gasses. Properly-tuned, you may see as much as a 1000 RPM gain or more. If the engine's kept rich of peak RPM, no damage should occur.

Ok so if I open up exhaust outlet we go more rpm.
Other than that Fx91! I would like 18k BUT the prop.
Exaust timeing is next.
Cheer's Bob.

I have mine on a Macs 15 CC tuned pipe mounted in a Sig King Kobra, it turns a 13x8 at about 12300 up from somwhere in the mid 11000's. Somewhere in the glow engine forum I posted which part numbers I used and how much was cut off the header. But I'm running it pretty rich. Has great power, though it's a gas guzzler.

18K RPM on a .91 FX would not be a reasonable RPM level. That's much too high, and well out of a reasonable RPM range for flying almost any airplane. We really doubt the engine will hold up very long if you're going to be running it at that RPM. At high RPM levels, you'll also have to go to a fuel with 22%-24% oil content to make sure the lower-end of the connecting rod gets enough lubrication...otherwise, it can seize and blow in seconds. We'd blown rods on fuel with 15% nitro and 18% oil content in ducted-fan engines...they'll run beautifully on 10% nitro and 24% oil. Oil should be a blend of castor and synthetic, preferably 50/50...more castor is better.