RE: model tech mach racer
I flew my 2 with a Magnum .52XLS with a Rossi short pipe and an OS .50SX with a Jett. Both used 9-7 props.
First, my testing on the black Mac's muffler is that it will cut your rpm, not increase it. The stock muffler without the baffle will be faster.
Second, a 10-7 will be a better prop for the .46AX, in my opinion. It's not a revver like the .50 or the TT .46.
Third, I always replace my pushrods on anything that can flutter with 4-40 size rods. The 2mm stuff that comes in the kit is pretty flexible.
Fourth, Mine were built without the gear and launched. Lots of drag on the gear.