Don't change too many things at one time. Just try a prop and see how it performs. Once you find a prop that works, then change the muffler and see if you can get an increase in performance. If the performance drops, then try another muffler. If you change too many things at once, you will never know where to go next. You will be just chasing your tail around in a circle.

Just because you are getting high rpm, doesn't mean you're getting the power. There is a balance between the rpm and the power curve.

RPMs are great, but if the prop isn't getting a large enough bite, you won't have the climb. However, if you are getting too much bite, the engine will bog down on the climb where you will lose the power you need to get to the top.

The less bite you get from the prop will get you more rpm just not the forward speed. However, too much bite will bog the engine down because it can't reach the tip of the power/torgue curve and you will again lose forward speed.

One method that can be used to find the right combination is to tie a fish scale to the tail of the plane. Try different props and see what the scale reads. Then try different mufflers and again see what the fish scale reads. Keep this up and you will find the right combination prop /muffler to get the most out of the plane.

Frank