[quote]ORIGINAL: DarZeelon
A Mac's OS.46FX, ready-to-use, full length pipe, will only be at its best with 10x6 and 10x7 props and with 10-15% nitro fuel.
Change anything and you could be out of the pipe's RPM range.

Dar,
Your statement is very misleading.

When I speak of using a tuned pipe; I'm assuming the user will adjust the header length for best operation. There is plenty of info about how to tune the pipe right here on RCU. Any of the macs 40 - 45 Pipes can be tuned for 10 to 18K RPM with a 46FX depending on the prop.

Since speed is what he is after; forget the 10" props with a 46FX.
Use a 9" or less diameter prop depending on the airframe's thrust requirement for takeoff. Use 15 to 20% Nitro (We're in the good ole USA Right!) Set the tuned pipe for about 17" (glow plug to end of converging cone) and use a prop pitch that limits RPM to about 15K on the ground.
With this setup you will be much faster than with a tuned muffler The tuned mufflers do have a wider band over which they add a boost but the boost they add is much smaller.